CTL1s and CTL2s are not associated with the acetylcholine production, but transport choline together via the blood–brain barrier. Only CTL2s occur on the brain side of the barrier. They also remove excess choline from the neurons back to blood. CTL1s occur only on the blood side of the barrier, but also on the membranes of astrocytes and neurons.OCT1s and OCT2s are not associated with the acetylcholine production. They transport choline with low affinity. OCT1s transport choline primarily in the liver and kidneys; OCT2s in kidneys and the brain. Storage
Choline is stored in the cell membranes and organelles as phospholipids, and inside cells as phosphatidylcholines and glycerophosphocholines. Excretion
Even at choline doses of 2–8 g, little choline is excreted into urine in humans. Excretion happens via transporters that occur within kidneys (see transport). Trimethylglycine is demethylated in the liver and kidneys to dimethylglycine (tetrahydrofolate receives one of the methyl groups). Methylglycine forms, is excreted into urine, or is demethylated to glycine. Function
Choline and its derivatives have many functions in humans and in other organisms. The most notable function is that choline serves as a synthetic precursor for other essential cell components and signalling molecules, such as phospholipids that form cell membranes, the neurotransmitter acetylcholine, and the osmoregulator trimethylglycine (betaine). Trimethylglycine in turn serves as a source of methyl groups by participating in the biosynthesis of S-adenosylmethionine. Phospholipid precursor
Choline is transformed to different phospholipids, like phosphatidylcholines and sphingomyelins. These are found in all cell membranes and the membranes of most cell organelles.
Acetylcholine is even present in the placenta and may help control cell proliferation and differentiation (increases in cell number and changes of multiuse cells into dedicated cellular functions) and parturition.Choline uptake into the brain is controlled by a low-affinity transporter located at the blood–brain barrier. Transport occurs when arterial plasma choline concentrations increase above 14 μmol/L, which can occur during a spike in choline concentration after consuming choline-rich foods. Neurons, conversely, acquire choline by both high- and low-affinity transporters. Choline is stored as membrane-bound phosphatidylcholine, which can then be used for acetylcholine neurotransmitter synthesis later. Acetylcholine is formed as needed, travels across the synapse, and transmits the signal to the following neuron. Afterwards, acetylcholinesterase degrades it, and the free choline is taken up by a high-affinity transporter into the neuron again. Uses
Choline chloride and choline bitartrate are used in dietary supplements. Bitartrate is used more often due to its lower hygroscopicity. Certain choline salts are used to supplement chicken, turkey and some other animal feeds. Some salts are also used as industrial chemicals: for example, in photolithography to remove photoresist. Choline theophyllinate and choline salicylate are used as medicines, as well as structural analogs, like methacholine and carbachol. Radiolabeled cholines, like 11C-choline, are used in medical imaging. Other commercially used salts include tricholine citrate and choline bicarbonate. Antagonists and inhibitors
Hundreds of choline antagonists and enzyme inhibitors have been developed for research purposes. Aminomethylpropanol is among the first ones used as a research tool. It inhibits choline and trimethylglycine synthesis.