Homocysteine export from cells cultured in the presence of physiological or superfluous levels of methionine: methionine loading of non-transformed, transformed, proliferating, and quiescent cells in culture

Mechanism of action and impact of thiol homeostasis on efficacy of an enzyme replacement therapy for classical homocystinuria

Homocystinuria (HCU) due to cystathionine beta-synthase (CBS) deficiency is characterized by elevated plasma and tissue homocysteine levels. There is no cure, but HCU is typically managed by methionine/protein restriction and vitamin B6 supplementation. Enzyme replacement therapy (ERT) based on human CBS has been developed and has shown significant efficacy correcting HCU phenotype in several mouse models by bringing plasma total homocysteine below the clinically relevant 100 μM threshold. As the reactive nature of homocysteine promotes disulfide formation and protein binding, and ERT is unable to normalize plasma total homocysteine levels, the mechanism of action of ERT in HCU remains to be further characterized. Here we showed that only a reduced homocysteine serves as a substrate for CBS and its availability restricts the homocysteine-degrading capacity of CBS. We also demonstrated that cells export homocysteine in its reduced form, which is efficiently metabolized by CBS in the culture medium. Availability of serine, a CBS co-substrate, was not a limiting factor in our cell-based model. Biological reductants, such as N-acetylcysteine, MESNA or cysteamine, increased the availability of the reduced homocysteine and thus promoted its subsequent CBS-based elimination. In a transgenic I278T mouse model of HCU, administration of biological reductants significantly increased the proportion of protein-unbound homocysteine in plasma, which improved the efficacy of the co-administered CBS-based ERT, as evidenced by significantly lower plasma total homocysteine levels. These results clarify the mechanism of action of CBS-based ERT and unveil novel pharmacological approaches to further increase its efficacy.

CRIF1 Deficiency Increased Homocysteine Production by Disrupting Dihydrofolate Reductase Expression in Vascular Endothelial Cells

Elevated plasma homocysteine levels can induce vascular endothelial dysfunction; however, the mechanisms regulating homocysteine metabolism in impaired endothelial cells are currently unclear. In this study, we deleted the essential mitoribosomal gene CR6 interacting factor 1 (CRIF1) in human umbilical vein endothelial cells (HUVECs) and mice to induce endothelial cell dysfunction; then, we monitored homocysteine accumulation. We found that CRIF1 downregulation caused significant increases in intracellular and plasma concentrations of homocysteine, which were associated with decreased levels of folate cycle intermediates such as 5-methyltetrahydrofolate (MTHF) and tetrahydrofolate (THF). Moreover, dihydrofolate reductase (DHFR), a key enzyme in folate-mediated metabolism, exhibited impaired activity and decreased protein expression in CRIF1 knockdown endothelial cells. Supplementation with folic acid did not restore DHFR expression levels or MTHF and homocysteine concentrations in endothelial cells with a CRIF1 deletion or DHFR knockdown. However, the overexpression of DHFR in CRIF1 knockdown endothelial cells resulted in decreased accumulation of homocysteine. Taken together, our findings suggest that CRIF1-deleted endothelial cells accumulated more homocysteine, compared with control cells; this was primarily mediated by the disruption of DHFR expression.

The Effects of Medicinal Plants and Bioactive Natural Compounds on Homocysteine

BACKGROUND

Among non-communicable diseases, cardiovascular diseases (CVDs) are the leading cause of mortality and morbidity in global communities. By 2030, CVD-related deaths are projected to reach a global rise of 25 million. Obesity, smoking, alcohol, hyperlipidemia, hypertension, and hyperhomocysteinemia are several known risk factors for CVDs. Elevated homocysteine is tightly related to CVDs through multiple mechanisms, including inflammation of the vascular endothelium. The strategies for appropriate management of CVDs are constantly evolving; medicinal plants have received remarkable attention in recent researches, since these natural products have promising effects on the prevention and treatment of various chronic diseases. The effects of nutraceuticals and herbal products on CVD/dyslipidemia have been previously studied. However, to our knowledge, the association between herbal bioactive compounds and homocysteine has not been reviewed in details. Thus, the main objective of this study is to review the efficacy of bioactive natural compounds on homocysteine levels according to clinical trials and animal studies.

RESULTS

Based on animal studies, black and green tea, cinnamon, resveratrol, curcumin, garlic extract, ginger, and soy significantly reduced the homocysteine levels. According to the clinical trials, curcumin and resveratrol showed favorable effects on serum homocysteine. In conclusion, this review highlighted the beneficial effects of medicinal plants as natural, inexpensive, and accessible agents on homocysteine levels based on animal studies. Nevertheless, the results of the clinical trials were not uniform, suggesting that more well-designed trials are warranted.

Proteomic exploration of cystathionine β-synthase deficiency: implications for the clinic

Introduction: Homocystinuria due to cystathionine β-synthase (CBS) deficiency, the most frequent inborn error of sulfur amino acid metabolism, is characterized biochemically by severely elevated homocysteine (Hcy) and related metabolites, such as Hcy-thiolactone and N-Hcy-protein. CBS deficiency reduces life span and causes pathological abnormalities affecting most organ systems in the human body, including the cardiovascular (thrombosis, stroke), skeletal/connective tissue (osteoporosis, thin/non-elastic skin, thin hair), and central nervous systems (mental retardation, seizures), as well as the liver (fatty changes), and the eye (ectopia lentis, myopia). Molecular basis of these abnormalities were largely unknown and available treatments remain ineffective. Areas covered: Proteomic and transcriptomic studies over the past decade or so, have significantly contributed to our understanding of mechanisms by which the CBS enzyme deficiency leads to clinical manifestations associated with it. Expert opinion: Recent findings, discussed in this review, highlight the involvement of dysregulated proteostasis in pathologies associated with CBS deficiency, including thromboembolism, stroke, neurologic impairment, connective tissue/collagen abnormalities, hair defects, and hepatic toxicity. To ameliorate these pathologies, pharmacological, enzyme replacement, and gene transfer therapies are being developed.

Targeted Metabolic Profiling of Methionine Cycle Metabolites and Redox Thiol Pools in Mammalian Plasma, Cells and Urine

The concentration of thiol and thioether metabolites in plasma has diagnostic value in genetic diseases of B-vitamin metabolism linked to methionine utilization. Among these, cysteine/cystine (Cys/CSSC) and glutathione/oxidized glutathione (GSH/GSSG) act as cellular redox buffers. A new LC-MS/MS method was developed for the simultaneous detection of cystathionine (Cysta), methionine (Met), methionine sulfoxide (MSO), creatinine and the reduced and oxidized pairs of homocysteine (Hcy/HSSH), cysteine (Cys/CSSC) and glutathione (GSH/GSSG). A one-step thiol-blocking protocol with minimal sample preparation was established to determine redox thiol pairs in plasma and cells. The concentrations of diagnostic biomarkers Hcy, Met, Cysta, and Cys in a cohort of healthy adults (n = 53) agreed with reference ranges and published values. Metabolite concentrations were also validated in commercial samples of human, mouse, rat and Beagle dog plasma and by the use of a standardized ERNDIM quality control. Analysis of fibroblasts, endothelial and epithelial cells, human embryonic stem cells, and cancer cell lines showed cell specificity for both the speciation and concentration of thiol and thioether metabolites. This LC-MS/MS platform permits the fast and simultaneous quantification of 10 thiol and thioether metabolites and creatinine using 40 µL plasma, urine or culture medium, or 500,000 cells. The sample preparation protocols are directly transferable to automated metabolomic platforms.

