Homocysteine is an intermediate amino acid utilized by our body for protein synthesis. It comes from the essential acid methionine. When methionine is activated, it provides methyl groups for many compounds, including betaine, choline, creatine, epinephrine, and melatonin. When methionine loses its methyl groups to form these various substances, homocysteine is formed. 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 heart problems. 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.
Homocysteine is a common amino acid (one of the building blocks that make up proteins) found in the blood and is acquired mostly from eating meat. High levels of homocysteine are related to the early development of heart and blood vessel disease. In fact, it is considered an independent risk factor for heart disease. High homocysteine is associated with low levels of vitamin B6, B12 and folate and renal disease. Researchers are currently trying to figure out whether reducing high homocysteine levels will actually reduce the risk of heart attack and stroke.
The importance of homocysteine is that higher levels in blood have been linked to a higher chance of developing vascular disease (heart attacks, strokes and blockage of the arteries to the legs). High homocysteine is also associated with a higher frequency of venous thrombosis (clots). These associations of homocysteine with vascular disease do not prove that homocysteine is a cause of the disease. They are in the category of circumstantial evidence, qualifying homocysteine as a strong risk factor and allowing the reasonable hypothesis that it is a cause of cardiovascular disease. The final link in proving a cause is to show that reducing levels of a risk factor leads to reduction in disease and these studies have not yet been completed for homocysteine.
Vascular disease (arteriosclerosis/narrowing of arteries) is a complex condition that has a number of contributing causes. The exact details of the overall process and the interactions of potentially causative factors are still being researched. The well known risk factors are cholesterol, smoking, hypertension (high blood pressure), diabetes and family history but these do not explain all the risk of heart disease. Clearly other factors are involved, and homocysteine is one of them.