# Homocysteine

Homocysteine (/ˌhoʊmoʊˈsɪstiːn/; symbol Hcy) is a non-proteinogenic α-amino acid. It is a homologue of the amino acid cysteine, differing by an additional methylene bridge (-CH$_2$-). It is biosynthesized from methionine by the removal of its terminal C$^ε$ methyl group. In the body, homocysteine can be recycled into methionine or converted into cysteine with the aid of vitamin B$_6$, B$_9$, and B$_{12}$.
High levels of homocysteine in the blood (hyperhomocysteinemia) is regarded as a marker of cardiovascular disease, likely working through atherogenesis, which can result in ischemic injury. Therefore, hyperhomocysteinemia is a possible risk factor for coronary artery disease. Coronary artery disease occurs when an atherosclerotic plaque blocks blood flow to the coronary arteries, which supply the heart with oxygenated blood.
Hyperhomocysteinemia has been correlated with the occurrence of blood clots, heart attacks, and strokes, although it is unclear whether hyperhomocysteinemia is an independent risk factor for these conditions. Hyperhomocysteinemia has also been associated with early-term spontaneous abortions and with neural tube defects.

## Structure

Homocysteine exists at neutral pH values as a zwitterion.

## Biosynthesis and biochemical roles

Homocysteine is biosynthesized naturally via a multi-step process. First, methionine receives an adenosine group from ATP, a reaction catalyzed by S-adenosyl-methionine synthetase, to give S-adenosyl methionine (SAM-e). SAM-e then transfers the methyl group to an acceptor molecule, (e.g., norepinephrine as an acceptor during epinephrine synthesis, DNA methyltransferase as an intermediate acceptor in the process of DNA methylation). The adenosine is then hydrolyzed to yield L-homocysteine. L-Homocysteine has two primary fates: conversion via tetrahydrofolate (THF) back into L-methionine or conversion to L-cysteine.

### Biosynthesis of cysteine

Mammals biosynthesize the amino acid cysteine via homocysteine. Cystathionine β-synthase catalyses the condensation of homocysteine and serine to give cystathionine. This reaction uses pyridoxine (vitamin B$_6$) as a cofactor. Cystathionine γ-lyase then converts this double amino acid to cysteine, ammonia, and α-ketobutyrate. Bacteria and plants rely on a different pathway to produce cysteine, relying on O-acetylserine.

### Methionine salvage

Homocysteine can be recycled into methionine. This process uses N5-methyl tetrahydrofolate as the methyl donor and cobalamin (vitamin B$_{12}$)-related enzymes. More detail on these enzymes can be found in the article for methionine synthase.

### Other reactions of biochemical significance

Homocysteine can cyclize to give homocysteine thiolactone, a five-membered heterocycle. Because of this "self-looping" reaction, homocysteine-containing peptides tend to cleave themselves by reactions generating oxidative stress.
Homocysteine also acts as an allosteric antagonist at Dopamine D$_2$ receptors.
It has been proposed that both homocysteine and its thiolactone may have played a significant role in the appearance of life on the early Earth.

## Homocysteine levels

Homocysteine levels typically are higher in men than women, and increase with age.
Common levels in Western populations are 10 to 12 μmol/L, and levels of 20 μmol/L are found in populations with low B-vitamin intakes or in the elderly (e.g., Rotterdam, Framingham).
It is decreased with methyl folate trapping, where it is accompanied by decreased methylmalonic acid, increased folate, and a decrease in formiminoglutamic acid. This is the opposite of MTHFR C677T mutations, which result in an increase in homocysteine.
Blood reference ranges for homocysteine: :
| Sex | Age | Lower limit | Upper limit | Unit | Elevated | Therapeutic target |
| Female | 12–19 years | 3.3 | 7.2 | μmol/L | > 10.4 μmol/L or > 140 μg/dl | < 6.3 μmol/L or < 85 μg/dL |
| Female | 12–19 years | 45 | 100 | μg/dL | > 10.4 μmol/L or > 140 μg/dl | < 6.3 μmol/L or < 85 μg/dL |
| Female | >60 years | 4.9 | 11.6 | μmol/L | > 10.4 μmol/L or > 140 μg/dl | < 6.3 μmol/L or < 85 μg/dL |
| Female | >60 years | 66 | 160 | μg/dL | > 10.4 μmol/L or > 140 μg/dl | < 6.3 μmol/L or < 85 μg/dL |
| Male | 12–19 years | 4.3 | 9.9 | μmol/L | > 11.4 μmol/L or > 150 μg/dL | < 6.3 μmol/L or < 85 μg/dL |
| Male | 12–19 years | 60 | 130 | μg/dL | > 11.4 μmol/L or > 150 μg/dL | < 6.3 μmol/L or < 85 μg/dL |
| Male | >60 years | 5.9 | 15.3 | μmol/L | > 11.4 μmol/L or > 150 μg/dL | < 6.3 μmol/L or < 85 μg/dL |
| Male | >60 years | 80 | 210 | μg/dL | > 11.4 μmol/L or > 150 μg/dL | < 6.3 μmol/L or < 85 μg/dL |
The ranges above are provided as examples only; test results always should be interpreted using the range provided by the laboratory that produced the result.

## Elevated homocysteine

Abnormally high levels of homocysteine in the serum, above 15 μmol/L, are a medical condition called hyperhomocysteinemia. This has been claimed to be a significant risk factor for the development of a wide range of diseases, in total more than 100 including thrombosis, neuropsychiatric illness, in particular dementia and fractures. It also is found to be associated with microalbuminuria (moderately increased albuminuria), which is a strong indicator of the risk of future cardiovascular disease and renal dysfunction. Vitamin B$_{12}$ deficiency, even when coupled with high serum folate levels, has been found to increase overall homocysteine concentrations as well.
Typically, hyperhomocysteinemia is managed with vitamin B$_6$, vitamin B$_9$, and vitamin B$_{12}$ supplementation. However, supplementation with these vitamins does not appear to improve cardiovascular disease outcomes.