Choline from dietary lecithins

A major feature of neurotransmitter synthesis is the observation that the neurotransmitters themselves are all nitrogenous substances and in most cases are ultimately derived from dietary protein (Fig. 3). A singular exception to this concept is acetylcholine. Choline can be derived Irom serine, although a significant portion is derived from dietary lecithin. While dietary protein is not directly converted into neurotransmitters, the amino acids that are derived from these proteins can be used unchanged (glycine, glutamate, or aspartatic acid), metabolized to specific derivatives (catecholamines, etc.),... 

Neuronal systems using acetylcholine as a neurotransinitter have been among those more widely studied. Acetylcholine was the first neurotransmitter isolated by scientists. The precursor of this neurotransmitter is choline, which is derived either from dietary lecithin or from intracellular metabolism. Choline can be formed from ethanolamine, the decarboxylated product of serine, via a complex series of reactions. The relative normal contributions of endogenous or exogenous choline to the precursor pool is not established. It is known that choline loading can substantially enhance the level of choline within the brain. 

TMA is normally formed from dietary choline and lecithin, but also from TMA N-oxide by intestinal bacteria (Fig. 7.4.2). Choline bound to lecithin is present most abundantly in egg yolk, liver, kidney legumes, soy beans and peas. TMA N-oxide is present in considerable amounts in marine fish, amounting to approximately... 

PC can come from the diet or be synthesized in the body. Dietary PC is cleaved by a pancreatic enzyme, phospholipase B, that catalyzes hydrolysis of the fatty add from carbon-2 (C-2) of the glycerol backbone. This hydrolysis produces a free fatty add and l-acyh3-phosphocholine glycerol. The latter compound is called lysolecithin, because its parent compound PC is known as lecithin. Lysoledthin and the free fatty acid are absorbed by the enterocyte, converted back to PC, and then packaged into chylomiorons that are released into the lymphatic system. A small fraction of the choline derived from dietary PC enters the bloodstream as free choline. 

This recessively inherited condition causes accumulation of trimethylamine (TMA) in body fluids. Its incidence is not known while marked cases appear to be quite rare, milder cases may be more frequent. TMA is formed in the gut by bacterial metabolism from dietary precursors such as lecithin and choline. 

Acetylcholine Precursors. Your nerve cells produce acetylcholine from certain dietary precursors (choline and lecithin). Many early studies tried dietary supplements of these precursors. A precedent for this approach was established using the dopamine precursor, L-DOPA, a well-established treatment for Parkinson s disease. Unfortunately, this approach is ineffective in dementia. It appears that the daily doses of these fatty acid precursors needed to have any discernible impact on acetylcholine levels far exceed what an individual can reasonably take in a day. This approach has therefore been abandoned. 

Choiine. Choline is a component of many biomem-hranes and plasma phospholipids. Dietary sources include eggs. fish, liver, milk, and vegetables. These sources provide choline primarily as the phospholipid lecithin. Lecithin is hydrolyzed to glycerophosphorylcholinc by the intestinal mucosa before absorption. The liver liberates choline. Choline can be biosynthesized by humans con.sequcntly. it cannot be con.sidcred a (rue vitamin. Biosynthesis involves methylation of cthanolamine. The methyl groups arc provided by methionine or by a reaction involving vitamin B12 and folic acid. Therefore, deficiencies can occur only if all methyl donors are excluded from the diet. 

Choline (N,N,N-trimethyl-y6-hydroxyethylamine) is an important constituent of phospholipids (lecithin is phosphatidylcholine) and of acetylcholine. It can be completely synthesized from serine (Chapter 19), but only in the form of phosphatidylserine and then only when the dietary supply of amino acids is adequate. Betaine (N,N,N-trimethylglycine) readily replaces dietary choline for all species. Choline is conserved by a salvage pathway. In the lung, this salvage route is the principal route for the synthesis of the phosphatidylcholine needed as a surfactant (see Chapter 19). 

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