Relationship between folate, vitamin B12, homocysteine, transaminase and mild cognitive impairment in China: a case-control study

To explore the association between the levels of serum folate, vitamin B₁₂, and homocysteine (Hcy), transaminase and mild cognitive impairment (MCI) in Chinese elderly. A case-control study was implemented between April and October 2016. Elderly participants aged ≥60 with and without MCI (n = 118 separately) were recruited from Community Health Center of Binhai New Area in Tianjin. Spearman's correlation analysis indicated that Hcy was significantly positively correlated with alanine transaminase (ALT) and aspartate aminotransferase (AST), and negative correlations were found among Hcy, Mini-Mental Status Examination score, Wechsler Adult Intelligence Scale-Revised by China intelligence quotient, folate and vitamin B₁₂. The associations among MCI and folate, vitamin B₁₂, Hcy and transaminase were assessed using multivariate logistic regression analyses. Lower folate levels and higher Hcy and ALT and AST levels were associated with MCI risk adjusted for multiple covariates. Increased ALT, AST, Hcy levels and lower folate levels were independently associated with the risk of MCI.

Homocysteine Modification in Protein Structure/Function and Human Disease

Epidemiological studies established that elevated homocysteine, an important intermediate in folate, vitamin B₁₂, and one carbon metabolism, is associated with poor health, including heart and brain diseases. Earlier studies show that patients with severe hyperhomocysteinemia, first identified in the 1960s, exhibit neurological and cardiovascular abnormalities and premature death due to vascular complications. Although homocysteine is considered to be a nonprotein amino acid, studies over the past 2 decades have led to discoveries of protein-related homocysteine metabolism and mechanisms by which homocysteine can become a component of proteins. Homocysteine-containing proteins lose their biological function and acquire cytotoxic, proinflammatory, proatherothrombotic, and proneuropathic properties, which can account for the various disease phenotypes associated with hyperhomocysteinemia. This review describes mechanisms by which hyperhomocysteinemia affects cellular proteostasis, provides a comprehensive account of the biological chemistry of homocysteine-containing proteins, and discusses pathophysiological consequences and clinical implications of their formation.

Methionine restriction leads to hyperhomocysteinemia and alters hepatic H2S production capacity in Fischer-344 rats

Dietary methionine restriction (MR) increases lifespan in several animal models. Despite low dietary intake of sulphur amino acids, rodents on MR develop hyperhomocysteinemia. On the contrary, MR has been reported to increase H₂S production in mice. Enzymes involved in homocysteine metabolism also take part in H₂S production and hence, in this study, the impact of MR on hyperhomocysteinemia and H₂S production capacity were investigated using Fischer-344 rats assigned either a control or a MR diet for 8 weeks. The MR animals showed elevated plasma homocysteine accompanied with a reduction in liver cysteine content and methylation potential. It was further found that MR decreased cystathionine-β-synthase (CBS) activity in the liver, however, MR increased hepatic cystathionine-γ-lyase (CGL) activity which is the second enzyme in the transsulfuration pathway and also participates in regulating H₂S production. The relative contribution of CGL in H₂S production increased concomitantly with the increased CGL activity. Additionally, hepatic mercaptopyruvate-sulphur-transferase (MPST) activity also increased in response to MR. Taken together, our results suggest that reduced CBS activity and S-Adenosylmethionine availability contributes to hyperhomocysteinimia in MR animals. Elevated CGL and MPST activities may provide a compensatory mechanism for maintaining hepatic H₂S production capacity in response to the decreased CBS activity.

Investigation of the effects of kisspeptin-10 in methionine-induced lipid peroxidation in testicle tissue of young rats

Disruption of the balance oxidants, antioxidants cause various pathophysiological conditions such as lipid peroxidation, protein degradation, or DNA damage. We have examined possible effects of kisspeptin-10 on the structural damage produced by methionine-induced lipid peroxidation in testicle tissue of young rats. Kisspeptin-10 did not significantly affect spermatogenic cells in seminiferous tubules. Testosterone levels decreased in the methionine group as compared with the control group but without statistical significance. Luteinizing hormone levels decreased in the methionine group as compared with the control group (P < 0.001). Catalase enzyme activity increased in the kisspeptin-10 group (P < 0.01) as compared with the other groups. Catalase mRNA expression was decreased in the methionine group as compared with the kisspeptin group (P < 0.001). Total superoxide dismutase enzyme activity and superoxide dismutase mRNA expression were increased in the kisspeptin group as compared with the methionine group (P < 0.05). In conclusion, kisspeptin treatment may protect the structure of spermatogenic cells against methionine-induced damage.

Homocysteine in Renal Injury

BACKGROUND

Homocysteine (Hcy) is an intermediate of methionine metabolism. Hyperhomocysteinemia (HHcy) can result from a deficiency in the enzymes or vitamin cofactors required for Hcy metabolism. Patients with renal disease tend to be hyperhomocysteinemic, particularly as renal function declines, although the underlying cause of HHcy in renal disease is not entirely understood.

SUMMARY

HHcy is considered a risk or pathogenic factor in the progression of chronic kidney disease (CKD) as well as the cardiovascular complications.

KEY MESSAGES

In this review, we summarize both clinical and experimental findings that reveal the contribution of Hcy as a pathogenic factor to the development of CKD. In addition, we discuss several important mechanisms mediating the pathogenic action of Hcy in the kidney, such as local oxidative stress, endoplasmic reticulum stress, inflammation and hypomethylation.

Role of homocysteinylation of ACE in endothelial dysfunction of arteries

The direct impact of de novo synthesis of homocysteine (Hcy) and its reactive metabolites, Hcy-S-S-Hcy and Hcy thiolactone (HCTL), on vascular function has not been fully elucidated. We hypothesized that Hcy synthesized within endothelial cells affects activity of angiotensin-converting enzyme (ACE) by direct homocysteinylation of its amino- and/or sulfhydryl moieties. This covalent modification enhances ACE reactivity toward angiotensin II (ANG II)-NADPH oxidase-superoxide-dependent endothelial dysfunction. Mesenteric and coronary arteries isolated from normal rats were incubated for 3 days with or without exogenous methionine (Met, 0.1-0.3 mM), a precursor to Hcy. Incubation of arteries in Met-free media resulted in time-dependent decreases in vascular Hcy formation. By contrast, vessels incubated with Met produced Hcy in a dose-dependent manner. There was a notably greater de novo synthesis of Hcy from endothelial than from smooth muscle cells. Enhanced levels of Hcy production significantly impaired shear stress-induced dilation and release of nitric oxide, events that are associated with elevated production of vascular superoxide. Each of these processes was attenuated by ANG II type I receptor blocker or ACE and NADPH oxidase inhibitors. In addition, in vitro exposure of purified ACE to Hcy-S-S-Hcy/HCTL resulted in formation of homocysteinylated ACE and an enhanced ACE activity. The enhanced ACE activity was confirmed in isolated coronary and mesenteric arteries that had been exposed directly to Hcy-S-S-Hcy/HCTL or after Met incubation. In conclusion, vasculature-derived Hcy initiates endothelial dysfunction that, in part, may be mediated by ANG II-dependent activation of NADPH oxidase in association with homocysteinylation of ACE.

