The metabolic intermediate homocysteine is an amino acid created by the single carbon chemistry of S-adenosyl-methionine. It can be converted back to methionine, or converted to cysteine or taurine via the transsulfuration pathway .
Homocysteine is attracting attention because a high level of blood serum homocysteine is now considered to be a marker of potential cardiovascular (risk factor for heart attack and stroke) disease. Note that, as a consequence of the chemistry in which homocysteine is involved, deficiencies of the vitamins folic acid, pyridoxine (B-6), or cobalamin (B-12) can lead to high homocysteine levels. A current area of research is whether high serum homocysteine itself is a problem, or merely an indicator of extant problems.
Although homocysteine can be converted back to methionine, there is no indication that dietary homocysteine contributes any homocysteine nutritionally to humans.
Elevations of homocysteine occur in the rare hereditary disease homocystinuria and in methyl-tetrahydrofolate-reductase deficiency. The latter is quite common and usually goes unnoticed, although there are reports that thrombosis and cardiovascular disease occurs more often in people with elevated homocysteine.