What enzymes are present in the human digestive system?
Human digestive system
Chewing, in which food is mixed with saliva, begins the mechanical process of digestion. This produces a bolus which can be swallowed down the esophagus to enter the stomach. Here it is mixed with gastric acid until it passes into the duodenum where it is mixed with a number of enzymes produced by the pancreas. Saliva also contains a catalytic enzyme called amylase which starts to act on food in the mouth. Another digestive enzyme called lingual lipase is secreted by some of the lingual papillae on the tongue and also from serous glands in the main salivary glands. Digestion is helped by the chewing of food carried out by the muscles of mastication, by the teeth, and also by the contractions of peristalsis, and segmentation. Gastric acid, and the production of mucus in the stomach, are essential for the continuation of digestion.
Porphyria
There are eight enzymes in the heme biosynthetic pathway, four of which—the first one and the last three—are in the mitochondria, while the other four are in the cytosol. Defects in any of these can lead to some form of porphyria.
Human digestive system
Teeth are complex structures made of materials specific to them. They are made of a bone-like material called dentin, which is covered by the hardest tissue in the body—enamel. Teeth have different shapes to deal with different aspects of mastication employed in tearing and chewing pieces of food into smaller and smaller pieces. This results in a much larger surface area for the action of digestive enzymes.
The teeth are named after their particular roles in the process of mastication—incisors are used for cutting or biting off pieces of food; canines, are used for tearing, premolars and molars are used for chewing and grinding. Mastication of the food with the help of saliva and mucus results in the formation of a soft bolus which can then be swallowed to make its way down the upper gastrointestinal tract to the stomach. 
The digestive enzymes in saliva also help in keeping the teeth clean by breaking down any lodged food particles.
Proteolysis
In human digestion, proteins in food are broken down into smaller peptide chains by digestive enzymes such as pepsin, trypsin, chymotrypsin, and elastase, and into amino acids by various enzymes such as carboxypeptidase, aminopeptidase, and dipeptidase. It is necessary to break down proteins into small peptides (tripeptides and dipeptides) and amino acids so they can be absorbed by the intestines, and the absorbed tripeptides and dipeptides are also further broken into amino acids intracellularly before they enter the bloodstream. Different enzymes have different specificity for their substrate; trypsin, for example, cleaves the peptide bond after a positively charged residue (arginine and lysine); chymotrypsin cleaves the bond after an aromatic residue (phenylalanine, tyrosine, and tryptophan); elastase cleaves the bond after a small non-polar residue such as alanine or glycine.
Digestive enzyme
Some carnivorous plants digestive enzymes:
Astacus astacus
Astacins are a family of digestive enzymes, discovered in the 1990s, which were first isolated from "A. astacus". More than 20 enzymes of this group have since been discovered in animals from "Hydra" to humans.
21-Hydroxylase
21-hydroxylase is a complex of three independent and identical enzyme subunits. Each subunit in the human enzyme consists of 13 􏰁􏰁α-helices and 9 ß-strands, formed into a triangular prism-like tertiary structure. The iron(III) heme group that defines the active site resides in the center of each subunit. The human enzyme binds one substrate at a time. In contrast, the well-characterized bovine enzyme can bind two substrates. The human and bovine enzyme share 80% amino acid sequence identity, but are structurally different, particularly in loop regions, and also evident in secondary structure elements.
Epoxyeicosatetraenoic acid
The EEQ-forming cytochromes are widely distributed in the tissues of humans and other mammals, including blood vessel endothelium, blood vessel atheroma placques, heart muscle, kidneys, pancreas, intestine, lung, brain, monocytes, and macrophages. These tissues are known to metabolize arachidonic acid to EETs; it has been shown or is presumed that they also metabolize EPA to EEQs. Note, however, that the CYP epoxygenases, similar to essentially all CYP450 enzymes, are involved in the metabolism of xenobiotics as well as endogenously-formed compounds; since many of these same compounds also induce increases in the levels of the epoxygenases, CYP oxygenase levels and consequently EEQ levels in humans vary widely and are highly dependent on recent consumption history; numerous other factors, including individual genetic differences, also contribute to the variability in CYP450 epoxygenase expression.
Digestion
In mammals, preparation for digestion begins with the cephalic phase in which saliva is produced in the mouth and digestive enzymes are produced in the stomach. Mechanical and chemical digestion begin in the mouth where food is chewed, and mixed with saliva to begin enzymatic processing of starches. The stomach continues to break food down mechanically and chemically through churning and mixing with both acids and enzymes. Absorption occurs in the stomach and gastrointestinal tract, and the process finishes with defecation.
Amphetamine
CYP2D6, dopamine β-hydroxylase (DBH), flavin-containing monooxygenase 3 (FMO3), butyrate-CoA ligase (XM-ligase), and glycine N-acyltransferase (GLYAT) are the enzymes known to metabolize amphetamine or its metabolites in humans. Amphetamine has a variety of excreted metabolic products, including , , , benzoic acid, hippuric acid, norephedrine, and phenylacetone. Among these metabolites, the active sympathomimetics are , , and norephedrine. The main metabolic pathways involve aromatic para-hydroxylation, aliphatic alpha- and beta-hydroxylation, N-oxidation, N-dealkylation, and deamination. The known metabolic pathways, detectable metabolites, and metabolizing enzymes in humans include the following:
Human digestive system
There are diagnostic tools mostly involving the ingestion of barium sulphate to investigate disorders of the GI tract. These are known as upper gastrointestinal series that enable imaging of the pharynx, larynx, oesophagous, stomach and small intestine and lower gastrointestinal series for imaging of the colon.
Cytochrome c oxidase subunit I
The enzyme complex consists of 3-4 subunits (prokaryotes) up to 13 polypeptides (mammals) of which only the catalytic subunit (equivalent to mammalian subunit I (COI)) is found in all heme-copper respiratory oxidases. The presence of a bimetallic centre (formed by a high-spin heme and copper B) as well as a low-spin heme, both ligated to six conserved histidine residues near the outer side of four transmembrane spans within COI is common to all family members. In contrast to eukaryotes the respiratory chain of prokaryotes is branched to multiple terminal oxidases. The enzyme complexes vary in heme and copper composition, substrate type and substrate affinity. The different respiratory oxidases allow the cells to customize their respiratory systems according to a variety of environmental growth conditions.
D-amino acid oxidase
While D-amino acid oxidase differs to some extent between various organisms, the structure is basically the same across most eukaryotes, excluding plants. This enzyme is a flavoprotein belonging to the FAD dependent oxidoreductase family, and acts on the CH-NH group of D-amino acid donors with oxygen as acceptor. It is also considered a peroxisomal enzyme containing FAD as a cofactor. Each DAO monomer has an FAD-binding domain (FBD) containing a Rossman fold, and a substrate-binding domain (SBD) that also forms an interface with the other monomer in the protein. DAO exists as a dimer, with each monomer containing both an FBD and SBD. Each monomer is composed of 347 amino acids in human DAO, though among other eukaryotes the protein can range from 345 to 368 amino acids long. In human DAO, the two monomers are connected in a head-to-head fashion. DAO of other organisms, such as yeast, can be present as head-to-tail dimers. The hDAAO gene is found on chromosome 12 and contains 11 exons.
