Homocysteine metabolism and various consequences of folate deficiency

Homocysteine is a neurotoxic non-proteinogenic amino acid, an abnormal increase of which in plasma has been implicated in many pathological conditions including cardiovascular diseases, neural tube defects and is now recognized and Alzheimer's disease. Homocysteine elimination is regulated by the transmethylation and the transsulfuration pathways and is modulated by folate, a member of the B-vitamin family. A metabolic product of folate, 5 methyltetrahydrofolate, provides a methyl group that is used to reconvert homocysteine back to methionine through the transmethylation pathway. The efficiency of folate metabolism has an impact on the availability of S-adenosylmethionine (SAM), a compound that is known to activate homocysteine flux through the transsulfuration pathway. SAM is also necessary for utilization of the antioxidant glutathione via glutathione S-transferase. In this review, I will elaborate on different biochemical reactions that are implicated in the regulation of homocysteine elimination through the transmethylation and the transsulfuration pathways and on various consequences of folate deficiency on homocysteine metabolism.