Homocysteine, oxidative stress, and vascular disease

In vitro effect of homocysteine on some parameters of oxidative stress in rat hippocampus

Homocystinuria is an inherited metabolic disease characterized biochemically by increased blood and brain levels of homocysteine caused by severe deficiency of cystathionine beta-synthase activity. Affected patients present mental retardation, seizures, and atherosclerosis. Oxidative stress plays an important role in the pathogenesis of many neurodegenerative and vascular diseases, such Alzheimer's disease, stroke, and atherosclerosis. However, the mechanisms underlying the neurological damage characteristic of homocystinuria are still poorly understood. To evaluate the involvement of oxidative stress on the neurological dysfunction present in homocystinuria, we measured thiobarbituric acid reactive substances (TBARS), an index of lipid peroxidation, and total radical-trapping antioxidant potential (TRAP) and antioxidant enzyme activities (superoxide dismutase, catalase, and glutathione peroxidase) in rat hippocampus in the absence (controls) or in the presence of homocysteine (10-500 microM) in vitro. We demonstrated that homocysteine significantly increases TBARS and decreases TRAP, both in a dose-dependent manner, but did not change antioxidant enzymes. Our results suggest that oxidative stress is involved in the neurological dysfunction of homocystinuria. However, further studies are necessary to confirm and extend our findings to the human condition and also to determine whether antioxidant therapy may be of benefit to these patients.

Changes in markers of vascular injury in response to transient hyperhomocysteinemia

The purpose of this study was to test whether transient increases in homocysteine would promote changes in markers of endothelial injury, cellular fibronectin (cFN), and soluble vascular cell adhesion molecule 1 (sVCAM-1). Homocysteine, cFN, and sVCAM-1 concentrations increased significantly in response to a methionine load by 6 hours in human subjects. However, no correlation was observed between homocysteine and cFN or sVCAM-1. To directly test whether homocysteine can injure endothelial cells, human umbilical vein endothelial cells (HUVEC) were incubated with increasing concentrations of homocysteine, plasma, or serum from hyperhomocysteinemic mice or from the methionine-loaded test subjects. cFN release was increased from endothelial cells cultured with plasma (but not serum) of hyperhomocysteinemic transgenic mice or from methionine-loaded human subjects. These data suggest that very high homocysteine concentrations can promote endothelial injury; however, this effect is likely mediated by secondary effects that include a factor(s) present in plasma that affects endothelial cells.

Inhibition of Na(+),K(+)-ATPase activity in hippocampus of rats subjected to acute administration of homocysteine is prevented by vitamins E and C treatment

In the present study we evaluated the effect of acute homocysteine (Hcy) administration on Na(+),K(+)-ATPase activity, as well as on some parameters of oxidative stress such as total radical-trapping antioxidant potential (TRAP) and on activities of antioxidant enzymes catalase (CAT), superoxide dismutase and glutathione peroxidase in rat hippocampus. Results showed that Hcy significantly decreased TRAP, Na(+),K(+)-ATPase and CAT activities, without affecting the activities of superoxide dismutase and glutathione peroxidase. We also verified the effect of chronic pretreatment with vitamins E and C on the reduction of TRAP, Na(+),K(+)-ATPase and CAT activities caused by Hcy. Vitamins E and C per se did not alter these parameters, but prevented the reduction of TRAP, Na(+),K(+)-ATPase and CAT activities caused by Hcy. Our results indicate that oxidative stress is probably involved in the pathogenesis of homocystinuria and that reduction of Na(+),K(+)-ATPase activity may be related to the neuronal dysfunction found in homocystinuric patients.

Reduction of Na(+),K(+)-ATPase activity in hippocampus of rats subjected to chemically induced hyperhomocysteinemia

Hyperhomocysteinemia occurs in homocystinuria, an inherited metabolic disease clinically characterized by thromboembolic episodes and a variable degree of neurological dysfunction whose pathophysiology is poorly known. In this study, we induced elevated levels of homocysteine (Hcy) in blood (500 microM), comparable to those of human homocystinuria, and in brain (60 nmol/g wet tissue) of young rats by injecting subcutaneously homocysteine (0.3-0.6 micromol/g of body weight) twice a day at 8-hr intervals from the 6th to the 28th postpartum day. Controls received saline in the same volumes. Na(+),K(+)-ATPase and Mg(2+)-ATPase activities were determined in the hippocampus of treated Hcy- and saline-treated rats. Chronic administration of Hcy significantly decreased (40%) Na(+),K(+)-ATPase activity but did not alter Mg(2+)-ATPase activity. Considering that Na(+),K(+)-ATPase plays a crucial role in the central nervous system, our results suggest that the brain dysfunction found in homocystinuria may be related to the reduction of brain Na(+),K(+)-ATPase activity.

