Phosphatidylcholines are structurally important part of the cell membranes. In humans 40–50% of their phospholipids are phosphatidylcholines.Choline phospholipids also form lipid rafts in the cell membranes along with cholesterol. The rafts are centers, for example for receptors and receptor signal transduction enzymes.Phosphatidylcholines are needed for the synthesis of VLDLs: 70–95% of their phospholipids are phosphatidylcholines in humans.Choline is also needed for the synthesis of pulmonary surfactant, which is a mixture consisting mostly of phosphatidylcholines. The surfactant is responsible for lung elasticity, that is for lung tissues ability to contract and expand. For example, deficiency of phosphatidylcholines in the lung tissues has been linked to acute respiratory distress syndrome.Phosphatidylcholines are excreted into bile and work together with bile acid salts as surfactants in it, thus helping with the intestinal absorption of lipids. Acetylcholine synthesis
Choline is needed to produce acetylcholine. This is a neurotransmitter which plays a necessary role in muscle contraction, memory and neural development, for example. Nonetheless, there is little acetylcholine in the human body relative to other forms of choline. Neurons also store choline in the form of phospholipids to their cell membranes for the production of acetylcholine. Source of trimethylglycine
In humans, choline is oxidized irreversibly in liver mitochondria to glycine betaine aldehyde by choline oxidases. This is oxidized by mitochondrial or cytosolic betaine-aldehyde dehydrogenases to trimethylglycine. Trimethylglycine is a necessary osmoregulator. It also works as a substrate for the BHMT-enzyme, which methylates homocysteine to methionine. This is a S-adenosylmethionine (SAM) precursor. SAM is a common reagent in biological methylation reactions.
It is able to induce choline deficiency that in turn results in fatty liver in rodents. Diethanolamine is another such compound, but also an environmental pollutant. N-cyclohexylcholine inhibits choline uptake primarily in brains. Hemicholinium-3 is a more general inhibitor, but also moderately inhibits choline kinases. More specific choline kinase inhibitors have also been developed. Trimethylglycine synthesis inhibitors also exist: carboxybutylhomocysteine is an example of a specific BHMT inhibitor.The cholinergic hypothesis of dementia has not only lead to medicinal acetylcholinesterase inhibitors, but also to a variety of acetylcholine inhibitors. Examples of such inhibiting research chemicals include triethylcholine, homocholine and many other N-ethyl derivates of choline, which are false neurotransmitter analogs of acetylcholine. Choline acetyltransferase inhibitors have also been developed. History
Discovery
In 1849, Adolph Strecker was the first to isolate choline from pig bile. In 1852, L. Babo and M. Hirschbrunn extracted choline from white mustard seeds and named it sinkaline. In 1862, Strecker repeated his experiment with pig and ox bile, calling the substance choline for the first time after the Greek word for bile, chole, and identifying it with the chemical formula C5H13NO. In 1850, Theodore Nicolas Gobley extracted from the brains and roe of carps a substance he named lecithin after the Greek word for egg yolk, lekithos, showing in 1874 that it was a mixture of phosphatidylcholines.In 1865, Oscar Liebreich isolated "neurine" from animal brains. The structural formulas of acetylcholine and Liebreichs "neurine" were resolved by Adolf von Baeyer in 1867.