(+-)-Homocysteine

130.78 Ų [M+H]+ [CCS Type: DT; Method: stepped-field]

130.2 Ų [M+H]+

Several groups of investigators have reported a positive impact of supplementation with vitamins B6, B12, and folic acid, administered alone or jointly, in patients with homocystinuria. Folic acid appears to be useful in most subjects while very high doses of B6 (100 mg or more daily) also seem to have broad utility. The efficacy of supplemental B12 may likewise hinge on baseline B12 status. Lower homocysteine levels in people who eat breakfast cereals may reflect the fact that such cereals are frequently enriched with B vitamins. High intakes of betaine (6 grams or more daily) have been used successfully to treat genetic homocystinuria in humans.

Chronically high levels of homocysteine (hyperhomocysteinemia) is associated with increased incidence of cardiovascular disease and Alzheimer's disease. Elevations in plasma homocysteine are commonly found as a result of vitamin deficiencies, polymorphisms of enzymes of methionine metabolism, and renal disease. Homocysteine is an independent cardiovascular disease (CVD) risk factor. This includes heart attack, stroke, peripheral atherosclerosis, and venous thromboembolism (the blockage of a blood vessel by a migrating clot). Furthermore, the risk associated with homocysteine appears to increase throughout the normal range of concentrations; each 1 micromolar rise in the concentration of homocysteine in the blood corresponds to an increase of about 10% in cardiovascular risk. A high level of homocysteine makes a person more prone to endothelial injury, which leads to vascular inflammation, which in turn may lead to atherogenesis, which can result in ischemic injury. High levels of homocysteine limit the early development of heart and blood vessel disease. High homocysteine is associated with low levels of vitamin B6, B12, and folate and renal disease .Chronically high levels of homocysteine are associated with at least 6 inborn errors of metabolism including: Cystathionine Beta-Synthase Deficiency, Homocystinuria due to defect of N(5,10)-methylene THF deficiency, Homocystinuria-megaloblastic anemia due to defect in cobalamin metabolism, Methionine Adenosyltransferase Deficiency, Methylenetetrahydrofolate reductase deficiency and S-Adenosylhomocysteine (SAH) Hydrolase Deficiency. Homocystinuria is characterized by about 20-fold incrase over the normal concentration—is associated with greatly increased risk for premature vascular disease, sometimes leading to strokes or heart attacks in teenagers.

Nitrosylation converts homocysteine (Hcy) into a methionine analogue, S-nitroso-Homocysteine, which can substitute for methionine in protein synthesis in biological systems. In humans, homocyteine-thiolactone modifies proteins posttranslationally by forming adducts in which homocysteine is linked by amide bonds to epsilon-amino group of protein lysine residues (Hcy-epsilonN-Lys-protein). Levels of homocystine bound by amide or peptide linkages (Homocysteine-N-protein) in human plasma proteins are directly related to plasma 'total homocysteine' levels. Homocysteine-N-hemoglobin and Homocysteine-N-albumin constitute a major pool of homocysteine in human blood, larger than 'total homocysteine' pool. Homocysteine-thiolactone is present in human plasma. Modification with Homocysteine-thiolactone leads to protein damage and induces immune response. Autoantibodies that specifically recognize the Homocysteine-epsilonN-Lys-epitope on Homocysteine-thiolactone-modified proteins occur in humans. The ability of Homocysteine to interfere with protein biosynthesis, which causes protein damage, induces cell death and elicits immune response, is likely a key contributor to the toxicity of homocysteine (A15343). Uremic toxins such as homocysteine are actively transported into the kidneys via organic ion transporters (especially OAT3). Increased levels of uremic toxins can stimulate the production of reactive oxygen species. This seems to be mediated by the direct binding or inhibition by uremic toxins of the enzyme NADPH oxidase (especially NOX4 which is abundant in the kidneys and heart) (A7868). Reactive oxygen species can induce several different DNA methyltransferases (DNMTs) which are involved in the silencing of a protein known as KLOTHO. KLOTHO has been identified as having important roles in anti-aging, mineral metabolism, and vitamin D metabolism. A number of studies have indicated that KLOTHO mRNA and protein levels are reduced during acute or chronic kidney diseases in response to high local levels of reactive oxygen species (A7869).

Moderately high levels of homocysteine produce no overt symptoms. Extremely high levels in adults over extended periods of times can lead to the usual symptoms of heart disease (high blood pressure, atherosclerosis, shortness of breath, poor exercise tolerance, etc.). Individuals with genetic forms of homocysteinuria experience such symptoms as dislocation of the lenses in the eyes, nearsightedness, abnormal blood clots, osteoporosis (weakening of the bones), learning disabilities and developmental problems. As a uremic toxin, high levels of homocysteine in adults can cause uremic syndrome. Uremic syndrome may affect any part of the body and can cause nausea, vomiting, loss of appetite, and weight loss. It can also cause changes in mental status, such as confusion, reduced awareness, agitation, psychosis, seizures, and coma. Abnormal bleeding, such as bleeding spontaneously or profusely from a very minor injury can also occur. Heart problems, such as an irregular heartbeat, inflammation in the sac that surrounds the heart (pericarditis), and increased pressure on the heart can be seen in patients with uremic syndrome. Shortness of breath from fluid buildup in the space between the lungs and the chest wall (pleural effusion) can also be present.

In the body, dietary methionine is converted to homocysteine. In a series of metabolic steps, the enzyme cystathionine b-synthase (CBS) irreversibly generates a substance called cystathionine from homocysteine. The rate at which homocysteine is generated from methionine and then converted to cystathionine is evidently determined by the habitual dietary intake of methionine. L-Homocysteine has two primary fates: conversion via tetrahydrofolate (THF) back into L-methionine or conversion to L-cysteine. Homocysteine can cyclize to give homocysteine thiolactone, a five-membered heterocycle, a reaction catalyzed by methionyl-transfer RNA synthetase.