Pretreatment with vitamins E and C prevent the impairment of memory caused by homocysteine administration in rats

Homocystinuria is a metabolic disorder caused by deficiency of cystathione beta-synthase activity leading to tissue accumulation of homocysteine (Hcy); affected patients present neurological dysfunction. Considering that Hcy induces free radical formation and that memory is impaired by oxidative stress, in the present study we investigated the effect of an acute administration of Hcy on retrieval of step-down inhibitory avoidance in adult rats. The action of vitamins E and C on the effects produced by Hcy was also tested. Adult Wistar rats were pretreated for 1 week with daily i.p. administration of saline (control group) and vitamins E and C (vitamin E 40 mg/kg and vitamin C 100 mg/kg). Hcy (11 mmol/kg) or an equivalent volume of 0.9% saline were administered 1 h before training, 1 h before testing, or immediately after training sessions. Memory was significantly impaired in Hcy-treated group, whereas the rats chronically treated with vitamins E and C had this effect prevented. Present data strongly indicate that Hcy administration impairs memory, an effect probably mediated by oxidative stress since treatment with vitamins E and C prevented amnesia. Assuming the possibility that this might occur in the human condition, reported results may be relevant to explain, at least in part, neurologic dysfunction associated with homocystinuria.

Inhibition of Na+, K+-ATPase activity by the metabolites accumulating in homocystinuria

Homocystinuria is an inborn error of sulfur amino acid metabolism characterized predominantly by vascular and nervous system dysfunction. In this study we determined the in vitro effects of homocysteine and methionine, metabolites which accumulate in homocystinuria, on Na+, K+-ATPase, and Mg2+-ATPase activities in synaptic membranes from the hippocampus of rats. The results showed that both metabolites significantly inhibit Na+, K+-ATPase but not Mg2+-ATPase activity at concentrations usually observed in plasma of homocystinuric patients. Furthermore, incubation of hippocampal homogenates with homocysteine also elicited an inhibition of the enzyme activity which was however prevented by the simultaneous addition of cysteine to the medium. In addition, cysteine or methionine per se did not modify the two enzymatic activities. These findings indicate that oxidation of critical groups in the enzyme may possibly be involved in homocysteine inhibitory effect. Moreover, kinetic studies performed to investigate the interaction between homocysteine and methionine on Na+, K+-ATPase inhibition suggested a common site for the two amino acids in the enzyme. Considering the critical role exerted by Na+, K+-ATPase in brain, it is proposed that the inhibition provoked by homocysteine and methionine on the enzyme activity may be possibly related to the brain dysfunction characteristic of homocystinuria.

Long-term improvement in homocysteine levels and arterial endothelial function after 1-year folic acid supplementation

PURPOSE

Hyperhomocysteinemia, a risk factor for atherosclerosis, is associated with endothelial dysfunction that can be improved with short-term folic acid supplementation. The current study aimed to assess whether folic acid supplementation could produce longer-term improvements in homocysteine levels and arterial endothelial function.

SUBJECTS AND METHODS

Twenty-nine healthy adults with hyperhomocysteinemia were selected from 89 volunteers enrolled in a community-based atherosclerosis screening project. All subjects were given folic acid (10 mg/d) for 1 year. Fasting plasma homocysteine levels were measured by high-performance liquid chromatography. Arterial endothelial function was measured as flow-mediated dilation of the brachial artery using high-resolution B-mode ultrasound.

RESULTS

Folic acid supplementation for 1 year was associated with a significant increase in mean (+/-SD) plasma folate levels (24 +/- 5 nmol/L to 40 +/- 5 nmol/L; P < 0.001) and a significant decline in homocysteine levels (9.0 +/- 1.7 micromol/L to 7.9 +/- 2.0 micromol/L; P < 0.001). Flow-mediated dilation also improved significantly, from 7.4% +/- 2.0% to 8.9% +/- 1.5% (P <0.0001), but there was no change in nitroglycerin-induced (endothelium-independent) responses.

CONCLUSIONS

These results demonstrate that long-term folic acid improves arterial endothelial function and has potential implications for the prevention of atherosclerosis in adults with hyperhomocysteinemia.

