Homocysteine
The metabolic intermediate homocysteine is an amino acid created by the single carbon chemistry of S-adenosyl-methionine. It can be reconstituted back to methionine, or converted to cysteine or taurine via the transsulfuration pathway.Note: need homocysteine structure about here.
Homocysteine is attracting attention because high levels of blood serum homocysteine are now considered to be markers of potential heart problems. Note that, as a consequence of the chemistry that homocysteine is involved in, that deficiencies of folic acid, or 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 in higher concentrations, 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.
Elevated homocysteine
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.