Organophosphorus acid anhydrolase
Organophosphorus acid anhydrolase (OPAA) is an enzyme that been shown to be particularly effective in detoxifying organophosphorus-containing compounds, such as deadly nerve gas used in chemical warfare. The enzyme is found in a diverse range of organisms, including protozoa, squid and clams, mammals, and soil bacteria. A highly active form of the enzyme is typically isolated from the marine bacteria "Alteromonas undina" for laboratory study. This form is both halophilic and thermophilic, making it particularly useful for detoxification applications. A slightly less active variant of OPAA has also been isolated in mung beans and slime mold duckweed.
Human digestive system
A major digestive organ is the stomach. Within its mucosa are millions of embedded gastric glands. Their secretions are vital to the functioning of the organ.
Discovery and development of phosphodiesterase 5 inhibitors
The human genome contains at least 21 genes involved in determining the intracellular levels of cAMP and cGMP by the expression of phosphodiesterase proteins or PDE’s. These PDE’s are grouped into at least 11 functional subfamiles, named PDE1-PDE11. PDEs are enzymes that hydrolyze cyclic adenosine 3,5-monophosphate (cAMP) and cyclic guanosine 3,5-monophospahate (cGMP), which are intracellular second messengers, into AMP and GMP. These second messengers control many physiological processes.
The cAMP is formed from ATP by the enzyme adenylyl cyclase and cGMP is formed from GTP by the enzyme guanylyl cyclase which are either membrane bound or soluble in the cytosol. When soluble it functions as a receptor for nitric oxide (NO) (see figure 1).
Formation of cGMP initiates several reactions in the body including influence on cGMP ion channels, cGMP binding proteins and protein kinase G (PKG). The effect on PKG reduces levels of calcium leading to relaxation of smooth muscles (see figure 2). 
The PDE5 enzyme is specific for cGMP which means it only hydrolyzes cGMP but not cAMP. The selectivity is mediated through an intricate network of hydrogen bonding which is favorable for cGMP but unfavorable for cAMP in PDE5.
By inhibition of PDE5 enzyme the cGMP concentration will be raised and can therefore increase the relaxation of smooth muscles. PDE5 has only one subtype, PDE5A, of which there are 4 isoforms in humans called PDE5A1-4. The difference in PDE5A1-3 isoforms is only in the 5´ end of the mRNA and corresponding N-terminal of the protein.
Lactose intolerance
Lactase enzymes similar to those produced in the small intestines of humans are produced industrially by fungi of the genus "Aspergillus". The enzyme, β-galactosidase, is available in tablet form in a variety of doses, in many countries without a prescription. It functions well only in high-acid environments, such as that found in the human gut due to the addition of gastric juices from the stomach. Unfortunately, too much acid can denature it, so it should not be taken on an empty stomach. Also, the enzyme is ineffective if it does not reach the small intestine by the time the problematic food does. Lactose-sensitive individuals can experiment with both timing and dosage to fit their particular needs.
Diamine oxidase
The highest levels of DAO expression are observed in the digestive tract and the placenta. In humans, a certain subtype of cells of the placenta, namely the extravillous trophoblasts, express the enzyme and secrete it into the blood stream of a pregnant woman. Lowered diamine oxidase values in maternal blood in early pregnancy might be an indication for trophoblast-related pregnancy disorders like early-onset preeclampsia. Normally the enzyme is not or only very scarce present in the blood circulation of humans, but it increases vastly in pregnant women suggesting a protective mechanism against adverse histamine. It is also secreted by eosinophils. In case of a shortage of diamine oxidase in the human body, it may appear as an allergy or histamine intolerance.
Human digestive system
At the same time that protein is being digested, mechanical churning occurs through the action of peristalsis, waves of muscular contractions that move along the stomach wall. This allows the mass of food to further mix with the digestive enzymes. Gastric lipase secreted by the chief cells in the fundic glands in the gastric mucosa of the stomach, is an acidic lipase, in contrast with the alkaline pancreatic lipase. This breaks down fats to some degree though is not as efficient as the pancreatic lipase.
Enoyl CoA isomerase
The enzymes of the hydratase/isomerase or crotonase superfamily are typically trimeric disks dimerized into hexamers. The wide range of their substrate-enzyme specificity derives from the variations in the distances between the trimeric disks and their orientation. However, the human mitochondrial enoyl-CoA isomerase is a trimer and orients the fatty acid tail in a completely different direction from that seen in the hexamers. The trimeric disk of peroxisomal enzymes in "Saccharomyces cerevisiae" is displayed in figure 3.
Retinoid
Several enzymes that are present in the intestinal lumen may be involved in the hydrolysis of dietary retinyl esters. Cholesterol esterase is secreted into the intestinal lumen from the pancreas and has been shown, in vitro, to display retinyl ester hydrolase activity. In addition, a retinyl ester hydrolase that is intrinsic to the brush-border membrane of the small intestine has been characterized in the rat as well as in the human. The different hydrolyzing enzymes are activated by different types of bile salts and have distinct substrate specificities. For example, whereas the pancreatic esterase is selective for short-chain retinyl esters, the brush-border membrane enzyme preferentially hydrolyzes retinyl esters containing a long-chain fatty acid such as palmitate or stearate. Retinol enters the absorptive cells of the small intestine, preferentially in the all-trans-retinol form.
Efavirenz
Efavirenz is broken down in the liver by enzymes that belong to the cytochrome P450 system, which include both CYP2B6 and CYP3A4. Efavirenz is a substrate of these enzymes and can decrease the metabolism of other drugs that require the same enzymes. However, efavirenz also induces these enzymes, which means the enzyme activity is enhanced and the metabolism of other drugs broken down by CYP2B6 and CYP3A4 can be increased. People who are taking both efavirenz and other drugs metabolized by the same enzymes might need the dose of their drugs to be increased or decreased.
Enzyme
An important function of enzymes is in the digestive systems of animals. Enzymes such as amylases and proteases break down large molecules (starch or proteins, respectively) into smaller ones, so they can be absorbed by the intestines. Starch molecules, for example, are too large to be absorbed from the intestine, but enzymes hydrolyze the starch chains into smaller molecules such as maltose and eventually glucose, which can then be absorbed. Different enzymes digest different food substances. In ruminants, which have herbivorous diets, microorganisms in the gut produce another enzyme, cellulase, to break down the cellulose cell walls of plant fiber.
Human digestive system
The pylorus, the lowest section of the stomach which attaches to the duodenum via the pyloric canal, contains countless glands which secrete digestive enzymes including gastrin. After an hour or two, a thick semi-liquid called chyme is produced. When the pyloric sphincter, or valve opens, chyme enters the duodenum where it mixes further with digestive enzymes from the pancreas, and then passes through the small intestine, where digestion continues. When the chyme is fully digested, it is absorbed into the blood. 95% of absorption of nutrients occurs in the small intestine. Water and minerals are reabsorbed back into the blood in the colon of the large intestine, where the environment is slightly acidic. Some vitamins, such as biotin and vitamin K produced by bacteria in the gut flora of the colon are also absorbed.
Amanita ocreata
Amatoxins consist of at least eight compounds with a similar structure, that of eight amino-acid rings; of those found in "A. ocreata", α-amanitin is the most prevalent and along with β-amanitin is likely to be responsible for the toxic effects. The major toxic mechanism is the inhibition of RNA polymerase II, a vital enzyme in the synthesis of messenger RNA (mRNA), microRNA, and small nuclear RNA (snRNA). Without mRNA, essential protein synthesis and hence cell metabolism stop and the cell dies. The liver is the principal organ affected, as it is the first organ encountered after absorption by the gastrointestinal tract, though other organs, especially the kidneys, are susceptible to the toxins.