Homocysteine and arterial disease. Experimental mechanisms

Hyperhomocysteinemia (hH(e)) in the general population is associated with incidence and progression of arterial occlusive disease, although the underlying mechanisms are not well defined. Current research supports a role for homocysteine (H(e))-mediated endothelial damage and endothelial dysfunction. This mechanism appears to be a key factor in subsequent impaired endothelial-dependent vasoreactivity and decreased endothelium thromboresistance. These consequences may predispose hyperhomocysteinemic vessels to the development of increased atherogenesis. Additional mechanisms of H(e)-mediated vascular pathology, including protein homocysteinylation and vascular smooth muscle cell proliferation may also play a role. Continued investigation into the mechanisms contributing to H(e) toxicity will provide further insight into the processes by which hH(e) may increase atherosclerosis.

Homocysteine increases monocyte and T-cell adhesion to human aortic endothelial cells

Although hyperhomocysteinemia has been recognized as an independent risk factor for atherosclerosis, its mechanism(s) are not well understood. Because chemotaxis and accumulation of leukocytes such as monocytes and T cells have been demonstrated to be critical events in the initiation and development of atherosclerosis, we investigated the effect of homocysteine (HCY) on U937 monocytic cells- and Jurkat T-cell-human aortic endothelial cell (HAEC) interactions under inflammatory cytokine-stimulated conditions. When HAEC were pretreated with HCY followed by stimulation with IL-1 beta, U937 and Jurkat T-cell adhesion to HAEC increased in a dose-dependent manner. The significant increase in U937 cell adhesion to HAEC was also observed when U937 cells were treated with HCY or when both cell types were treated with HCY. We also demonstrated that HCY increases endothelial surface expression and mRNA level of adhesion molecules, VCAM-1 and E-selectin. Attenuation of Jurkat T-cell and U937 cell adhesion to HAEC by monoclonal antibodies directed to specific adhesion molecules demonstrated that both VCAM-1 and E-selectin are involved in Jurkat T-cell adhesion, and VCAM-1 in U937 cell adhesion. Supplementation of HAEC with vitamin E was effective in preventing HCY-stimulated Jurkat T-cell adhesion and VCAM-1 and E-selectin expression in HAEC. These results indicate that HCY-mediated leukocyte-endothelial cell interaction is one potential mechanism by which homocysteinemia may lead to the development of atherosclerosis under inflammatory conditions. Dietary antioxidants such as vitamin E may attenuate HCY-stimulated activation of the endothelium and may help reduce the risk of vascular disease associated with hyperhomocysteinemia.

Cyclin A transcriptional suppression is the major mechanism mediating homocysteine-induced endothelial cell growth inhibition

Previously, it was reported that homocysteine (Hcy) specifically inhibits the growth of endothelial cells (ECs), suppresses Ras/mitogen-activated protein (MAP) signaling, and arrests cell growth at the G1/S transition of the cell cycle. The present study investigated the molecular mechanisms underlying this cell-cycle effect. Results showed that clinically relevant concentrations (50 μM) of Hcy significantly inhibited the expression of cyclin A messenger RNA (mRNA) in ECs in a dose- and time-dependent manner. G1/S-associated molecules that might account for this block were not changed, because Hcy did not affect mRNA and protein expression of cyclin D1 and cyclin E. Cyclin D1- and E-associated kinase activities were unchanged. In contrast, cyclin A–associated kinase activity and CDK2 kinase activity were markedly suppressed. Nuclear run-on assay demonstrated that Hcy decreased the transcription rate of the cyclin A gene but had no effect on the half-life of cyclin A mRNA. In transient transfection experiments, Hcy significantly inhibited cyclin A promoter activity in endothelial cells, but not in vascular smooth muscle cells. Finally, adenovirus-transduced cyclin A expression restored EC growth inhibition and overcame the S phase block imposed by Hcy. Taken together, these findings indicate that cyclin A is a critical functional target of Hcy-mediated EC growth inhibition.