Vitamin B-6 restriction reduces the production of hydrogen sulfide and its biomarkers by the transsulfuration pathway in cultured human hepatoma cells

BACKGROUND

Pyridoxal 5'-phosphate (PLP) functions as a coenzyme in many cellular processes including one-carbon metabolism and the interconversion and catabolism of amino acids. PLP-dependent enzymes, cystathionine β-synthase and cystathionine γ-lyase, function in transsulfuration but also have been implicated in the production of the endogenous gaseous signaling molecule hydrogen sulfide (H2S) concurrent with the formation of the biomarkers lanthionine and homolanthionine.

OBJECTIVE

Our objective was to determine if H2S production and concurrent biomarker production is affected by vitamin B-6 restriction in a cell culture model.

METHODS

We used cultured human hepatoma cells and evaluated static intracellular profiles of amino acids and in vivo kinetics of H2S biomarker formation. Cells were cultured for 6 wk in media containing concentrations of pyridoxal that represented severe vitamin B-6 deficiency (15 nmol/L pyridoxal), marginal deficiency (56 nmol/L pyridoxal), adequacy (210 nmol/L pyridoxal), and standard medium formulation providing a supraphysiologic pyridoxal concentration (1800 nmol/L pyridoxal).

RESULTS

Intracellular concentrations of lanthionine and homolanthionine in cells cultured at 15 nmol/L pyridoxal were 50% lower (P < 0.002) and 47% lower (P < 0.0255), respectively, than observed in cells cultured at 1800 nmol/L pyridoxal. Extracellular homocysteine and cysteine were 58% and 46% higher, respectively, in severely deficient cells than in adequate cells (P < 0.002). Fractional synthesis rates of lanthionine (P < 0.01) and homolanthionine (P < 0.006) were lower at 15 and 56 nmol/L pyridoxal than at both higher pyridoxal concentrations. The rate of homocysteine remethylation and the fractional rate of homocysteine production from methionine were not affected by vitamin B-6 restriction. In vitro studies of cell lysates using direct measurement of H2S also had a reduced extent of H2S production in the 2 lower vitamin B-6 conditions.

CONCLUSION

In view of the physiologic roles of H2S, these results suggest a mechanism that may be involved in the association between human vitamin B-6 inadequacy and its effects on human health.

Hyperhomocysteinemia and cardiovascular disease: The nutritional perspectives

Several members of the vitamin B-complex family are known to participate in the normal metabolism of homocysteine (Hcy). Leaving aside the genetic determinants of hyperhomocysteinemia (HHC), the deficiencies of these vitamins can also result in HHC. The situation of sustained and long standing HHC is likely to be prevalent in population groups with low/average socio-economic status, geriatric population and alcohol abusers. If not corrected by supplementation, these population groups certainly are more vulnerable to develop atherosclerosis (AS) and subsequently, cardiovascular disease (CVD). Hyperhomocysteinemia per se and/or HHC-induced oxidative stress result(s) in chronic chemical endothelial injury/dysfunction, smooth muscle proliferation, prothrombotic state and oxidation of low density lipoproteins (LDL) leading to diverse cardiovascular complications. In the first decade of the new millennium, major research efforts would be directed towards understanding the basic mechanism of HHC-induced oxidative stress and the pathophysiology of HHC-induced CVD, culminating in the evolution of hitherto unknown therapeutic strategies such as nutriceuticals and oxidant-antidotes.

The nutrigenetics of hyperhomocysteinemia: quantitative proteomics reveals differences in the methionine cycle enzymes of gene-induced versus diet-induced hyperhomocysteinemia

Hyperhomocysteinemia has long been associated with atherosclerosis and thrombosis and is an independent risk factor for cardiovascular disease. Its causes include both genetic and environmental factors. Although homocysteine is produced in every cell as an intermediate of the methionine cycle, the liver contributes the major portion found in circulation, and fatty liver is a common finding in homocystinuric patients. To understand the spectrum of proteins and associated pathways affected by hyperhomocysteinemia, we analyzed the mouse liver proteome of gene-induced (cystathionine beta-synthase (CBS)) and diet-induced (high methionine) hyperhomocysteinemic mice using two-dimensional difference gel electrophoresis and Ingenuity Pathway Analysis. Nine proteins were identified whose expression was significantly changed by 2-fold (p < or = 0.05) as a result of genotype, 27 proteins were changed as a result of diet, and 14 proteins were changed in response to genotype and diet. Importantly, three enzymes of the methionine cycle were up-regulated. S-Adenosylhomocysteine hydrolase increased in response to genotype and/or diet, whereas glycine N-methyltransferase and betaine-homocysteine methyltransferase only increased in response to diet. The antioxidant proteins peroxiredoxins 1 and 2 increased in wild-type mice fed the high methionine diet but not in the CBS mutants, suggesting a dysregulation in the antioxidant capacity of those animals. Furthermore, thioredoxin 1 decreased in both wild-type and CBS mutants on the diet but not in the mutants fed a control diet. Several urea cycle proteins increased in both diet groups; however, arginase 1 decreased in the CBS(+/-) mice fed the control diet. Pathway analysis identified the retinoid X receptor signaling pathway as the top ranked network associated with the CBS(+/-) genotype, whereas xenobiotic metabolism and the NRF2-mediated oxidative stress response were associated with the high methionine diet. Our results show that hyperhomocysteinemia, whether caused by a genetic mutation or diet, alters the abundance of several liver proteins involved in homocysteine/methionine metabolism, the urea cycle, and antioxidant defense.

Public health significance of elevated homocysteine

Homocysteine is a sulfur amino acid whose metabolism stands at the intersection of two pathways: remethylation, which requires folic acid and vitamin B12 coenzymes; and transsulfuration, which requires pyridoxal-5'-phosphate, the vitamin B6 coenzyme. Data from a number of laboratories suggest that mild elevations of homocysteine in plasma are a risk factor for occlusive vascular disease. In the Framingham studies, we have shown that plasma homocysteine concentration is inversely related to the intake and plasma levels of folate and vitamin B6 as well as vitamin B12 plasma levels. Almost two-thirds of the prevalence of high homocysteine is attributable to low vitamin status or intake. Elevated homocysteine concentrations in plasma are a risk factor for prevalence of extracranial carotid-artery stenosis > or = 25% in both men and women. Prospectively elevated plasma homocysteine is associated with increased total and cardiovascular mortality, increased incidence of stroke, increased incidence of dementia and Alzheimer's disease, increased incidence of bone fracture, and higher prevalence of chronic heart failure. It was also shown that elevated plasma homocysteine is a risk factor for preeclampsia and maybe neural tube defects (NTD). This multitude of relationships between elevated plasma homocysteine and diseases that afflict the elderly, pregnant women, and the embryo points to the existence ofa common denominator which may be responsible for these diseases. Whether this denominator is homocysteine itself or homocysteine is merely a marker, remains to be determined.

