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 (-CH2-). It is biosynthesized from methionine by the removal of its terminal Cεmethyl group. In the body, homocysteine can be recycled into methionine or converted into cysteine with the aid of vitamin B6, B9, and B12.[3]
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.[4][5]
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.[6] Hyperhomocysteinemia has also been associated with early-term spontaneous abortions[7] and with neural tube defects.[8]
Structure
Zwitterionic form of (S)-homocysteine (left) and (R)-homocysteine (right)
Homocysteine exists at neutral pH values as a zwitterion.
Biosynthesis and biochemical roles
Two of homocysteine's main biochemical roles (homocysteine is seen in the left middle of the image). It can be synthesized from methionine and then converted back to methionine via the SAM cycle or used to create cysteine and alpha-ketobutyrate.
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 B6) 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.[11]
Homocysteine can be recycled into methionine. This process uses N5-methyl tetrahydrofolate as the methyl donor and cobalamin (vitamin B12)-related enzymes. More detail on these enzymes can be found in the article for methionine synthase.
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.[14]
Homocysteine levels
Total plasma homocysteine
Homocysteine levels typically are higher in men than women, and increase with age.[15][16]
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).[17][18]
It is decreased with methyl folate trapping, where it is accompanied by decreased methylmalonic acid, increased folate, and a decrease in formiminoglutamic acid.[19] This is the opposite of MTHFR C677T mutations, which result in an increase in homocysteine.[citation needed]
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.
Abnormally high levels of homocysteine in the serum, above 15 μmol/L, are a medical condition called hyperhomocysteinemia.[23] This has been claimed to be a significant risk factor for the development of a wide range of diseases, in total more than 100[24] including thrombosis,[25] neuropsychiatric illness,[26][27][28][29] in particular dementia[30] and fractures.[31][32] It also is found to be associated with microalbuminuria, which is a strong indicator of the risk of future cardiovascular disease and renal dysfunction.[33] Vitamin B12 deficiency, even when coupled with high serum folate levels, has been found to increase overall homocysteine concentrations as well.[34]
Typically, hyperhomocysteinemia is managed with vitamin B6, vitamin B9, and vitamin B12 supplementation.[35] However, supplementation with these vitamins does not appear to improve cardiovascular disease outcomes.[36]
References
^ abChalcraft, Kenneth R.; Lee, Richard; Mills, Casandra; Britz-McKibbin, Philip (2009). "Virtual Quantification of Metabolites by Capillary Electrophoresis-Electrospray Ionization-Mass Spectrometry: Predicting Ionization Efficiency Without Chemical Standards". Analytical Chemistry. 81 (7): 2506–2515. doi:10.1021/ac802272u. PMID19275147.
^Allen, Milton J.; Steinman, Harry G. (1952). "The Electrolytic Reduction of Homocystine at a Controlled Reference Potential". Journal of the American Chemical Society. 74 (15): 3932–3933. doi:10.1021/ja01135a502.
^Nelen WL, Blom HJ, Steegers EA, den Heijer M, Thomas CM, Eskes TK (2000). "Homocysteine and folate levels as risk factors for recurrent early pregnancy loss". Obstet Gynecol. 95 (4): 519–24. doi:10.1016/s0029-7844(99)00610-9. PMID10725483. S2CID26125655.
^Nygård, O; Vollset, SE; Refsum, H; Stensvold, I; Tverdal, A; Nordrehaug, JE; Ueland, M; Kvåle, G (Nov 15, 1995). "Total plasma homocysteine and cardiovascular risk profile. The Hordaland Homocysteine Study". JAMA: The Journal of the American Medical Association. 274 (19): 1526–33. doi:10.1001/jama.274.19.1526. PMID7474221.
^Selhub, J.; Jacques, P. F.; Bostom, A. G.; Wilson, P. W.; Rosenberg, I. H. (2000). "Relationship between plasma homocysteine and vitamin status in the Framingham study population. Impact of folic acid fortification". Public Health Reviews. 28 (1–4): 117–145. ISSN0301-0422. PMID11411265.
^Scott, JohnM.; Weir, DonaldG. (15 August 1981). "THE METHYL FOLATE TRAP: A physiological response in man to prevent methyl group deficiency in kwashiorkor (methionine deficiency) and an explanation for folic-acid-induced exacerbation of subacute combined degeneration in pernicious anaemia". The Lancet. 318 (8242): 337–340. doi:10.1016/S0140-6736(81)90650-4. ISSN0140-6736. PMID6115113. S2CID29977127.
^Smach, MA; Jacob, N; Golmard, JL; Charfeddine, B; Lammouchi, T; Ben Othman, L; Dridi, H; Bennamou, S; Limem, K (2011). "Folate and homocysteine in the cerebrospinal fluid of patients with Alzheimer's disease or dementia: a case control study". European Neurology. 65 (5): 270–8. doi:10.1159/000326301. PMID21474939. S2CID7689901.