Cyclin A transcriptional suppression is the major mechanism mediating homocysteine-induced endothelial cell growth inhibition

Previously, it was reported that homocysteine (Hcy) specifically inhibits the growth of endothelial cells (ECs), suppresses Ras/mitogen-activated protein (MAP) signaling, and arrests cell growth at the G1/S transition of the cell cycle. The present study investigated the molecular mechanisms underlying this cell-cycle effect. Results showed that clinically relevant concentrations (50 μM) of Hcy significantly inhibited the expression of cyclin A messenger RNA (mRNA) in ECs in a dose- and time-dependent manner. G1/S-associated molecules that might account for this block were not changed, because Hcy did not affect mRNA and protein expression of cyclin D1 and cyclin E. Cyclin D1- and E-associated kinase activities were unchanged. In contrast, cyclin A–associated kinase activity and CDK2 kinase activity were markedly suppressed. Nuclear run-on assay demonstrated that Hcy decreased the transcription rate of the cyclin A gene but had no effect on the half-life of cyclin A mRNA. In transient transfection experiments, Hcy significantly inhibited cyclin A promoter activity in endothelial cells, but not in vascular smooth muscle cells. Finally, adenovirus-transduced cyclin A expression restored EC growth inhibition and overcame the S phase block imposed by Hcy. Taken together, these findings indicate that cyclin A is a critical functional target of Hcy-mediated EC growth inhibition.

Homocyst(e)ine and coronary heart disease: pharmacoeconomic support for interventions to lower hyperhomocyst(e)inaemia

Homocyst(e)ine, a sulphur-containing amino acid, is an intermediate formed during the metabolism of the essential amino acid methionine. Biological and epidemiological evidence suggest that elevated plasma levels of homocyst(e)ine are a risk factor for atherosclerosis and coronary heart disease (CHD). In the general US population, hyperhomocyst(e)inaemia is common and most often due to mild nutritional deficiencies in the B vitamins (folic acid, vitamin B(12) and vitamin B(6)). While high homocyst(e)ine levels can be effectively lowered using folic acid and other B vitamins, it is unknown whether such vitamin therapy will lead to clinical benefits. Given that strategies for homocyst(e)ine-lowering are safe and inexpensive, however, even small reductions in CHD risk will be highly cost effective. Thus, it may be prudent for patients to ensure an adequate daily intake of dietary folic acid and other B vitamins and for physicians to screen high-risk adults such as those with established CHD as we await definitive results from ongoing clinical trials.

Oxidative stress and cardiovascular disease in type 2 diabetes: the role of antioxidants and pro-oxidants

Oxidative stress occurs when there is an imbalance between free radical production and antioxidant capacity. This may be due to increased free radical formation in the body and/or loss of normal antioxidant defenses. Oxidative stress has been associated with the development of cardiovascular disease. The role of antioxidants in the primary and secondary prevention of coronary heart disease is currently under study. Although epidemiologic evidence indicates that antioxidants may decrease cardiovascular risk, clinical trial data are not conclusive. Information regarding the use and benefits of antioxidants in persons with diabetes is limited. Persons with diabetes may be more prone to oxidative stress because hyperglycemia depletes natural antioxidants and facilitates the production of free radicals. In addition, other factors such as homocysteine, insulin resistance, and aging may be contributory. This article highlights landmark clinical trials that have examined the cardioprotective effect of antioxidants. Because these trials have not been designed to study persons with diabetes, and clinical trial data for this group are not available, correlational studies are also presented. Finally, the concept of oxidative stress, the antioxidant and pro-oxidant factors that may contribute to oxidative stress, and the consequences of oxidative stress in persons with type 2 diabetes are presented. Key words: antioxidants, clinical trials,

Inhibition of rat brain Na+, K+-ATPase activity induced by homocysteine is probably mediated by oxidative stress

The objective of the present study was to investigate the effects of preincubation of hippocampus homogenates in the presence of homocysteine or methionine on Na+, K+-ATPase and Mg2+-ATPase activities in synaptic membranes of rats. Homocysteine significantly inhibited Na+, K+-ATPase activity, whereas methionine had no effect. Mg2+-ATPase activity was not altered by the metabolites. We also evaluated the effect of incubating glutathione, cysteine, dithiothreitol, trolox, superoxide dismutase and GM1 ganglioside alone or incubation with homocysteine on Na+, K+-ATPase activity. Tested compounds did not alter Na+, K+-ATPase and Mg2+-ATPase activities, but except for trolox, prevented the inhibitory effect of homocysteine on Na+, K+-ATPase activity. These results suggest that inhibition of this enzyme activity by homocysteine is possibly mediated by free radicals and may contribute to the neurological dysfunction found in homocystinuric patients.