Hyperhomocysteinemia: association with renal transsulfuration and redox signaling in rats

Despite substantial evidence indicating the association of hyperhomocysteinemia (hHcys) and end-stage renal disease (ESRD), the pathogenic role of increased plasma homocysteine (Hcys) levels in the progression of ESRD remains unclear. This review will briefly summarize recent findings regarding the role of hHcys in the development of glomerulosclerosis, the association of hHcys with reduced renal transsulfuration and Hcys-induced changes of redox signaling in the development of glomerulosclerosis in rat kidneys. Based on these results, it is concluded that hHcys is implicated in glomerular sclerosis in hypertension, elevated plasma Hcys in Dahl salt-sensitive (SS) hypertensive rats is due to downregulation of cystathionine beta-synthase (CBS) expression and consequent abnormality of transsulfuration in the kidney compared with normotensive rats. Hcys-induced superoxide (O(2)(*-)) production by activation of NADPH oxidase as a triggering mechanism contributes to the effects of Hcys on the homeostasis of extracellular matrix and consequent sclerosis in the glomeruli, and NADPH oxidase activation by Hcys is associated with enhanced Rac GTPase activity.

Homocysteine and Parkinson's disease: a dangerous liaison?

Homocysteine, a sulphur-containing amino acid formed by demethylation of methionine, is involved in numerous processes of methyl group transfer, all playing pivotal roles in the biochemistry of the human body. Increased levels of plasma homocysteine (hyperhomocysteinemia) - which may result from a deficiency of folate, vitamin B6 or B12 or mutations in enzymes regulating the catabolism of homocysteine - are associated with a wide range of clinical manifestations, mostly affecting the central nervous system (e.g., mental retardation, cerebral atrophy and epileptic seizures). Recent evidence suggests that changes in the metabolic fate of homocysteine, leading to hyperhomocysteinemia, may also play a role in the pathophysiology of neurodegenerative disorders, particularly Parkinson's disease (PD). The nervous system might be particularly sensitive to homocysteine, due to the excitotoxic-like properties of the amino acid. However, experimental findings have shown that homocysteine does not seem to posses direct, cytotoxic activity, while the amino acid has proven able to synergize with more specific neurotoxic insults. Hyperhomocysteinemia has been repeatedly reported in PD patients; the increase, however, seems mostly related to the methylated catabolism of l-Dopa, the main pharmacological treatment of PD. Therefore, hyperhomocysteinemia may not be specific to movement disorders or other neurological diseases, the condition being, in fact, rather the result of the combinations of different factors, mainly metabolic, but also genetic and pharmacological, intervening in the neurodegenerative process.

Homocysteine Down-regulates Cellular Glutathione Peroxidase (GPx1) by Decreasing Translation

Hyperhomocysteinemia contributes to vascular dysfunction and an increase in the risk of cardiovascular disease. An elevated level of homocysteine in vivo and in cell culture systems results in a decrease in the activity of cellular glutathione peroxidase (GPx1), an intracellular antioxidant enzyme that reduces hydrogen peroxide and lipid peroxides. In this study, we show that homocysteine interferes with GPx1 protein expression without affecting transcript levels. Expression of the selenocysteine (SEC)-containing GPx1 protein requires special translational cofactors to "read-through" a UGA-stop codon that specifies SEC incorporation at the active site of the enzyme. These factors include a selenocysteine incorporation sequence (SECIS) in the 3'-untranslated region of the GPx1 mRNA and cofactors involved in the biosynthesis and translational insertion of SEC. To monitor SEC incorporation, we used a reporter gene system that has a UGA codon within the protein-coding region of the luciferase mRNA. Addition of either the GPx1 or GPx3 SECIS element in the 3'-untranslated region of the luciferase gene stimulated read-through by 6-11-fold in selenium-replete cells; absence of selenium prevented translation. To alter cellular homocysteine production, we used methionine in the presence of aminopterin, a folate antagonist, co-administered with hypoxanthine and thymidine (HAT/Met). This treatment increased homocysteine levels in the media by 30% (p < 0.01) and decreased GPx1 enzyme activity by 45% (p = 0.0028). HAT/Met treatment decreased selenium-mediated read-through significantly (p < 0.001) in luciferase constructs containing the GPx1 or GPx3 SECIS element; most importantly, the suppression of selenium-dependent read-through was similar whether an SV40 promoter or the GPx1 promoter was used to drive transcription of the SECIS-containing constructs. Furthermore, HAT/Met had no effect on steady-state GPx1 mRNA levels but decreased GPx1 protein levels, suggesting that this effect is not transcriptionally mediated. These data support the conclusion that homocysteine decreases GPx1 activity by altering the translational mechanism essential for the synthesis of this selenocysteine-containing protein.

Downstream effects on human low density lipoprotein of homocysteine exported from endothelial cells in an in vitro system

A model system is presented using human umbilical vein endothelial cells (HUVECs) to investigate the role of homocysteine (Hcy) in atherosclerosis. HUVECs are shown to export Hcy at a rate determined by the flux through the methionine/Hcy pathway. Additional methionine increases intracellular methionine, decreases intracellular folate, and increases Hcy export, whereas additional folate inhibits export. An inverse relationship exists between intracellular folate and Hcy export. Hcy export may be regulated by intracellular S-adenosyl methionine rather than by Hcy. Human LDLs exposed to HUVECs exporting Hcy undergo time-related lipid oxidation, a process inhibited by the thiol trap dithionitrobenzoate. This is likely to be related to the generation of hydroxyl radicals, which we show are associated with Hcy export. Although Hcy is the major oxidant, cysteine also contributes, as shown by the effect of glutamate. Finally, the LDL oxidized in this system showed a time-dependent increase in uptake by human macrophages, implying an upregulation of the scavenger receptor. These results suggest that continuous export of Hcy from endothelial cells contributes to the generation of extracellular hydroxyl radicals, with associated oxidative modification of LDL and incorporation into macrophages, a key step in atherosclerosis. Factors that regulate intracellular Hcy metabolism modulate these effects.