Succinate-semialdehyde dehydrogenase
Succinate-semialdehyde dehydrogenase is found in organisms ranging across the tree of life from bacteria to humans. It is important in the degradation of γ-aminobutyric acid in humans, and deficiency of the enzyme causes serious health effects (succinic semialdehyde dehydrogenase deficiency).
Acetylation
To date, six different NATs have been found in humans - NatA, NatB, NatC, NatD, NatE and NatF. Each of these different enzyme complexes is specific for different amino acids or amino acid sequences which is shown in the following table.
ALDH2
The ALDH2 gene is about 44 kbp in length and contains at least 13 exons which encode 517 amino acid residues. Except for the signal NH2-terminal peptide, which is absent in the mature enzyme, the amino acid sequence deduced from the exons coincided with the reported primary structure of human liver ALDH2. Several introns contain Alu repetitive sequences. A TATA-like sequence (TTATAAAA) and a CAAT-like sequence (GTCATCAT) are located 473 and 515 bp, respectively, upstream from the translation initiation codon.
The enzyme encoded by the human ALDH2 gene is a tetrameric enzyme that contains three domains; two dinucleotide-binding domains and a three-stranded beta-sheet domain. The active site of ALDH2 is divided into two halves by the nicotinamide ring of NAD. Adjacent to the A-side (Pro-R) of the nicotinamide ring is a cluster of three cysteines (Cys301, Cys302 and Cys303) and adjacent to the B-side (Pro-S) are Thr244, Glu268, Glu476 and an ordered water molecule bound to Thr244 and Glu476. Although there is a recognizable Rossmann fold, the coenzyme-binding region of ALDH2 binds NAD in a manner not seen in other NAD-binding enzymes. The positions of the residues near the nicotinamide ring of NAD suggest a chemical mechanism whereby Glu268 functions as a general base through a bound water molecule. The sidechain amide nitrogen of Asn169 and the peptide nitrogen of Cys302 are in position to stabilize the oxyanion present in the tetrahedral transition state prior to hydride transfer. The functional importance of residue Glu487 now appears to be due to indirect interactions of this residue with the substrate-binding site via Arg264 and Arg475.
Intestinal gland
The enterocytes in the small intestinal mucosa contain digestive enzymes that digest specific foods while they are being absorbed through the epithelium. These enzymes include peptidase, sucrase, maltase, lactase and intestinal lipase. This is in contrast to the gastric glands of the stomach where chief cells secrete pepsinogen.
Enoyl-CoA hydratase
ECH of rats is a hexameric protein (this trait is not universal, but human enzyme is also hexameric), which leads to the efficiency of this enzyme as it has 6 active sites. This enzyme has been discovered to be highly efficient, and allows people to metabolize fatty acids into energy very quickly. In fact this enzyme is so efficient that the rate for short chain fatty acids is equivalent to that of diffusion-controlled reactions.
Human digestive system
The parietal cells in the fundus of the stomach, produce a glycoprotein called intrinsic factor which is essential for the absorption of vitamin B12. Vitamin B12 (cobalamin), is carried to, and through the stomach, bound to a glycoprotein secreted by the salivary glands - transcobalamin I also called haptocorrin, which protects the acid-sensitive vitamin from the acidic stomach contents. Once in the more neutral duodenum, pancreatic enzymes break down the protective glycoprotein. The freed vitamin B12 then binds to intrinsic factor which is then absorbed by the enterocytes in the ileum.
Epoxyeicosatrienoic acid
The cytochrome P450 (CYP) superfamily of enzymes is distributed broadly throughout bacteria, archaea, fungi, plants, animals, and even viruses (see Cytochrome P450). The superfamily comprises more than 11,000 genes categorized into 1,000 families. Humans have 57 putatively active CYP genes and 58 CYP pseudogenes; only a relatively few of the active CYP genes code for EET-forming epoxygenases, i.e. protein enzymes with the capacity to attach atomic oxygen (see Allotropes of oxygen#Atomic oxygen) to the carbon-carbon double bonds of unsaturated long chain fatty acids such as arachidonic acid. The CYP epoxygenases fall into several subfamilies including CYP1A, CYP2B, CYP2C, CYP2E, CYP2J, and within the CYP3A sub family, CYP3A4; in humans, CYP2C8, CYP2C9, CYP2C19, CYP2J2, and possibly CYP2S1 isoforms are the main producers of EETs although CYP2C9, CYP2C18, CYP3A4, CYP4A11, CYP4F8, and CYP4F12 are capable of producing the EETs and may do so in certain tissues. The CYP epoxygenases can epoxidize any of the double bounds in arachidonic acid but most of them are relatively selective in that they make appreciable amounts of only one or two EETs with 11,12-EET and 14,15-EET accounting for 67%-80% of the product made by the cited CYP epoxidases as well as the main EETs made by mammalian tissues. CYP2C9, CYP2J9, and possibly the more recently characterized CYP2S1 appear to be the main produces of the EETs in humans with CYP2C9 being the main EET producer in vascular endothelial cells and CYP2J9 being highly expressed (although less catalytically active than CYP2C) in heart muscle, kidneys, pancreas, lung, and brain. CYP2S1 is expressed in macrophages, liver, lung, intestine, and spleen and is abundant in human and mouse atherosclerosis (i.e. Atheroma) plaques as well as inflamed tonsils.
ETEs are commonly produced by the stimulation of specific cell types. The stimulation causes arachidonic acid to be released from the "sn-2" position of cellular phospholipids through the action of Phospholipase A2-type enzymes and subsequent attack of the released arachidonic acid by a CYP epoxidase. In a typical example of this mechanism, bradykinin or acetylcholine acting through their respective Bradykinin receptor B2 and muscarinic acetylcholine receptor M1 or muscarinic acetylcholine receptor M3 stimulate vascular endothelial cells to make and release EETs.
Enzyme
In multicellular eukaryotes, cells in different organs and tissues have different patterns of gene expression and therefore have different sets of enzymes (known as isozymes) available for metabolic reactions. This provides a mechanism for regulating the overall metabolism of the organism. For example, hexokinase, the first enzyme in the glycolysis pathway, has a specialized form called glucokinase expressed in the liver and pancreas that has a lower affinity for glucose yet is more sensitive to glucose concentration. This enzyme is involved in sensing blood sugar and regulating insulin production.
Human digestive system
The stomach is a major organ of the gastrointestinal tract and digestive system. It is a consistently J-shaped organ joined to the esophagus at its upper end and to the duodenum at its lower end.
Gastric acid (informally "gastric juice"), produced in the stomach plays a vital role in the digestive process, and mainly contains hydrochloric acid and sodium chloride. A peptide hormone, gastrin, produced by G cells in the gastric glands, stimulates the production of gastric juice which activates the digestive enzymes. Pepsinogen is a precursor enzyme (zymogen) produced by the gastric chief cells, and gastric acid activates this to the enzyme pepsin which begins the digestion of proteins. As these two chemicals would damage the stomach wall, mucus is secreted by innumerable gastric glands in the stomach, to provide a slimy protective layer against the damaging effects of the chemicals on the inner layers of the stomach.
