Homocysteine is an amino acid that, in humans, is elevated in vitamin and folate deficiency. It is also a uremic toxin and modifiable risk factor for cardiovascular disorder. Foods rich in methionine (e.g. meat, eggs) are digested and produce homocysteine. After intestinal absorption and entry into the bloodstream, methionine is transported to the hepatocyte where it undergoes three metabolic reactions. First, in a reaction catalysed by methionine adenosyltransferase, methionine reacts with ATP and water to form S-adenosylmethionine with phosphate and pyrophosphate byproducts. Then, S-adenosylmethionine forms S-adenosylhomocysteine in a reaction catalysed by dimethyladenosine transferase. Finally, S-adenosylhomocysteine is used with water to form homocysteine in a reaction catalysed by adenosylhomocysteinase with an adenosine byproduct. Homocysteine enters systemic circulation via an amino acid transporter where it then induces oxidative stress, which can lead to cell death. Though the mechanism is unclear, homocysteine is associated with Alzheimer’s disease and cardiovascular disorder, namely atherosclerosis.

Metabolism and Physiological Effects of Homocysteine References