Fig. 1

Schematic representation of homocysteine metabolism. Methionine is converted to S-adenosylmethionine (SAM) by methionine adenosyltransferase. SAM serves as a donor of methyl groups in various methylation reactions in which it is converted to S-adenosylhomocysteine (SAH). SAH is then hydrolyzed by S-adenosylhomocysteine hydrolase forming homocysteine (Hcy) and adenosine. Under normal physiological conditions, Hcy is catabolized by the transsulfuration and remethylation routes to form glutathione or methionine, respectively. Vitamin B12 is an essential cofactor of methionine synthase in the remethylation route. Adapted from [23]