Homocysteine in breast cyst fluid

BACKGROUND

Elevated plasma homocysteine concentrations have been reported in a variety of carcinoma, including those of the breast. The risk of breast cancer is higher in patients suffering from gross cystic disease. The breast cyst fluid contains unusual amounts of low- and high- molecular substances, including steroid hormones and their conjugates. The present study was undertaken to find out the presence of homocysteine in the fluid filling the cysts and have its concentration compared with other thiols, levels of Na+/K+ ratio and steroid hormones. Materials and methods Fourteen women suffering from gross cystic disease were enrolled in this study. Cystic concentrations of homocysteine (Hcy), cysteine (Cys), cysteinylglycine (Cys-Gly) and glutathione (GSH) were determined by high performance liquid chromatography, with fluorescence detection; estradiol (E2), progesterone, allopregnanolone and pregnenolone sulfate (PregS) by RIA methods.

RESULTS

Mean levels of Hcy, Cys, Cys-Gly, Na+/K+, E2 and PregS in the fluid filling the breast cysts were significantly higher than the corresponding plasma concentrations. In addition, a negative correlation was found between cystic Hcy and the Na+/K+ ratio (Rs = -0.72, P = 0.003) and positive correlations between cyst Hcy and estradiol (Rs = 0.64, P = 0.018) and Hcy and PregS (Rs = 0.60, P = 0.025). Conclusion The study provides the first evidence of thiol concentrations in the breast cyst fluid. The finding of a negative correlation between homocysteine and the Na+/K+ ratio support the idea that the homocysteine concentration in breast cysts might be used clinically as a marker for the development of breast cancer disease.

Suppression of LDL oxidation by garlic

It has been known for several decades that hypercholesterolemia is a major risk factor for atherosclerosis and that lowering of cholesterol can significantly reduce risk for cardiovascular diseases. More recently, oxidation of LDL has been recognized as playing an important role in the initiation and progression of atherosclerosis. Oxidized LDL, but not native LDL, promotes vascular dysfunction by exerting direct cytotoxicity toward endothelial cells, by increasing chemotactic properties for monocytes, by transforming macrophages to foam cells via scavenger-receptors and by enhancing the proliferation of various cell types, e.g., endothelial cells, monocytes and smooth muscle cells; all of these events are recognized as contributing to atherogenesis. In this paper, experimental evidence is presented that shows that several garlic compounds can effectively suppress LDL oxidation in vitro. Short-term supplementation of garlic in human subjects has demonstrated an increased resistance of LDL to oxidation. These data suggest that suppressed LDL oxidation may be one of the powerful mechanisms accounting for the antiatherosclerotic properties of garlic.

Therapeutic potential of dietary phase 2 enzyme inducers in ameliorating diseases that have an underlying inflammatory component

Many diseases associated with ageing have an underlying oxidative stress and accompanying inflammatory component, for example, Alzheimer's disease or atherosclerosis. Reviewed in this manuscript are: the role of oxidative stress in activating the transcription factor nuclear factor kappa B (NFκB), the role of NFκB in activating pro-inflammatory gene transcription, strong oxidants produced by cells, anti-oxidant defense systems, the central role of phase 2 enzymes in the anti-oxidant defense, dietary phase 2 enzyme inducers and evidence that dietary phase 2 enzymes decrease oxidative stress. It is likely that a diet containing phase 2 enzyme inducers may ameliorate or even prevent diseases that have a prominent inflammatory component to them. Research should be directed into the potential therapeutic effects of dietary phase 2 enzyme inducers in ameliorating diseases with an underlying oxidative stress and inflammatory component to them.Key words: Alzheimer's disease, atherosclerosis, diet, glutathione, inflammation, stroke.

Folate depletion and elevated plasma homocysteine promote oxidative stress in rat livers

This study was designed to determine whether nutritional folate depletion exerts hepatic oxidative stress in relation to elevated plasma homocysteine. To mimic various extents of folate depletion status in vivo, male Wistar rats were fed an amino acid-defined diet containing either 8 (control), 2, 0.5, or 0 mg folic acid/kg diet. After a 4-wk feeding period, the plasma and hepatic folate concentrations of the rats decreased significantly with each decrement of dietary folate. Folate depletion did not significantly affect two major liver antioxidants: reduced glutathione and alpha-tocopherol. Conversely, folate depletion decreased Cu-Zn superoxide dismutase and glutathione peroxidase activities, but had no effect on catalase activity in liver homogenates. Lipid peroxidation products, as measured by thiobarbituric acid-reactive substances, were significantly higher in livers of folate-depleted rats than in those of the controls. This occurrence of hepatic oxidative stress in folate-depleted rats was confirmed by demonstrating an increased susceptibility of livers of folate-depleted rats to lipid peroxidation induced by additional H2O2 or Fe(2+) treatments compared with the controls. Decreasing dietary folate intake resulted in graded increases in plasma homocysteine concentrations of folate-depleted rats. Elevated plasma homocysteine and decreased plasma and hepatic folate concentrations in folate-depleted rats were all strongly and significantly correlated with increased liver lipid peroxidation (/r/ > or = 0.58, P < 0.0003). These data demonstrate that folate depletion and elevated plasma homocysteine promote oxidative stress in rat livers.

