Three S-adenosylhomocysteines (SAHs) are formed as a byproduct.Choline can also be released from more complex choline containing molecules. For example, phosphatidylcholines (PC) can be hydrolyzed to choline (Chol) in most cell types. Choline can also be produced by the CDP-choline route, cytosolic choline kinases (CK) phosphorylate choline with ATP to phosphocholine (PChol). This happens in some cell types like liver and kidney. Choline-phosphate cytidylyltransferases (CPCT) transform PChol to CDP-choline (CDP-Chol) with cytidine triphosphate (CTP). CDP-choline and diglyceride are transformed to PC by diacylglycerol cholinephosphotransferase (CPT).In humans, certain PEMT-enzyme mutations and estrogen deficiency (often due to menopause) increase the dietary need for choline. In rodents, 70% of phosphatidylcholines are formed via the PEMT route and only 30% via the CDP-choline route. In knockout mice, PEMT inactivation makes them completely dependent on dietary choline. Absorption
In humans, choline is absorbed from the intestines via the SLC44A1 (CTL1) membrane protein via facilitated diffusion governed by the choline concentration gradient and the electrical potential across the enterocyte membranes. SLC44A1 has limited ability to transport choline: at high concentrations part of it is left unabsorbed. Absorbed choline leaves the enterocytes via the portal vein, passes the liver and enters systemic circulation. Gut microbes degrade the unabsorbed choline to trimethylamine, which is oxidized in the liver to trimethylamine N-oxide.Phosphocholine and glycerophosphocholines are hydrolyzed via phospholipases to choline, which enters the portal vein. Due to their water solubility, some of them escape unchanged to the portal vein.
It is hygroscopic and thus often encountered as a colorless viscous hydrated syrup that smells of trimethylamine (TMA). Aqueous solutions of choline are stable, but the compound slowly breaks down to ethylene glycol, polyethylene glycols, and TMA.Choline chloride can be made by treating TMA with 2-chloroethanol:
(CH3)3N + ClCH2CH2OH → (CH3)3N+CH2CH2OH · Cl–The 2-chloroethanol can be generated from ethylene oxide. Choline has historically been produced from natural sources, such as via hydrolysis of lecithin. Metabolism
Biosynthesis
In plants, the first step in de novo biosynthesis of choline is the decarboxylation of serine into ethanolamine, which is catalyzed by a serine decarboxylase. The synthesis of choline from ethanolamine may take place in three parallel pathways, where three consecutive N-methylation steps catalyzed by a methyl transferase are carried out on either the free-base, phospho-bases, or phosphatidyl-bases. The source of the methyl group is S-adenosyl-L-methionine and S-adenosyl-L-homocysteine is generated as a side product. In humans and most other animals, de novo synthesis of choline is via the phosphatidylethanolamine N-methyltransferase (PEMT) pathway, but biosynthesis is not enough to meet human requirements. In the hepatic PEMT route, 3-phosphoglycerate (3PG) receives 2 acyl groups from acyl-CoA forming a phosphatidic acid. It reacts with cytidine triphosphate to form cytidine diphosphate-diacylglycerol. Its hydroxyl group reacts with serine to form phosphatidylserine which decarboxylates to ethanolamine and phosphatidylethanolamine (PE) forms. A PEMT enzyme moves three methyl groups from three S-adenosyl methionines (SAM) donors to the ethanolamine group of the phosphatidylethanolamine to form choline in the form of a phosphatidylcholine.