The S-adenosyl homocysteine hydrolase inhibitor 3-deaza-adenosine prevents oxidative damage and cognitive impairment following folate and vitamin E deprivation in a murine model of age-related, oxidative stress-induced neurodegeneration

Deficiencies in folate promote neurodegeneration and potentiate the influence of other risk factors for neurodegeneration. This is accomplished at least in part by increasing levels of the neurotoxin homocysteine (HC). The S-adenosyl homocysteine (SAH) hydrolase inhibitor 3-deaza-adenosine (DZA) prevents HC accumulation following folate deprivation. We tested the ability of dietary supplementation with DZA to counteract the deleterious influence of folate deprivation. Folate deficiency has previously been shown to potentiate the impact of apolipoprotein E (ApoE); ApoE-/- mice deprived of folate demonstrated increased oxidative damage in brain tissue and impaired cognitive performance as compared to normal mice or to ApoE-/- mice receiving folate. Herein, we demonstrate that dietary supplementation with DZA prevented both the increase in oxidative damage and impaired cognition characteristic of ApoE-/- mice following folate deprivation. These findings suggest that manipulation of the methionine cycle by DZA can counteract folate deficiency. Because folate deprivation, increased HC, and apolipoprotein E deficiency are all risk factors for Alzheimer's disease, these findings also underscore that DZA might be useful in a therapeutic approach to delay neurodegeneration in Alzheimer's disease.

Homocysteine metabolism in renal disease

Hyperhomocysteinemia, a new cardiovascular risk factor, occurs in 85-100% of patients with end-stage renal disease. The exact mechanism by which renal function is linked to plasma homocysteine has not been definitively established. There is reasonably good clinical evidence that hyperhomocysteinemia in itself does not cause renal insufficiency. Two, not mutually exclusive, hypotheses are that in renal failure: i) homocysteine disposal is impaired in the kidneys themselves and ii) extra-renal homocysteine metabolism is defective, possibly due to uremic toxins. Several methods have been applied to investigate kidney and whole-body sulfur amino acid metabolism in healthy subjects and in patients with different degrees of renal failure. Arteriovenous extraction studies have not found a significant homocysteine disposal in the human kidney. Methods to study whole-body homocysteine metabolism have included measurement of plasma metabolites, calculation of plasma homocysteine elimination after oral loading and the use of stable isotope techniques with methionine tracers. The results implicate a decreased homocysteine clearance instead of an increased production as the cause of hyperhomocysteinemia in renal failure, but the exact site of the impaired clearance remains controversial.

Attenuation of homocysteine-induced endothelial dysfunction by exercise training

Hyperhomocysteinemia is an independent risk factor for the development of cardiovascular disease. Exposure of endothelial cells to elevated levels of homocysteine (HCY) results in decreased availability of nitric oxide (NO) and impaired vascular function, both of which are early events in atherogenesis. Exercise training improves vascular function by increasing endothelial NO production secondary to an increase in the enzyme responsible for its synthesis, endothelial nitric oxide synthase (eNOS). We hypothesized that exercise training would increase endothelial NO production, which would attenuate the endothelial dysfunction associated with HCY exposure. Rats were randomly assigned to either sedentary (SED) or exercise (EX) groups. The exercise regimen consisted of treadmill running at 20-25 m/min, 15% grade, 30 min/day, 5 day/week for 6 weeks. Aortic rings obtained from SED and EX trained rats were incubated with 2 mM HCY for 120 min, then exposed to norepinephrine (NE 100 nM) to induce vasoconstriction. Once a stable contraction plateau was achieved, rings were exposed to increasing concentrations of the receptor-mediated endothelium-dependent vasodilator acetylcholine (ACh; 0.1, 1, 10, 100 nM). This procedure was repeated with the non-receptor-mediated endothelium-dependent vasodilator A-23187 (0.1, 1, 10, 100 nM), and the endothelium-independent vasodilator, NaNO(2) (0.1, 1, 10, 100 muM). In addition, eNOS protein content and eNOS enzyme activity were determined. Aortic rings obtained from exercise trained rats demonstrated significantly (P<0.05) greater relaxation to both ACh and A-23187 in comparison to aortic rings obtained from SED rats following exposure to HCY. Additionally, exercise training increased aortic eNOS protein content and activity. Our data demonstrate that exercise training improves endothelium-dependent vasorelaxation following HCY exposure and this may be due, at least in part, to elevated levels of eNOS protein and an increase in eNOS activity. These results suggest the possible role exercise may play in attenuating the endothelial dysfunction associated with hyperhomocysteinemia.

Homocysteine and methylmalonic acid in diagnosis and risk assessment from infancy to adolescence

The concentration of total homocysteine (tHcy) in serum and plasma is elevated in both folate and cobalamin deficiencies, whereas methylmalonic acid (MMA) in serum, plasma, or urine is a specific marker of cobalamin function. The combined measurement of both metabolites is useful for the diagnosis and follow-up of these deficiency states. In addition, tHcy is elevated under various pathologic states (eg, renal failure), and hyperhomocysteinemia is associated with an increased risk of cardiovascular disease, cognitive dysfunction, and adverse pregnancy outcomes. The diagnostic utility of tHcy and MMA concentrations as markers of folate and cobalamin deficiencies in healthy and diseased children has been documented. This article briefly summarizes the biochemical background of tHcy and MMA and the associations of tHcy and MMA with various disease states and focuses on novel data obtained in infants, children, and adolescents, with emphasis on cobalamin status in infants. The utility of tHcy and MMA as indicators of cobalamin and folate deficiencies in adults can be extended to infants and older children. Furthermore, as in adults, tHcy is related to unhealthy lifestyle factors and is a risk factor for vascular disease. High MMA concentrations in newborns, occasionally denoted as benign methylmalonic aciduria, may reflect impaired cobalamin function.

Role of abnormal methionine metabolism in alcoholic liver injury

Methionine catabolism occurs mostly in the liver through the formation of S-adenosylmethionine (SAM) in a reaction catalyzed by methionine adenosyltransferase (MAT). S-adenosylmethionine is the principal biologic methyl donor, a precursor for polyamines, and in liver, it is also a precursor for reduced glutathione (GSH). Liver-specific and non-liver-specific MAT are products of two different genes, MAT1A and MAT2A, respectively. Mature liver expresses MAT1A, whereas MAT2A is expressed in extrahepatic tissues and induced during liver growth and de-differentiation. The type of MAT expressed by the cell affects the steady-state SAM level, DNA methylation, and growth rate. This has been demonstrated further by using the MAT1A knockout mouse model in which hepatic SAM and GSH levels decrease, the liver becomes larger and more susceptible to injury, and steatohepatitis develops spontaneously. Altered methionine metabolism in alcoholic liver disease results in decreased transmethylation and transsulfuration, changes that may play important pathogenic roles. Major changes include a relative switch in MAT expression; decreased hepatic SAM, GSH, and DNA methylation levels; decreased homocysteine metabolism; and hyperhomocysteinemia. Consequences of hepatic DNA hypomethylation include increased expression of c-myc and DNA strand break accumulation. One possible consequence of hyperhomocysteinemia is increased fibrogenesis. Abnormal methionine metabolism may also occur in Kupffer cells, which express both forms of MAT. If SAM levels also decrease in these cells, this may contribute to the induction of tumor necrosis factor (TNF) expression and release. In summary, altered hepatic methionine metabolism can have serious consequences that affect not only hepatocytes, but also hepatic stellate and Kupffer cells. These changes can lead to impaired antioxidant defense, altered gene expression, promotion of fibrogenesis, and even hepatocarcinogenesis.