Drug interaction
An example of this is shown in the following table for the CYP1A2 enzyme, which is the most common enzyme found in the human liver. The table shows the substrates (drugs metabolized by this enzyme) and the inductors and inhibitors of its activity:
Exoenzyme
In eukaryotic cells, exoenzymes are manufactured like any other enzyme via protein synthesis, and are transported via the secretory pathway. After moving through the rough endoplasmic reticulum, they are processed through the Golgi apparatus, where they are packaged in vesicles and released out of the cell. In humans, a majority of such exoenzymes can be found in the digestive system and are used for metabolic breakdown of macronutrients via hydrolysis. Breakdown of these nutrients allows for their incorporation into other metabolic pathways.
Glutaminase
Humans express 4 isoforms of glutaminase. "GLS1" encodes 2 types of kidney-type glutaminase with a high activity and low Km. "GLS2" encodes 2 forms of liver-type glutaminase with a low activity and allosteric regulation. 
Glutaminases belong to a larger family that includes serine-dependent beta-lactamases and penicillin-binding proteins. Many bacteria have two isozymes. This model is based on selected known glutaminases and their homologs within prokaryotes, with the exclusion of highly derived (long-branch) and architecturally varied homologs, so as to achieve conservative assignments. A sharp drop in scores occurs below 250, and cutoffs are set accordingly. The enzyme converts glutamine to glutamate, with the release of ammonia. Members tend to be described as glutaminase A (glsA), where B (glsB) is unknown and may not be homologous (as in "Rhizobium etli"; some species have two isozymes that may both be designated A (GlsA1 and GlsA2).
Hepatotoxicity
A group of enzymes located in the endoplasmic reticulum, known as cytochrome P-450, is the most important family of metabolizing enzymes in the liver. Cytochrome P-450 is the terminal oxidase component of an electron transport chain. It is not a single enzyme, but rather consists of a closely related family of 50 isoforms; six of them metabolize 90% of drugs. There is a tremendous diversity of individual P-450 gene products, and this heterogeneity allows the liver to perform oxidation on a vast array of chemicals (including almost all drugs) in phase 1. Three important characteristics of the P-450 system have roles in drug-induced toxicity:
Each of the P-450 proteins is unique and accounts (to some extent) for the variation in drug metabolism between individuals. Genetic variations (polymorphism) in P-450 metabolism should be considered when patients exhibit unusual sensitivity or resistance to drug effects at normal doses. Such polymorphism is also responsible for variable drug response among patients of differing ethnic backgrounds.
Many substances can influence the P-450 enzyme mechanism. Drugs interact with the enzyme family in several ways. Drugs that modify cytochrome P-450 enzyme are referred to as either inhibitors or inducers. Enzyme inhibitors block the metabolic activity of one or several P-450 enzymes. This effect usually occurs immediately. On the other hand, inducers increase P-450 activity by increasing its synthesis. Depending on the inducing drug's half life, there is usually a delay before enzyme activity increases.
Casomorphin
Digestive enzymes can break casein down into peptides that have some biological activity in cells and in laboratory animals though conclusive causal effects on humans have not been established. Although they have not yet been proven to be formed in the human digestive system, there are associations between serum levels of β-casomorphins and the risk of sudden infant death syndrome, histamine release, stimulation of food intake, specifically high-fat foods, and lymphocytosis.
Inulin
Inulin is indigestible by the human enzymes ptyalin and amylase, which are adapted to digest starch. As a result, it passes through much of the digestive system intact. Only in the colon do bacteria metabolise inulin, with the release of significant quantities of carbon dioxide, hydrogen, and/or methane. Inulin-containing foods can be rather gassy, in particular for those unaccustomed to inulin, and these foods should be consumed in moderation at first.
Human digestive system
There are three main divisions – the left gastric artery, the common hepatic artery and the splenic artery.
Essential amino acid
The amino acids that are essential in the human diet were established in a series of experiments led by William Cumming Rose. The experiments involved elemental diets to healthy male graduate students. These diets consisted of cornstarch, sucrose, butterfat without protein, corn oil, inorganic salts, the known vitamins, a large brown "candy" made of liver extract flavored with peppermint oil (to supply any unknown vitamins), and mixtures of highly purified individual amino acids. The main outcome measure was nitrogen balance. Rose noted that the symptoms of nervousness, exhaustion, and dizziness were encountered to a greater or lesser extent whenever human subjects were deprived of an essential amino acid.
Assimilation (biology)
Most foods are composed of largely indigestible components depending on the enzymes and effectiveness of an animal's digestive tract. The most well-known of these indigestible compounds is cellulose; the basic chemical polymer in the makeup of plant cell walls. Most animals, however, do not produce cellulase; the enzyme needed to digest cellulose. However some animal and species have developed symbiotic relationships with cellulose-producing bacteria (see termites and metamonads.) This allows termites to use the energy-dense cellulose carbohydrate. Other such enzymes are known to significantly improve bio-assimilation of nutrients. Because of the use of bacterial derivatives, enzymatic dietary supplements now contain such enzymes as amylase, glucoamylase, protease, invertase, peptidase, lipase, lactase, phytase, and cellulase. These enzymes improve the overall bioassimilation in the digestive tract but are still not proven to increase bloodstream bioavailability. 
The enzymes break the bigger substances in some foods into smaller molecules so they can go through the rest of the digestive tract more easily.
Digestive system of gastropods
Two diverticular glands open into the stomach, and secrete enzymes that help to break down the food. In the more primitive species, these glands may also absorb the food particles directly and digest them intracellularly.
Discovery and development of gastrointestinal lipase inhibitors
The lipase inhibitors lipstatin and orlistat act locally in the intestinal tract. They are minimally absorbed in the circulation because of their lipophilicity. Hence, they do not affect systemic lipases. 
The mechanism of lipase inhibitors in fat digestion is shown in figure 1. These inhibitors bind covalently as an ester to the serine hydroxyl group at the active site on pancreatic- and gastric lipases and form a stable complex.
This results in a conformational change in the enzyme which causes exposing of the catalytic active site. When the active site is exposed, the hydroxyl group on the serine residue is acylated. This leads to irreversible inactivation of the enzyme. 
The inactive lipase is incapable of hydrolysing fats into absorbable fatty acids and monoglycerides, therefore triglycerides are excreted undigested with faeces. With this mode of action calorie uptake from fat in food is limited, hence body weight is reduced. The main role of lipase inhibitors is therefore to inhibit lipases in the gastrointestinal tract, but they do not have significant activity against proteases, amylases or other digestive enzymes.
Human digestive system
The lower gastrointestinal tract (GI), includes the small intestine and all of the large intestine. The intestine is also called the bowel or the gut. The lower GI starts at the pyloric sphincter of the stomach and finishes at the anus. The small intestine is subdivided into the duodenum, the jejunum and the ileum. The cecum marks the division between the small and large intestine. The large intestine includes the rectum and anal canal.
Drug interaction
Cytochrome P450 is a very large family of haemoproteins (hemoproteins) that are characterized by their enzymatic activity and their role in the metabolism of a large number of drugs. Of the various families that are present in human beings the most interesting in this respect are the 1, 2 and 3, and the most important enzymes are CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4.
