Abstract
Methylenetetrahydrofolate reductase (MTHFR) plays a major role in regulating levels of homocysteine (HC), and the C677T mutation of this enzyme results in significantly decreased activity. This MTHFR C677T genotype is associated with elevated plasma HC and an increased risk of cardiovascular (CV) disease. The HC levels are set by individual laboratories, but in general, a level 60 mmol/L (808 mg/dL) considered severely elevated. Homocysteine is a sulfhydryl-containing amino acid intermediate, which results from the demethylation of methionine, an essential amino acid that comes from proteins in the diet and also from endogenous protein metabolism. Metabolism of HC occurs by either a transsulfuration pathway or 1 of the 2 remethylation pathways. With the transsulfuration pathway, cystathionine b synthase and cystathionase are required and both need vitamin B6 as a cofactor. The first remethylation pathway requires transfer of a methyl group from 5-methyltetrahydrofolate (MTHF) to HC. This first remethylation pathway involves methionine synthetase, which needs vitamin B12 and folate. Therefore, specific vitamin B deficiencies (B6, B12, and folate) could theoretically result in increased plasma HC levels. The second remethylation pathway utilizes an irreversible methyl group transfer to HC from betaine.
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