Oxidant stress in the vasculature

Vascular disease and vasomotor responses are largely influenced by oxidant stress. Superoxide is generated via the cellular oxidase systems, xanthine oxidase, and NADH/NADPH oxidases. Once formed, superoxides participate in a number of reactions, yielding various free radicals such as hydrogen peroxide, peroxynitrite, oxidized low-density lipoprotein, or hypochlorous acid. Numerous cellular antioxidant systems exist to defend against oxidant stress; glutathione and the enzymes superoxide dismutase and glutathione peroxidase are critical for maintaining the redox balance of the cell. However, the redox state is disrupted by certain vascular diseases. It appears that oxidant stress both promotes and is induced by diseases such as hypertension, atherosclerosis, and restenosis as well as by certain risk factors for coronary artery disease including hyperlipidemia, diabetes, and cigarette smoking. Once oxidant stress is invoked, characteristic pathophysiologic features ensue, namely adverse vessel reactivity, vascular smooth muscle cell proliferation, macrophage adhesion, platelet activation, and lipid peroxidation.

Homocysteine

Homocysteine is a sulfur-containing amino acid generated through the demethylation of methionine. It is largely catabolized by trans-sulfuration to cysteine, but it may also be remethylated to methionine. Regulation of homocysteine is dependent on nutrient intake, especially folate, vitamins B6 and B12. It is also controlled by individual genetic differences in how vitamins are utilized as cofactors in the reactions controlling homocysteine metabolism. In excess quantities, homocysteine is thought to be thrombophilic and to damage the vascular endothelium. Total plasma homocysteine (tHcy) is now established as a clinical risk factor for coronary artery disease, as well as other arterial and venous occlusive disease in adult populations. These effects are probably related to its role as a teratogen in the pathogenesis of neural tube defects--genetic variants causing hyperhomocysteinemia are associated with both neural tube defects in susceptible pregnancies and with risks for vaso-occlusive disease in later years. Considerable care must be taken in assaying tHcy. Plasma should be separated shortly after collection to avoid artifactual increases due to synthesis by blood cells in vitro. tHcy concentrations must be interpreted in light of the fact that serum albumin, urate, creatinine, and vitamin concentrations may be important analytical covariates. Moreover, concentrations are age- and sex-dependent and are altered by renal function, hormonal status, drug intake, and a variety of other common clinical factors. Why then is homocysteine now of such great clinical and scientific interest? If the homocysteine moiety itself is important in the pathogenesis of vaso-occlusive disease, then simple treatment of hyperhomocysteinemia with vitamins should lead to a significant reduction in disease risk. Such a possibility lies behind the growing momentum to recommend increased supplements of folate and B vitamins to at-risk populations and patient groups today.

The effect of cyst(e)ine on the auto-oxidation of homocysteine

This study examines the effect of cysteine on the auto-oxidation of homocysteine, a process that has been implicated in the pathologic mechanism of hyperhomocystinemia with respect to arteriosclerosis and vascular disease. It is shown that homocysteine autoxidizes at a much slower rate than cysteine, but that low concentrations of cysteine or cystine dramatically accelerate homocysteine oxidation and increase the rate of homocysteine-dependent oxygen consumption. It is proposed that the major role of homocysteine is to reduce cystine to cysteine, and that cysteine autoxidation is the mechanism by which thiol-dependent oxidative stress occurs.

Hyperhomocysteinemia and premature vascular occlusive disease

Hyperhomocysteinemia has recently been identified as an independent risk factor for arterial and venous occlusive disease. In particular, hyperhomocysteinemia has been associated with premature vascular disease, and may act synergistically with other risk factors. Two young patients with severe premature vascular disease, one venous and one arterial, have significantly elevated homocysteine levels. In addition to appropriate anti-coagulant therapy, these patients receive B6 and folate vitamin therapy which normalizes the homocysteine levels. While this course of therapy is prudent, no prospective clinical trials have yet demonstrated that reduction of homocysteine levels correlates with a decreased cardiovascular risk.