The majority of the enzymes are also involved in the metabolism of endogenous substances, such as steroids or sex hormones, which is also important should there be interference with these substances. As a result of these interactions the function of the enzymes can either be stimulated (enzyme induction) or inhibited (enzyme inhibition).
Pancreas
The cells are filled with granules containing the digestive enzymes. These are secreted in an inactive form termed zymogens or proenzymes. When released into the duodenum, they are activated by the enzyme enteropeptidase present in the lining of the duodenum. The proenzymes are cleaved, creating a cascade of activating enzymes: enteropeptidase activates the proenzyme trypsinogen by cleaving it to form trypsin. The free trypsin then cleaves the rest of the trypsinogen, as well as chymotrypsinogen to its active form chymotrypsin.
Metabolome
Scientists at the University of Alberta have been systematically characterizing specific biofluid metabolomes including the serum metabolome, the urine metabolome, the cerebrospinal fluid (CSF) metabolome and the saliva metabolome. These efforts have involved both experimental metabolomic analysis (involving NMR, GC-MS, ICP-MS, LC-MS and HPLC assays) as well as extensive literature mining. According to their data, the human serum metabolome contains at least 4200 different compounds (including many lipids), the human urine metabolome contains at least 3000 different compounds (including 100s of volatiles and gut microbial metabolites), the human CSF metabolome contains nearly 500 different compounds while the human saliva metabolome contains approximately 400 different metabolites, including many bacterial products.
Abdomen
Functionally, the human abdomen is where most of the alimentary tract is placed and so most of the absorption and digestion of food occurs here. The alimentary tract in the abdomen consists of the lower esophagus, the stomach, the duodenum, the jejunum, ileum, the cecum and the appendix, the ascending, transverse and descending colons, the sigmoid colon and the rectum. Other vital organs inside the abdomen include the liver, the kidneys, the pancreas and the spleen.
TNO intestinal model
TNO (gastro-) Intestinal Models (“TIM”) are model systems mimicking the digestive tract. The models are dynamic computer controlled multi-compartmental systems with adjustable parameters for the physiological conditions of the stomach and intestine. Temperature, peristalsis, bile secretion, secretion of saliva, stomach and pancreas enzymes are all fully adjustable. 
The TIM systems are being used to study the behavior of oral products during transit through the stomach, the small intestine and large intestine. Commonly performed studies concern the digestibility of food and food components, the bioaccessibility for absorption of pharmaceutical compounds, proteins, fat, minerals and (water- and fat-soluble) vitamins.
Digestive enzyme
In the human digestive system, the main sites of digestion are the oral cavity, the stomach, and the small intestine. Digestive enzymes are secreted by different exocrine glands including:
Catabolite repression
An important enzyme from the phosphotransferase system called Enzyme II A (EIIA) plays a central role in this mechanism. There are different catabolite-specific EIIA in a single cell, even though different bacterial groups have specificities to different sets of catabolites. In enteric bacteria one of the EIIA enzymes in their set is specific for glucose transport only. When glucose levels are high inside the bacteria, EIIA mostly exists in its unphosphorylated form. This leads to inhibition of adenylyl cyclase and lactose permease, therefore cAMP levels are low and lactose can not be transported inside the bacteria.
Human digestive system
The largest structure of the digestive system is the gastrointestinal tract (GI tract). This starts at the mouth and ends at the anus, covering a distance of about nine (9) metres.
Human digestive system
Food enters the mouth where the first stage in the digestive process takes place, with the action of the tongue and the secretion of saliva. The tongue is a fleshy and muscular sensory organ, and the very first sensory information is received via the taste buds in the papillae on its surface. If the taste is agreeable, the tongue will go into action, manipulating the food in the mouth which stimulates the secretion of saliva from the salivary glands. The liquid quality of the saliva will help in the softening of the food and its enzyme content will start to break down the food whilst it is still in the mouth. The first part of the food to be broken down is the starch of carbohydrates (by the enzyme amylase in the saliva).
Aconitine
Aconitine is metabolized by cytochrome P450 isozymes (CYPs).
There has been research in 2011 in China to investigate in-depth the CYPs involved in aconitine metabolism in human liver microsomes. It has been estimated that more than 90 percent of currently available human drug metabolism can be attributed to eight main enzymes (CYP 1A2, 2C9, 2C8, 2C19, 2D6, 2E1, 3A4, 3A5). The researchers used recombinants of these eight different CYPs and incubated it with aconitine. To initiate the metabolism pathway the presence of NADPH was needed. 
Six CYP-mediated metabolites (M1–M6) were found by liquid-chromatography, these six metabolites were characterized by mass-spectrometry. The six metabolites and the involved enzymes are summarized in the following table:
Selective inhibitors were used to determine the involved CYPs in the aconitine metabolism. The results indicate that aconitine was mainly metabolized by CYP3A4, 3A5 and 2D6. CYP2C8 and 2C9 had a minor role to the aconitine metabolism, whereas CYP1A2, 2E1 and 2C19 did not produce any aconitine metabolites at all. The proposed metabolic pathways of aconitine in human liver microsomes and the CYPs involved to it are summarized in the table above.
Human digestive system
There are three pairs of main salivary glands and between 800 and 1,000 minor salivary glands, all of which mainly serve the digestive process, and also play an important role in the maintenance of dental health and general mouth lubrication, without which speech would be impossible. The main glands are all exocrine glands, secreting via ducts. All of these glands terminate in the mouth. The largest of these are the parotid glands—their secretion is mainly serous. The next pair are underneath the jaw, the submandibular glands, these produce both serous fluid and mucus. The serous fluid is produced by serous glands in these salivary glands which also produce lingual lipase. They produce about 70% of the oral cavity saliva. The third pair are the sublingual glands located underneath the tongue and their secretion is mainly mucous with a small percentage of saliva.
GLYAT
The glycine-N-acyltransferase protein conjugates glycine with acyl-CoA substrates in the mitochondria primarily in liver and kidney. The glycine N-acyltransferase enzyme is involved in the detoxification of a wide range of xenobiotic and endogenous metabolites. These include benzoic acid, a compound found in fruits and vegetables and used in medicine and foodstuffs as a preservative; salicylic acid, a metabolite of aspirin; and several endogenous metabolites. The diversity is demonstrated by the wide range of acylglycines excreted in the urines of patients with defects of organic acid metabolism. No defect of glycine N-acyltransferase has yet been described, but it has been demonstrated that there is significant inter individual variation in glycine conjugation capacity. Human glycine N-acyltransferase isoform a is a 296 amino acid protein translated from mRNA transcript splice variant 1. It is encoded by exons 2 to 6 of the mRNA transcript.
Pancreas
The pancreas plays a vital role in the digestive system. It secretes a fluid that contains enzymes into the duodenum. These enzymes help to break down carbohydrates (usually starch), proteins and lipids (fats). This role is called the "exocrine" role of the pancreas. Cells are arranged in clusters called "acini". Secretes into the middle of the acinus, which accumulates in intralobular ducts, which drain to the main pancreatic duct, which drains directly into the duodenum.