Vitamin supplementation normalizes total plasma homocysteine concentration but not plasma homocysteine redox status in patients with acute coronary syndromes and hyperhomocysteinemia

Despite the growing evidence that elevated total homocysteine (tHcy) in plasma is a cardiovascular risk factor, the mechanism underlying the vascular injury is still unknown. Studies are difficult due to the fact that little is known about the formation of different homocysteine species in vivo. In the present study we have investigated the different fractions of tHcy in 21 patients with acute coronary syndromes and elevated concentration of plasma tHcy. A subgroup of the patients (n=16) was investigated before and after a 3 months study period with or without vitamin supplementation (folic acid 5 mg, pyridoxine 40 mg and cyanocobalamin 1 mg once daily). A major finding is that these patients had a lowered ratio (0.95%) between the concentration of reduced homocysteine (HcyH) and tHcy compared to controls (1.38%). A low ratio HcyH/tHcy in plasma in combination with elevated plasma tHcy concentrations might reflect increased oxidative activity or decreased reducing capacity in plasma from the patients. Another main finding in the present study is that, although vitamin supplementation of these patients normalized plasma tHcy, the ratio between HcyH and tHcy did not normalize. Since substantial evidence indicates that progression of arteriosclerosis is related to enhanced oxidant activity, the premature vascular disease associated with increased plasma tHcy concentration might be due to increased oxidative activity and the elevated plasma tHcy concentration may only reflect the increased oxidative stress.

The kidney and homocysteine metabolism

Homocysteine (Hcy) is an intermediate of methionine metabolism that, at elevated levels, is an independent risk factor for vascular disease and atherothrombosis. Patients with renal disease, who exhibit unusually high rates of cardiovascular morbidity and death, tend to be hyperhomocysteinemic, particularly as renal function declines. This observation and the inverse relationship between Hcy levels and GFR implicate the kidney as an important participant in Hcy handling. The normal kidney plays a major role in plasma amino acid clearance and metabolism. The existence in the kidney of specific Hcy uptake mechanisms and Hcy-metabolizing enzymes suggests that this role extends to Hcy. Dietary protein intake may affect renal Hcy handling and should be considered when measuring Hcy plasma flux and renal clearance. The underlying cause of hyperhomocysteinemia in renal disease is not entirely understood but seems to involve reduced clearance of plasma Hcy. This reduction may be attributable to defective renal clearance and/or extrarenal clearance and metabolism, the latter possibly resulting from retained uremic inhibitory substances. Although the currently available evidence is not conclusive, it seems more likely that a reduction in renal Hcy clearance and metabolism is the cause of the hyperhomocysteinemic state. Efforts to resolve this important issue will advance the search for effective Hcy-lowering therapies in patients with renal disease.

Homocysteine-induced decrease in endothelin-1 production is initiated at the extracellular level and involves oxidative products

The increased cardiovascular risk associated with hyperhomocysteinemia has been partly related to homocysteine (Hcy)-induced endothelial cell dysfunction. However, the intra or extracellular starting point of the interaction between Hcy and endothelial cells, leading to cellular dysfunction, has not yet been identified. We investigated the effects of both intracellular and extracellular Hcy accumulation on endothelin-1 (ET-1) synthesis by cultured human endothelial cells. Incubation of cultures with methionine (1.0 mmol x L(-1)) for 2 h induced a slight increase in cellular Hcy content but no change in ET-1 production. Incubation of cells with Hcy (0.2 mmol x L(-1)) led to a significant fall in ET-1 generation, accompanied by a significant increase in cellular Hcy content. Addition of the amino-acid transport system L substrate 2-amino-2-norbornane carboxylic acid had no effect on the Hcy-induced decrease in ET-1 production but significantly inhibited the Hcy-induced increase in the cellular Hcy content. Incubation of cells with a lower Hcy concentration (0.05 mmol x L(-1)) also reduced ET-1 production without increasing the cellular Hcy content. Co-incubation with extracellular free-radical inhibitors (superoxide dismutase, catalase and mannitol) markedly reduced the effect of Hcy on ET-1 production. Thus, it is extracellular Hcy accumulation that triggers the decrease in ET-1 production by endothelial cells through oxidative products.

The importance of hyperhomocysteinemia as a risk factor for diseases: an overview

Hyperhomocysteinemia is the result of a disturbed methionine metabolism. It results from enzyme and/or vitamin deficiency. Epidemiological studies have proven, that hyperhomocysteinemia is a risk factor for atherosclerotic cardiovascular diseases, stroke, peripheral arterial occlusive disease and venous thrombosis. Conflicting results come from prospective studies. Trials which are now in progress may clarify the "causality" of high homocysteine concentrations and will assess the value of homocysteine-lowering therapy. The induction of the atherogenic process by hyperhomocysteinemia seems to be associated with an alteration of endothelial and smooth muscle cell function leading to an accelerated formation of reactive oxygen species. An increased endothelial expression of adhesion molecules will then lead to an enhanced deposition of oxidized LDL in the vessel wall with the formation of foam cells. Additionally, hyperhomocysteinemia interferes with the coagulation system and thus also has prothrombotic effects. There is a high prevalence of hyperhomocysteinemia as a sign of a vitamin deficiency in elderly subjects which strongly increases with age. Elderly people have a high frequency of vitamin B12 deficiency which can be diagnosed more reliably by the measurement of serum methylmalonic acid (MMA) level than by serum vitamin B12. Subjects following a strict vegetarian diet also have a high prevalence of hyperhomocysteinemia caused by functional vitamin B12 deficiency (increased MMA level). Last but not least, hyperhomocysteinemia is a factor in the pathogenesis of neural tube defects and pre-eclampsia. An early diagnosis of vitamin B12 deficiency is important for the prevention of neurological damages. Homocysteine should be measured in patients with a history of atherothrombotic vessel diseases, in patients with diabetes or hyperlipidemia, in renal patients, in obese subjects, in elderly people, in postmenopausal women, and in early pregnancy. A specific diagnosis of an underlying vitamin deficiency is important for adequate treatment. Individuals with homocysteine level >12 micromol/l should increase and/or supplement their dietary intake of vitamins.

Current concepts in the pathogenesis of alcoholic liver injury

Alcoholic liver disease (ALD) develops as a consequence of priming and sensitizing mechanisms rendered by cross-interactions of primary mechanistic factors and secondary risk factors. This concept, albeit not novel, is becoming widely accepted by the field, and more research is directed toward identifying and characterizing the interfaces of the cross-interactions to help understand individual predisposition to the disease. Another pivotal development is the beginning of cell type-specific research to elucidate specific contributions not only of hepatocytes, but also of hepatic macrophages, liver-associated lymphocytes, sinusoidal endothelial cells, and hepatic stellate cells to sensitizing and priming mechanisms. In particular, the critical role of hepatic macrophages has been highlighted and the priming mechanisms concerning this paracrine effect have been proposed. Glutathione depletion in hepatocyte mitochondria is considered the most important sensitizing mechanism. One of the contributing factors is decreased methionine metabolism. Remaining key questions include how altered methionine metabolism contribute to the pathogenesis of ALD; how cross-talk among nonparenchymal liver cells or between nonparenchymal cells and hepatocytes leads to ALD; how dysfunctional mitochondria determine the type of cell death in ALD; and what secondary factors are critical for the development of advanced ALD such as alcoholic hepatitis and cirrhosis.

