Nonprotein Amino Acids

There are several amino acids found in biological systems that are not incorporated into proteins. Ornithine and citrulline, for example, are intermediates of the urea cycle; GABA is a neurotransmitter.

Homocysteine is an intermediate in the transul-phuration pathway for the conversion of methio-nine to cysteine. Homocystinuria is an inborn error of metabolism that is characterized by the accumulation of high concentrations of homocys-teine, and this leads to severe cardiovascular disease at an early age. However, there is a much more common mutation in the enzyme 5,10-methylenetetrahydrofolate reductase that causes a moderate increase in plasma homocysteine concentration in more than 10% of the population. A high plasma homocysteine concentration appears to be an independent risk factor for cardiovascular disease in the population as a whole, although the mechanism is not known. Supplementing the diet with folic acid is often effective in reducing plasma homocysteine concentration because methyltetra-hydrofolate is a substrate for the remethylation of homocysteine by the vitamin B12-dependent enzyme methionine synthase. An inverse relationship has been observed between plasma homocys-teine concentration and folate status in many studies, and this has led to the proposal that plasma homocysteine concentration may be used as a biomarker of folate intake.

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