PDK2
The Pyruvate Dehydrogenase (PDH) complex must be tightly regulated due to its central role in general metabolism. Within the complex, there are three serine residues on the E1 component that are sites for phosphorylation; this phosphorylation inactivates the complex. In humans, there have been four isozymes of Pyruvate Dehydrogenase Kinase that have been shown to phosphorylate these three sites: PDK1, PDK2, PDK3, and PDK4. PDK2 has been identified as the most abundant isoform in human tissues. Through many studies, it has been made clear that the activity of this enzyme is essential, even at rest, to regulate glycolysis/carbodydrate oxidation and producing metabolites for oxidative phosphorylation and the electron transport chain. These studies have illustrated that the kinetics of the PDK isoform population, specifically PDK2, is more important in determining PDH activity than measuring PDK activity.
Human digestive system
The diaphragm is an important part of the body's digestive system. The muscular diaphragm separates the thoracic cavity from the abdominal cavity where most of the digestive organs are located. The suspensory muscle attaches the ascending duodenum to the diaphragm. This muscle is thought to be of help in the digestive system in that its attachment offers a wider angle to the duodenojejunal flexure for the easier passage of digesting material. The diaphragm also attaches to, and anchors the liver at its bare area. The esophagus enters the abdomen through a hole in the diaphragm at the level of T10.
Phenylacetylglutamine
The metabolism and conjugation of phenylacetate with glutamine in the liver involves amino acid acetylation carried out by the enzyme phenylacetyltranferase or glutamine N-acetyl transferase. The enzyme catalyzes the reaction of the substrates phenylacetyl-CoA and L-glutamine to produce CoA and alpha-N-phenylacetyl-L-glutamine and phenylacetic acid. The catalytic enzyme has been isolated in the human liver mitochondria. Furthermore, phenylacetylglutamine has been found in human urine, but not in the excretory material of rats, dogs, cats, monkeys, sheep, or horses. Throughout the metabolic process, phenylacetylglutamine is bound and conjugated by free-plasma in the kidney to remove excess nitrogen through its excretion in the urine.
Digestive enzyme
Of note is the division of function between the cells covering the stomach. There are four types of cells in the stomach:
Endomembrane system
Lysosomes are organelles that contain hydrolytic enzymes that are used for intracellular digestion. The main functions of a lysosome are to process molecules taken in by the cell and to recycle worn out cell parts. The enzymes inside of lysosomes are acid hydrolases which require an acidic environment for optimal performance. Lysosomes provide such an environment by maintaining a pH of 5.0 inside of the organelle. If a lysosome were to rupture, the enzymes released would not be very active because of the cytosol's neutral pH. However, if numerous lysosomes leaked the cell could be destroyed from autodigestion.
Bile salt-dependent lipase
Bile salt-dependent lipase (or BSDL), also known as carboxyl ester lipase (or CEL) is an enzyme produced by the adult pancreas and aids in the digestion of fats. Bile salt-stimulated lipase (or BSSL) is an equivalent enzyme found within breast milk. BSDL has been found in the pancreatic secretions of all species in which it has been looked for. BSSL, originally discovered in the milk of humans and various other primates, has since been found in the milk of many animals including dogs, cats, rats, and rabbits.
Digestion
There are at least five hormones that aid and regulate the digestive system in mammals. There are variations across the vertebrates, as for instance in birds. Arrangements are complex and additional details are regularly discovered. For instance, more connections to metabolic control (largely the glucose-insulin system) have been uncovered in recent years.
Exocrine pancreatic insufficiency
Exocrine pancreatic insufficiency (EPI) is the inability to properly digest food due to a lack of digestive enzymes made by the pancreas. EPI is found in humans afflicted with cystic fibrosis and Shwachman–Diamond syndrome, and is common in dogs. EPI is caused by a progressive loss of the pancreatic cells that make digestive enzymes; loss of digestive enzymes leads to maldigestion and malabsorption of nutrients from normal digestive processes.
Gingivitis
Current research shows that activity levels of the following enzymes in saliva samples are associated with periodontal destruction: aspartate aminotransferase (AST), alanine aminotransferase (ALT), gamma glutamyl transferase (GGT), alkaline phosphatase (ALP), and acid phosphatase (ACP). Therefore, these enzyme biomarkers may be used to aid in the diagnosis and treatment of gingivitis and periodontitis.
Glutathione peroxidase
Mammalian GPx1, GPx2, GPx3, and GPx4 have been shown to be selenium-containing enzymes, whereas GPx6 is a selenoprotein in humans with cysteine-containing homologues in rodents. GPx1, GPx2, and GPx3 are homotetrameric proteins, whereas GPx4 has a monomeric structure. As the integrity of the cellular and subcellular membranes depends heavily on glutathione peroxidase, its antioxidative protective system itself depends heavily on the presence of selenium.
20-Hydroxyeicosatetraenoic acid
In humans, the CYP4 ω-hydroxylases include CYP4A11, CYP4F2, and CYP4F3 with the predominant 20-HETE-synthesizing enzymes being CYP4F2, which is the major 20-HETE producing enzyme in the human kidney, followed by CYP4A11. CYP4F3 is expressed as two distinct enzymes, CYP4F3A and CYP4F3B, due to alternative splicing of a single pre-mRNA precursor molecule; CYP4F3A is mostly expressed in leukocytes, CYP4F3B mostly in the liver. Human CYP4Z1, which is expressed in a limited range of tissues such as human breast and ovary, may also metabolize arachidonic acid to 20-HETE while human CYP4A22, once considered as contributing to 20-HETE production, is now regarded as being metabolically inactive. Finally, CYP2U1, the only member of the human CYP2U subfamily, is highly expressed in brain and thymus and to lesser extents in numerous other tissues such as kidney, lung and heart. CYP2U1 protein is also highly expressed, compared to several other cytochrome P450 enzymes, in malignant breast tissue; the MCF-7 human breast cancer cell line express messenger RNA for this cytochrome.
Nutrition
Animal intestines contain a large population of gut flora. In humans, the four dominant phyla are Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria. They are essential to digestion and are also affected by food that is consumed. Bacteria in the large intestine perform many important functions for humans, including breaking down and aiding in the absorption of fermentable fiber, stimulating cell growth, repressing the growth of harmful bacteria, training the immune system to respond only to pathogens, producing vitamin B, and defending against some infectious diseases. "Probiotics" refers to the idea of deliberately consuming live bacteria in an attempt to change the bacterial population in the large intestine, to the health benefit of the host human or animal. "Prebiotic (nutrition)" refers to the idea that consuming a bacterial energy source such as soluble fiber could support the population of health-beneficial bacteria in the large intestine. There is not yet a scientific consensus as to health benefits accruing from probiotics or prebiotics.
Pancreatic enzymes (medication)
Pancreatin contains the pancreatic enzymes trypsin, amylase and lipase. A similar mixture of enzymes is sold as pancrelipase, which contains more active lipase enzyme than does pancreatin. The trypsin found in pancreatin works to hydrolyze proteins into oligopeptides; amylase hydrolyzes starches into oligosaccharides and the disaccharide maltose; and lipase hydrolyzes triglycerides into fatty acids and glycerols. Pancreatin is an effective enzyme supplement for replacing missing pancreatic enzymes, and aids in the digestion of foods in cases of pancreatic insufficiency.