Novel Approach for the Determination of the Redox Status of Homocysteine and Other Aminothiols in Plasma from Healthy Subjects and Patients with Ischemic Stroke

Background: Plasma “redox” status can be assessed by measurements of reduced (r)-, free (f)-, oxidized (ox)-, and protein-bound (b)-homocysteine (Hcy) plus the related aminothiols cysteine, cysteinylglycine (CysGly), and glutathione (GSH), but sample collection has been complex. The redox status has not been determined in ischemic stroke patients and may provide increased understanding of its role in pathogenesis. We wished to examine the feasibility of this measurement in samples collected in readily available acidic sodium citrate.

Methods: We measured aminothiols and their stability in stabilized protein-free filtrate using acidic sodium citrate (BioPool® StabilyteTM, pH 4.3) vs EDTA whole blood. Before analysis, plasma samples were also ultrafiltered to obtain a protein-free filtrate. The concentrations of total Hcy (tHcy), fHcy, and rHcy and their related aminothiols, cysteine, cysteinylglycine, and glutathione were simultaneously determined on acidic sodium-citrated blood using reversed-phase HPLC with fluorescence detection. Bound and oxidized aminothiols were calculated by difference using the concentrations of the total, free, and reduced fractions. Using this approach, we compared the redox status in newly diagnosed ischemic stroke patients (n = 20) and healthy age- and sex-matched subjects (n = 20).

Results: tHcy, tCys, tCysGly, and tGSH concentrations in whole blood with Stabilyte were stable for 8 h; the reduced fraction of each aminothiol was stable for 4 h. Recovery in the protein-free filtrate was 90–100% for all reduced thiols in acidified sodium-citrated blood. Patients with ischemic stroke had higher plasma tHcy, fHcy, bHcy, rHcy, and oxHcy (P &lt;0.0005) and higher plasma t-, f-, r-, and oxCys (P &lt;0.05). t-, b-, and rCysGly concentrations were lower in the stroke patients (P &lt;0.05), as were t-, b-, and oxGSH (P &lt;0.005).

Conclusions: Collection of blood in acidic sodium citrate (BioPool Stabilyte) permits the determination of the redox status of Hcy and its related aminothiols, which may add to the understanding of their relationship to the etiology of cerebrovascular disease.

Is there a role of cyclosporine A on total homocysteine export from human renal proximal tubular epithelial cells?

BACKGROUND

Immunosuppressive therapy may influence homocysteine metabolism in allograft recipients. We examined whether cyclosporine A influences the in vitro formation of homocysteine as determined by the measurement of total homocysteine (tHcy) concentrations in supernatants of human renal proximal tubule epithelial cells (hRPTEC), an important site of homocysteine metabolism.

METHODS

Cells were incubated with and without vitamins in the presence of low or high methionine concentrations at different cyclosporine A concentrations for 24, 48 and 72 hours (N = 7 for each experiment). The concentration of tHcy in culture supernatants was measured by a fluorescence polarization immunoassay. Data were analyzed by four-way ANOVA, three-way ANOVA and t tests.

RESULTS

The Hcy export from hRPTEC (tHcy in the culture supernatant) was 2.69 micromol/L during standard cell culture conditions at time point 24 hours and increased by 28.3% at 48 hours and by 44.6% at 72 hours. Comparisons of tHcy levels in culture supernatants over time by four way ANOVA showed that cyclosporine A at 200 or 800 ng/mL had no influence on tHcy export from hRPTEC (P = 0.67991). In contrast, the presence of vitamins in the medium (P = 0.000001), in vitro methionine loading (P < 0.000001), and prolonged incubation time (P < 0.000001) were associated with an increase of tHcy export from hRPTEC. Significant interactions in this analysis were "vitamins x methionine" (P = 0.001804), "vitamins x time" (P = 0.001478), "methionine x time" (P < 0.000001), and "vitamins x methionine x time" (P = 0.018128), pointing to a combined effect of vitamins in the presence of high methionine concentrations at the later time points.

CONCLUSION

Our study shows that hRPTEC export Hcy into the cell culture medium, an effect that is enhanced by in vitro methionine loading and modulated by the presence of vitamins. Cyclosporine A had no major influence on Hcy export from tubule cells. Therefore, our findings do not support the assumption that cyclosporine A elevates total homocysteine plasma levels in organ transplant patients.

Elevated plasma homocysteine levels in patients on isotretinoin therapy for cystic acne

BACKGROUND

The use of Isotretinoin (Iso) for cystic acne (CA) therapy includes marked side-effects such as dyslipidemia, increased liver enzymes, and reduction of biotinidase activity. Moreover, Homocysteine (Hcy), an amino acid, is metabolized in the liver requiring folate, vitamin B6, vitamin B12, and the activity of enzymes, i.e. cystathionine-beta-synthase. Increased blood levels of Hcy are associated with premature occlusive vascular disease.

OBJECTIVE

The aim of this study was the evaluation of Hcy levels and the responsible vitamins for its metabolism in patients with CA on Iso treatment.

METHODS AND RESULTS

Twenty-eight patients with CA were submitted to laboratory examinations before (Value 1) and after (Value 2) 45 days on Iso (0.5 mg/kg/24 h) therapy. Blood levels of Hcy and vitamin B6 were evaluated by HPLC methods, and folate and vitamin B12 using a commercial Kit. Hcy levels (Value 1 = 7.86 +/- 1.6 micromol/L; Value 2 = 13.65 +/- 3.3 micromol/L; P < 0.001) were statistically significantly increased in patients on treatment. Vitamins were unaltered, and lipids and liver enzymes increased. Significant correlation between Hcy levels, vitamins, and liver enzymes was found. Methionine loading tests performed in nine patient-volunteers showed an abnormal response post-treatment.

CONCLUSIONS

It is suggested that the elevated Hcy levels in patients after 45 days on Iso therapy could be due either to the 'inhibition' of cystathionine-beta-synthase by the drug and/or their liver dysfunction. Daily vitamin supplementation along with frequent evaluations of Hcy blood levels are recommended for the prevention of a premature occlusive vascular disease.

