Homocysteine

Homocysteine is an uncommon amino acid. The molecular formula is HOO=CCH(NH2)(CH2)2SH.

Catabolism
Homocysteine is produced by the hydrolysis of the common amino acid methionine.


 * Methionine + Water → Homocysteine + Methanol

Homocysteine can also be produced by the hydrolysis of S-adenosyl-L-homocysteine.


 * S-adenosyl-L-homocysteine + water ↔ adenosine + homocysteine

This reaction is catalyzed by the enzyme S-adenosyl-L-homocysteine hydrolase. This enzyme is involved in normal T-cell and macrophage function.

Atherosclerosis
Elevated levels of serum homocysteine is associated with an increased risk of coronary artery disease. Among individuals with type 2 diabetes elevated plasma homocysteine levels are associated with an increased risk of macroangiopathy and nephropathy. It is unclear if this increased risk is due to the homocysteine itself, the methanol which is produced as a by-product of homocysteine biosynthesis, or some other mechanism which is correlated with homocysteine biosynthesis. Some authors believe that methanol causes arteriosclerosis. Other authors believe that arteriosclerosis is caused by homocysteine via some direct but yet to be explained mechanism.

Chronic infection
Elevated serum homocysteine levels, in spite of normal dietary methionine intake, may therefore suggest a chronic infection.