Description | Elevated level of the nonprotein amino acid homocysteine (Hcy) is a risk factor for cardiovascular diseases, neurodegenerative diseases, and neural tube defects. Metabolic conversion of Hcy to a chemically reactive metabolite, Hcy-thiolactone, catalyzed by methionyl-tRNA synthetase is the first step in a pathway that contributes to Hcy toxicity in humans. (PMID 16702349)
The only known source of Hcy in the human body is dietary protein methionine; subsequent examinations of individual dietary amino acids have led to the conclusion that methionine, ingested in excess, is the most toxic amino acid.
(Harper AE, Benevenga NJ, Wohlheuter RM. Effects of ingestion of disproportionate amounts of amino acids. Physiol Rev. 1970;50: 428 - 58; Benevenga NJ, Steele RD. Adverse effects of excessive consumption of amino acids. Annu Rev Nutr. 1984; 4:157-81)
Animals fed high-protein or high-methionine diets for 2 years developed hyperhomocysteinemia and evidence of vascular disease (Fau D, Preret J, Hadjiisky P. Effects of ingestion of high protein or excess methionine diets by rats for two years. J Nutr. 1988; 118:128-33)
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