Higher export rate of homocysteine in a human endothelial cell line than in other human cell lines

Even mild hyperhomocysteinemia is associated with premature vascular disease. Despite the growing evidence that plasma homocysteine is a cardiovascular risk factor, the mechanism behind the vascular injuries is still unknown. Information about the metabolism of homocysteine is, therefore, essential for an understanding of its role in atherogenesis. In the present study we have, therefore, investigated the export mechanism of homocysteine. In HeLa cell lines the release of homocysteine was found to be a continuous process, which was increased in the presence of copper ions. High cell density led to a lowered release of homocysteine, probably due to a more extensive metabolism of the intracellular homocysteine. It was also found that HeLa cells were able to take up extracellularly released homocysteine and use it in the cellular metabolism. The ratio between intracellular homocysteine and the total amount of homocysteine is a measure of the ability of the cell to export the intracellularly produced homocysteine. The ratio also reflects the reuse of extracellular homocysteine. Under basal conditions, endothelial cells exported most of the intracellularly produced homocysteine and exhibited a very low concentration of homocysteine intracellularly, low reusage of exported homocysteine and consequently a low ratio in comparison with HeLa and hepatoma cell lines. After addition of homocysteine, all cell lines exhibited similar ratios. Thus, the intracellular homocysteine concentration in endothelial cells is more influenced by the extracellular concentration of homocysteine than is the intracellular concentration in HeLa and hepatoma cells. It may be speculated that this phenomenon could be associated with an increased sensitivity of endothelial cells to homocysteine and explain the association between hyperhomocysteinemia and vascular disease.

Urinary excretion measurement of cysteine and homocysteine in the form of their S-pyridinium derivatives by high-performance liquid chromatography with ultraviolet detection

Several human diseases, in particular metabolic disorders, often lead to the accumulation of characteristic metabolites in plasma, urine and cells. The selected diseases of this type include cystinuria and homocystinuria. In the typical laboratory diagnosis of these two diseases, a positive nitroprusside test is followed by quantitative analysis of urine cysteine and homocysteine in order to differentiate between cystinuria and homocystinuria. A sensitive and reproducible assay for total urine cysteine and homocysteine has been developed. The essential steps in the assay include conversion of disulphides to free thiols with tributylphosphine, conjugation of the thiols with 2-chloro-1-methyl pyridinium iodide, separation of S-pyridinium derivatives of cysteine and homocysteine from other endogenous urine thiol derivatives by reversed-phase high-performance liquid chromatography, and detection and quantitation by spectrophotometry. The method has a sensitivity of 4 pmol and is reproducible, intra- and inter-day coefficients of variation are from 1.37 to 4.14% and from 2.38 to 5.01%, respectively. The mean concentration of total urine cysteine and homocysteine in healthy donors (7 men and 7 women) were for women. 92.0 +/- 45.8 and 16.4 +/- 4.8 respectively, and for men 120.9 +/- 46.6 and 21.5 +/- 7.4 nmol/ml, respectively. Total urine homocysteine represents approximately 17.7% of cysteine in the urine of normal individuals.

Homocysteine and vascular dysfunction

Elevated plasma levels of homocysteine and disulfide adducts of homocysteine (collectively termed "homocyst(e)ine") are associated with increased risk of thrombotic and atherosclerotic vascular disease. It is still not evident, however, whether moderately elevated plasma homocyst(e)ine concentration per se is a cause, or rather just a marker for an associated condition that may predispose to development of vascular disease or its complications. This distinction has important clinical consequences, since dietary intervention to lower plasma homocyst(e)ine has been proposed as a global strategy to decrease the prevalence of vascular disease. Studies of cultured cells in vitro have led to the hypothesis that homocysteine may predispose to vascular disease by altering the normally antithrombotic and vasoprotective phenotype of vascular endothelium, perhaps through a mechanism that involves generation of peroxides and other reactive oxygen species. Recent findings in animal and human models of moderate hyperhomocyst(e)inemia provide support for some aspects of this hypothesis. Endothelial dysfunction in hyperhomocyst(e)inemia may contribute to development of atherosclerosis and predispose to complications such as thrombosis and vasospasm. Important questions to be addressed in future investigations include the relative importance of homocysteine versus associated conditions (such as folate deficiency) in the etiology of vascular dysfunction, the role of homocysteine-induced oxidant stress, and the potential benefits of lowering plasma homocyst(e)ine levels through dietary supplementation with B vitamins.

Response of the methionine synthase system to short-term culture with homocysteine and nitrous oxide and its relation to methionine dependence

We compared the metabolic response of a methionine(Met)-dependent (P60) human glioma cell line with that of a Met-independent variant (P60H) when cultured in a homocysteine (Hcy) medium and exposed to N2O. In Hcy medium (without Met), remethylation of Hcy in P60H cells was enhanced and supported growth, whereas remethylation was low in P60 cells, which failed to thrive under these conditions. Both cell types seemed to contain adequate amounts of folates and total cobalamin (Cbl). P60 cells showed increased total and methylcobalamin (CH3Cbl) content after the shift to a Hcy medium, but the high, stable level of CH3Cbl detected in P60H cells was not attained. Further metabolic differences were induced by N2O exposure, which markedly reduced Met-synthase activity in cell-free extracts in both cell lines and completely blocked intact-cell Hcy remethylation in P60, whereas Hcy remethylation was only partly inhibited in P60H cells cultured in Met medium. The residual Hcy remethylation in P60H cells may be related to only a moderate depletion of CH3Cbl. The resulting high CH3Cbl level relative to Met-synthase activity during N2O exposure was even higher in Hcy medium. These findings in P60H cells probably reflect increased provision of Cbl to support Hcy remethylation under metabolic strain. The inability of P60 to furnish CH3Cbl to the enzyme may explain both the Met-dependent phenotype and the increased sensitivity of Hcy remethylation to N2O exposure in these cells.

Homocysteine and coronary atherosclerosis

Homocysteine is increasingly recognized as a risk factor for coronary artery disease. An understanding of its metabolism and of the importance of vitamins B6 and B12 and folate as well as enzyme levels in its regulation will aid the development of therapeutic strategies that, by lowering circulating concentrations, may also lower risk. Possible mechanisms by which elevated homocysteine levels lead to the development and progression of vascular disease include effects on platelets, clotting factors and endothelium. This review presents the clinical and basic scientific evidence supporting the risk and mechanisms of vascular disease associated with elevated homocysteine concentrations as well as the results of preliminary therapeutic trials.

Is an elevated circulating maternal homocysteine concentration a risk factor for neural tube defects?

The mechanism by which folic acid supplementation reduces neural tube defect (NTD) incidence is unknown. Recent evidence suggests that mothers with NTD neonates have higher circulating homocysteine concentrations compared with controls. This presumably indicates impaired homocysteine remethylation, resulting in a possible methionine shortage at a crucial stage of fetal development.

The effect of blood sample aging and food consumption on plasma total homocysteine levels

The stability of homocysteine in whole blood and plasma was investigated. Total homocysteine concentrations in whole blood increased rapidly to values in excess of 180% of the basal concentration if whole blood was left at ambient temperature. Sodium fluoride partially inhibited homocysteine accumulation, while refrigeration inhibited homocysteine accumulation for at least 4 h. Since intracellular concentrations of homocysteine were low, the results indicate continued metabolism of L-methionine to homocysteine after the blood sample had been obtained. In contrast to whole blood, homocysteine was stable in plasma, even at room temperature. Food consumption (normal breakfast) resulted in significantly lower plasma homocysteine concentrations, which returned to pre-prandial concentrations 8 h later. The results indicate that both blood sampling and food intake should be rigorously standardized in epidemiological studies to elucidate the possible role of elevated circulating homocysteine concentrations in premature vascular disease.