Large intestine
The bacteria break down some of the fiber for their own nourishment and create acetate, propionate, and butyrate as waste products, which in turn are used by the cell lining of the colon for nourishment. No protein is made available. In humans, perhaps 10% of the undigested carbohydrate thus becomes available, though this may vary with diet; in other animals, including other apes and primates, who have proportionally larger colons, more is made available, thus permitting a higher portion of plant material in the diet. The large intestine produces no digestive enzymes — chemical digestion is completed in the small intestine before the chyme reaches the large intestine. The pH in the colon varies between 5.5 and 7 (slightly acidic to neutral).
Digestion
Digestive systems take many forms. There is a fundamental distinction between internal and external digestion. External digestion developed earlier in evolutionary history, and most fungi still rely on it. In this process, enzymes are secreted into the environment surrounding the organism, where they break down an organic material, and some of the products diffuse back to the organism. Animals have a tube (gastrointestinal tract) in which internal digestion occurs, which is more efficient because more of the broken down products can be captured, and the internal chemical environment can be more efficiently controlled.
Enoyl CoA isomerase
Although all three classes of enzymes have the same function, there is little overlap among their amino acid sequences. For example, only 40 out of 302 amino acid sequences (13%) are the same between monofunctional peroxisomal and mitochondrial enzymes in humans. In fact, in mammals, the peroxisomal enzyme has an extra N-terminal domain that is not present in the mitochondrial counterpart. Also, it has been found to be a subunit of the peroxisomal trifunctional enzyme (pTFE) and contributes only to minor cleavages of the fatty acid chain. In that sense, for many higher organisms, the mitochondrial enzyme is essential for deriving maximum energy from lipids and fueling muscles.
Amylase
Amylase () are enzymes that catalyse the hydrolysis of starch into sugars. Amylase is present in the saliva of humans and some other mammals, where it begins the chemical process of digestion. Foods that contain large amounts of starch but little sugar, such as rice and potatoes, may acquire a slightly sweet taste as they are chewed because amylase degrades some of their starch into sugar. The pancreas and salivary gland make amylase (alpha amylase) to hydrolyse dietary starch into disaccharides and trisaccharides which are converted by other enzymes to glucose to supply the body with energy. Plants and some bacteria also produce amylase. As "diastase", amylase is the first enzyme to be discovered and isolated (by Anselme Payen in 1833). Specific amylase proteins are designated by different Greek letters. All amylases are glycoside hydrolases and act on α-1,4-glycosidic bonds.
ENPP7
Differing from other ENPP members, ENPP7 seems only expressed in the intestinal mucosa in many species and additionally in human liver. In the intestinal tract, ENPP7 activity is low in the duodenum and colon but high in the middle of the jejunum. As an ecto enzyme, ENPP7 is located on the surface of the intestinal mucosa and is released in the lumen by bile salt and pancreatic trypsin. The enzyme expressed in human liver is released in the bile and delivered to the intestine.
Mitochondrial ribosome
Mitochondria contain around 1000 proteins in yeast and 1500 proteins in human organisms; however only 8 and 13 proteins are encoded in mtDNA in yeast and human, respectively. Most of mitochondrial proteins are synthesized via cytoplasmic ribosomes. Proteins that are the key components in the electron transport chain are translated in mitochondria.
Hydroxylamine oxidoreductase
Hydroxylamine oxidoreductase (HAO) is an enzyme found in the prokaryote "Nitrosomonas europaea." It plays a critically important role in the biogeochemical nitrogen cycle as part of the metabolism of ammonia-oxidizing bacteria.
Pancreatic enzymes (medication)
Pancreatic enzymes also known as pancrelipase and pancreatin, are commercial mixtures of amylase, lipase, and protease. They are used to treat malabsorption syndrome due to pancreatic problems. These pancreatic problems may be due to cystic fibrosis, surgical removal of the pancreas, long term pancreatitis, or pancreatic cancer among others. It is taken by mouth.
Common side effects include vomiting, abdominal pain, constipation, and diarrhea. Other side effects include perianal irritation and high blood uric acid. The enzymes are from pigs. Use is believed to be safe during pregnancy. The components are digestive enzymes similar to those normally produced by the human pancreas. They help the person digest fats, starches, and proteins.
Pancreatic enzymes have been used as medications since at least the 1800s. They are on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system. In the United Kingdom a typical month's supply costs the NHS about £11.64. In the United States a month of treatment typically costs $50 to $100.
CYP3A5
CYP3A4/3A5 are a group of heme-thiolate monooxygenases. In liver microsomes, this enzyme is involved in an NADPH-dependent electron transport pathway. It oxidizes a variety of structurally unrelated compounds, including steroids, fatty acids, and xenobiotics. Immunoblot analysis of liver microsomes showed that CYP3A5 is expressed as a 52.5-kD protein, whereas CYP3A4 migrates as a 52.0-kD protein. The human CYP3A subfamily, CYP3A4, CYP3A5, CYP3A7 and CYP3A43, is one of the most versatile of the biotransformation systems that facilitate the elimination of drugs (37% of the 200 most frequently prescribed drugs in the U.S.
Human digestive system
In the small intestine, the pH becomes crucial; it needs to be finely balanced in order to activate digestive enzymes. The chyme is very acidic, with a low pH, having been released from the stomach and needs to be made much more alkaline. This is achieved in the duodenum by the addition of bile from the gall bladder combined with the bicarbonate secretions from the pancreatic duct and also from secretions of bicarbonate-rich mucus from duodenal glands known as Brunner's glands. The chyme arrives in the intestines having been released from the stomach through the opening of the pyloric sphincter. The resulting alkaline fluid mix neutralises the gastric acid which would damage the lining of the intestine. The mucus component lubricates the walls of the intestine.
Digestion
Protein digestion occurs in the stomach and duodenum in which 3 main enzymes, pepsin secreted by the stomach and trypsin and chymotrypsin secreted by the pancreas, break down food proteins into polypeptides that are then broken down by various exopeptidases and dipeptidases into amino acids. The digestive enzymes however are mostly secreted as their inactive precursors, the zymogens. For example, trypsin is secreted by pancreas in the form of trypsinogen, which is activated in the duodenum by enterokinase to form trypsin. Trypsin then cleaves proteins to smaller polypeptides.
Human interactions with microbes
Microbes can form an endosymbiotic relationship with larger organisms. For example, the bacteria that live within the human digestive system contribute to human health through gut immunity, the synthesis of vitamins such as folic acid and biotin, and the fermentation of complex indigestible carbohydrates. Future drugs and food chemicals may need to be tested on the gut microbiota; it is already clear that probiotic supplements can promote health, and that gut microbes are affected by both diet and medicines.
Antioxidant
The glutathione system includes glutathione, glutathione reductase, glutathione peroxidases, and glutathione "S"-transferases. This system is found in animals, plants and microorganisms. Glutathione peroxidase is an enzyme containing four selenium-cofactors that catalyzes the breakdown of hydrogen peroxide and organic hydroperoxides. There are at least four different glutathione peroxidase isozymes in animals. Glutathione peroxidase 1 is the most abundant and is a very efficient scavenger of hydrogen peroxide, while glutathione peroxidase 4 is most active with lipid hydroperoxides. Surprisingly, glutathione peroxidase 1 is dispensable, as mice lacking this enzyme have normal lifespans, but they are hypersensitive to induced oxidative stress. In addition, the glutathione "S"-transferases show high activity with lipid peroxides. These enzymes are at particularly high levels in the liver and also serve in detoxification metabolism.
