Adverse effects of hyperhomocysteinemia and their management by folic acid
Institute of Biochemistry of Macromolecules, Second University of Naples, Italy.Mineral and Electrolyte Metabolism 02/1997; 23(3-6):174-8.
A moderate increase in plasma homocysteine is an independent risk factor for cardiovascular disease. Plasma homocysteine is frequently elevated in chronic renal failure and in uremic patients, and the major causes of death in these patients are cardiovascular accidents. Homocysteine metabolism and mechanisms of toxicity are reviewed. Homocysteine elevation in blood leads to the intracellular increase of its precursor, adenosylhomocysteine, a powerful inhibitor of adenosylmethionine-dependent transmethylations. In vitro evidence shows that this increase is reversible upon homocysteine removal. Membrane protein methylation levels are consistently reduced in erythrocytes of both chronic renal failure and hemodialysis patients. This widespread enzymatic methylation is a key step for the repair of molecular damage resulting from the spontaneous deamidation and isomerization reactions of susceptible residues in proteins. In agreement with these findings is the observation that the concentration of a stable side product, D-Asx, of the repair process is significantly lower in erythrocyte membrane proteins from hemodialysis patients than from controls, showing that the repair of damaged membrane proteins is actually defective. It has been shown that treatment with folates dramatically lowers plasma homocysteine, presumably by improving remethylation to methionine. This indicates that folates and/ or their active derivative, i.e., methyltetrahydrofolate, could be effective in ameliorating transmethylations as well.
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ABSTRACT: Homocysteine, a sulfur amino acid, is an important methionine derivative, which has been implicated in the pathogenesis of atherothrombosis. Although only observational, epidemiological studies are available at present, the evidence of an association between hyperhomocysteinemia and increased cardiovascular risk is quite strong and this is confirmed also in a population of chronic renal failure patients. From a biochemical standpoint at least three mechanisms have been summoned so far in order to explain homocysteine toxicity including: oxidation, hypomethylation, and acylation. Proteins are believed to play a crucial role as homocysteine molecular targets. Interference with the functions of several of such macromolecules has been so far described being mediated by any of the above mechanisms. Vitamins may positively influence homocysteine metabolism, thus facilitating the metabolic clearance of this compound. Therefore they are presently considered as potential means for reducing plasma levels of this amino acid and preventing vascular occlusions in hyperhomocysteinemic patients. These compounds, with special regard to folate, are eligible for interventional clinical trials, from which the definitive answer on the role of homocysteine in atherothrombosis is expected.Mineral and Electrolyte Metabolism 01/1999; 25(4-6):279-85. DOI:10.1159/000057460
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ABSTRACT: Most large observational studies available today establish that moderate hyperhomocysteinemia, either genetically or nutritionally determined, is an independent risk factor for myocardial infarction, stroke, and thromboembolic disease. This is also true for chronic renal failure patients, who exhibit a high prevalence of hyperhomocysteinemia (85-100%), which reaches high plasma concentrations (20-40 microM, while control values range between 8 and 12 microM). After a renal transplant, homocysteine levels decrease, but tend to be higher than normal. The cause of hyperhomocysteinemia in renal failure is still obscure, since recent data have questioned the previous notion that a net homocysteine renal extraction and/or excretion take place in man. No matter the cause of its increase, the sulfur amino acid homocysteine is thought to induce an increment in cardiovascular risk through three basic biochemical mechanisms: (1) homocysteine oxidation, with H2O2 generation; (2) hypomethylation through S-adenosylhomocysteine accumulation, and (3) protein acylation by homocysteine thiolactone. The final result is membrane protein damage, endothelial damage, and endothelial cell growth inhibition, among other effects. Hyperhomocysteinemia, in general, is susceptible of therapeutic intervention with the vitamins involved in its metabolism. Depending on the cause, vitamin B6, vitamin B12, betaine, and/or folic acid can be effectively utilized. Chronic renal failure patients benefit from folic acid in high dosage: 1-2 mg are usually not effective ('relative folate resistance'), while 5-15 mg reduce homocysteine levels to a 'normative' range (<15 microM) in a substantial group of patients. Good results are also obtained in transplant patients, best with a combination of folic and vitamin B6. The results of the interventional trials focusing on the possible reduction in cardiovascular risk after homocysteine-lowering therapy, both in the general population and in end-stage renal disease, are expected soon, as well as the genetic and biochemical studies in suitable models, with the aim to clarify the cause-effect link suggested by the numerous observational and basic science studies.Mineral and Electrolyte Metabolism 04/1999; 25(1-2):95-9. DOI:10.1159/000057428
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ABSTRACT: The methylation of carrier-free 74As-arsenite by liver cytosol of Flemish Giant rabbits is highly susceptible to additions of trace elements. In vitro supplementation of essential trace elements like zinc (Zn2+), vanadium (V5+), iron (Fe2+), copper (Cu2+) and selenate was shown to increase the methylation efficiency. Trivalent metal ions (e.g. Al3+, Cr3+ and Fe3+), Hg2+, Tl+ and SeO3(2-) had a deleterious effect. The inhibitory effect of EDTA, oxime and many divalent cations (Ca2+, Mg2+, Sr2+, ...) suggest a co-factor role for a specific divalent metal ion, possibly Zn2+. Chelating agents used in clinical treatment of acute and chronic inorganic arsenic poisoning lower the methylation capacity of cytosol by rendering the trivalent arsenic unavailable for the methyltransferase enzymes. S-adenosylhomocysteine and periodate-oxidized adenosine, inhibitors of s-adenosylmethionine dependent methylation pathways, inhibit the methylation of arsenite. Pyrogallol, a catechol-O-methyltransferase inhibitor, blocks the action of arsenite- and monomethylarsonic methyltransferase enzymes, suggesting a close structural relationship between the active sites of the different enzymes. Some uraemic toxins, namely oxalate, p-cresol, hypoxanthine, homocysteine and myo-inositol, inhibit arsenic methylation.Drug and Chemical Toxicology 12/1999; 22(4):613-28. DOI:10.3109/01480549908993171 · 1.23 Impact Factor
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