Homocysteine

It's the metabolite of the amino acid methionine. Homocysteine may be converted back to methionine with tetrahydrofolate (THF) as a cofactor, and THF is generated with the aid of vitamin B12. Hence, Hyperhomocyteinemia can be a result of vitamin B12 and/or folate deficiency, and has been linked to cardiovascular diseases. See vitamin B12 and Folate deficiency.