Human digestive system
Early in embryonic development, the embryo has three germ layers and abuts a yolk sac. During the second week of development, the embryo grows and begins to surround and envelop portions of this sac. The enveloped portions form the basis for the adult gastrointestinal tract. Sections of this foregut begin to differentiate into the organs of the gastrointestinal tract, such as the esophagus, stomach, and intestines.
Didier Raoult
This new approach to biodiversity is applied to the human microbiota. Thus, in the study described in the journal Nature Microbiology in 2016, nearly 1,000 samples from the human digestive tract (stool, stomach, small intestine, and colon) were analyzed, 1 170 different bacteria present in the digestive tract could be grown, including 247 completely new bacterial species. In addition, 269 bacteria that were known only in the environment were isolated for the first time in humans, and 250 bacteria that had already been isolated in humans but never in the digestive tract. All these new bacteria species are available in international collections of strains (Collection de Souches de l'Unité des Rickettsies, and Deutsche Sammlung von Mikroorganismen und Zellkulturen).
Human digestive system
A number of problems including malnutrition and anemia can arise from malabsorption, the abnormal absorption of nutrients in the GI tract. Malabsorption can have many causes ranging from infection, to enzyme deficiencies such as exocrine pancreatic insufficiency. It can also arise as a result of other gastrointestinal diseases such as coeliac disease. Coeliac disease is an autoimmune disorder of the small intestine. This can cause vitamin deficiencies due to the improper absorption of nutrients in the small intestine. The small intestine can also be obstructed by a volvulus, a loop of intestine that becomes twisted enclosing its attached mesentery. This can cause mesenteric ischemia if severe enough.
Equine anatomy
In the stomach, assorted acids and the enzyme pepsin break down food. Pepsin allows for the further breakdown of proteins into amino acid chains. Other enzymes include resin and lipase. Additionally, the stomach absorbs some water, as well as ions and lipid-soluble compounds.
Digestive enzyme
Complex food substances that are taken by animals and humans must be broken down into simple, soluble, and diffusible substances before they can be absorbed. In the oral cavity, salivary glands secrete an array of enzymes and substances that aid in digestion and also disinfection. They include the following:
4-Ipomeanol
However, in humans the reactivity of CYP4B1 is different and it does not activate 4-IPO. The CYP enzymes CYP1A2 and CYP3A4 are active in the liver and are similar to the rodent CYP4B1. 4-IPO is thus metabolically activated by CYP1A2 and CYP3A4 in humans. These two enzymes are both part of several pathways involved in the drug metabolism. As both CYPs are mainly active in the liver, 4-IPO causes hepatotoxicity in humans.
Enterocyte
Enterocytes, or intestinal absorptive cells, are simple columnar epithelial cells found in the small intestine. A glycocalyx surface coat contains digestive enzymes. Microvilli on the apical surface increase surface area for the digestion and transport of molecules from the intestinal lumen. The cells also have a secretory role.
Digestive enzyme
Digestive enzymes are a group of enzymes that break down polymeric macromolecules into their smaller building blocks, in order to facilitate their absorption by the body. Digestive enzymes are found in the digestive tracts of animals (including humans) and in the traps of carnivorous plants, where they aid in the digestion of food, as well as inside cells, especially in their lysosomes, where they function to maintain cellular survival. Digestive enzymes of diverse specificities are found in the saliva secreted by the salivary glands, in the secretions of cells lining
Lipase
Most lipases act at a specific position on the glycerol backbone of a lipid substrate (A1, A2 or A3)(small intestine). For example, human pancreatic lipase (HPL), which is the main enzyme that breaks down dietary fats in the human digestive system, converts triglyceride substrates found in ingested oils to monoglycerides and two fatty acids.
Human digestive system
The pancreas is also the main source of enzymes for the digestion of fats and proteins. Some of these are released in response to the production of CKK in the duodenum. (The enzymes that digest polysaccharides, by contrast, are primarily produced by the walls of the intestines.) The cells are filled with secretory granules containing the precursor digestive enzymes. The major proteases, the pancreatic enzymes which work on proteins, are trypsinogen and chymotrypsinogen. Elastase is also produced. Smaller amounts of lipase and amylase are secreted. The pancreas also secretes phospholipase A2, lysophospholipase, and cholesterol esterase. The precursor zymogens, are inactive variants of the enzymes; which avoids the onset of pancreatitis caused by autodegradation. Once released in the intestine, the enzyme enteropeptidase present in the intestinal mucosa activates trypsinogen by cleaving it to form trypsin; further cleavage results in chymotripsin.
Purinosome
The enzymes of the multi-step "de novo" purine biosynthesis pathway have been postulated to form a multi-enzyme complex to facilitate substrate channeling between each enzyme of the pathway. Slight variations of the pathway exists between phyla; however, there are 13 enzymes that can be considered part of this biosynthetic pathway. Several individual enzymatic functions have consolidated onto single bifunctional or trifunctional polypeptide chains in higher organisms, suggesting stable physical interactions exist between enzymes. The functional consolidation of steps 2,3, and 5 of the pathway into a single enzyme in higher organisms such as humans suggests physical local proximity of the enzyme for step 4 to the trifunctional enzyme.
Non-homologous isofunctional enzymes
A popular example of NISE is the Superoxide dismutase family of enzymes which contains three distinct forms (EC 1.15.1.1) 
CuZn (SOD1) superoxide dismutase was the first to be discovered and is a homodimer containing copper and zinc, found often in intracellular cytoplasmic spaces. FeMn(SOD2) is a tetramer produced by a leader peptide targeting the manganese containing enzyme only in mitochondrial spaces. Nickel superoxide (SOD3) is the most recently characterized and exists only in extracellular spaces.
Restriction enzyme
These enzymes are found in bacteria and archaea and provide a defence mechanism against invading viruses. Inside a prokaryote, the restriction enzymes selectively cut up "foreign" DNA in a process called "restriction digestion"; meanwhile, host DNA is protected by a modification enzyme (a methyltransferase) that modifies the prokaryotic DNA and blocks cleavage. Together, these two processes form the restriction modification system.
Molybdenum
Molybdenum is an essential trace dietary element. Four mammalian Mo-dependent enzymes are known, all of them harboring a pterin-based molybdenum cofactor (Moco) in their active site: sulfite oxidase, xanthine oxidoreductase, aldehyde oxidase, and mitochondrial amidoxime reductase. People severely deficient in molybdenum have poorly functioning sulfite oxidase and are prone to toxic reactions to sulfites in foods. The human body contains about 0.07 mg of molybdenum per kilogram of body weight, with higher concentrations in the liver and kidneys and lower in the vertebrae. Molybdenum is also present within human tooth enamel and may help prevent its decay.
Enzyme
The discovery that enzymes could be crystallized eventually allowed their structures to be solved by x-ray crystallography. This was first done for lysozyme, an enzyme found in tears, saliva and egg whites that digests the coating of some bacteria; the structure was solved by a group led by David Chilton Phillips and published in 1965. This high-resolution structure of lysozyme marked the beginning of the field of structural biology and the effort to understand how enzymes work at an atomic level of detail.
Gastrointestinal physiology
The digestive system has a complex system of motility and secretion regulation which is vital for proper function. This task is accomplished via a system of long reflexes from the central nervous system (CNS), short reflexes from the enteric nervous system (ENS) and reflexes from GI peptides working in harmony with each other.