A large-scale analysis of targeted metabolomics data from heterogeneous biological samples provides insights into metabolite dynamics

INTRODUCTION

We previously developed a tandem mass spectrometry-based label-free targeted metabolomics analysis framework coupled to two distinct chromatographic methods, reversed-phase liquid chromatography (RPLC) and hydrophilic interaction liquid chromatography (HILIC), with dynamic multiple reaction monitoring (dMRM) for simultaneous detection of over 200 metabolites to study core metabolic pathways.

OBJECTIVES

We aim to analyze a large-scale heterogeneous data compendium generated from our LC-MS/MS platform with both RPLC and HILIC methods to systematically assess measurement quality in biological replicate groups and to investigate metabolite abundance changes and patterns across different biological conditions.

METHODS

Our metabolomics framework was applied in a wide range of experimental systems including cancer cell lines, tumors, extracellular media, primary cells, immune cells, organoids, organs (e.g. pancreata), tissues, and sera from human and mice. We also developed computational and statistical analysis pipelines, which include hierarchical clustering, replicate-group CV analysis, correlation analysis, and case-control paired analysis.

RESULTS

We generated a compendium of 42 heterogeneous deidentified datasets with 635 samples using both RPLC and HILIC methods. There exist metabolite signatures that correspond to various phenotypes of the heterogeneous datasets, involved in several metabolic pathways. The RPLC method shows overall better reproducibility than the HILIC method for most metabolites including polar amino acids. Correlation analysis reveals high confidence metabolites irrespective of experimental systems such as methionine, phenylalanine, and taurine. We also identify homocystine, reduced glutathione, and phosphoenolpyruvic acid as highly dynamic metabolites across all case-control paired samples.

CONCLUSIONS

Our study is expected to serve as a resource and a reference point for a systematic analysis of label-free LC-MS/MS targeted metabolomics data in both RPLC and HILIC methods with dMRM.

Experimental hyperhomocysteinaemia: differences in tissue metabolites between homocystine and methionine feeding in a rat model

AIM

Hyperhomocysteinaemia, diagnosed by serum levels, is regarded as an independent risk indicator for cardiovascular events and is associated with various diseases. The pathomechanisms seem to be partly due to concentrations of homocysteine metabolites and their effect on the cellular transmethylation processes.

METHODS

We compared two common models for experimental hyperhomocysteinaemia - high methionine diet and homocystine-enriched diet - regarding their effects on tissue concentrations of homocysteine metabolites.

RESULTS

Both diets induced hyperhomocysteinaemia without affecting renal function or vitamine status. However, the tissue contents of homocysteine and its precursors S-adenosyl-homocysteine (SAH) and S-adenosyl-methionine exhibited major differences between both models. Transmethylation potential was elevated 1.7-fold in liver of rats fed the methionine diet, whereas it was unaltered after homocystine-enriched diet. Kidneys of rats fed the methionine diet did not show any alterations in tissue content of homocysteine and its precursors, whereas in the homocystine group homocysteine and the transmethylation inhibitor SAH were elevated from 23.1 +/- 10.4 to 78.0 +/- 26.0 nmol g(-1) and from 106 +/- 4 to 170 +/- 22 nmol g(-1) respectively. Homocysteine tissue content was elevated in the homocystine, but not in the methionine group.

CONCLUSIONS

Alterations to homocysteine metabolism are distinct in both models. These findings may explain divergent results, which have been published for these models of hyperhomocysteinaemia and which have resulted in controversial discussions in the past.

Dynamic regulation of MTHFR mRNA expression and C677T genotype modulate mortality in coronary artery disease patients after revascularization

INTRODUCTION

A large body of evidence links plasma homocysteine (Hcy) concentrations and cardiovascular disease. A common MTHFR polymorphism (C677T) leads to a variant with reduced activity and associated with increased Hcy levels. Coronary surgery precipitates a significant and sustained increase in the blood concentrations of Hcy and elevated levels of plasma Hcy have been associated to saphenous vein (SV) graft disease after CABG. However, the effects of MTHFR genotypes in the incidence of cardiovascular events after CABG have not been investigated prospectively. Here, we investigate whether MTHFR gene variants are associated with an increased cardiovascular risk in individuals submitted to CABG. We also propose a molecular mechanism to explain our findings.

METHODS

We performed MTHFR C677T genotypes in 558 patients with two or three vessel-disease and normal left ventricular function prospectively followed in the MASS II Trial, a randomized study to compare treatments for multivessel CAD and preserved left ventricle function. Follow-up time was 5 years. Survival curves were calculated with the Kaplan-Meier method, and evaluated with the log-rank statistic. We assessed the relationship between baseline variables and the composite end-point of death, myocardial infarction and refractory angina using a Cox proportional hazards survival model. Finally, using an ex-vivo organ culture we have reproduced the arterialization of SV implants by culturing human SV either under venous hemodynamic condition (flow: 5 mL/min; no pressure) or arterial hemodynamic condition (flow: 50 mL/min; pressure: 80 mm Hg) for 1 day. MTHFR gene expression was quantified by real time RT-PCR in 15 SV from different individuals in both experimental conditions.

RESULTS

There were no significant differences among individuals within each genotype group for baseline clinical characteristics. A statistically significant association between the TT genotype, associated with increased serum levels of Hcy, and cardiovascular mortality after 5 years was verified (p=0.007) in individuals submitted to CABG surgery. In addition, MTHFR TT genotype was still significantly associated with a 4.4 fold increased risk in cardiovascular outcomes (p=0.01) even after adjustment of a Cox multivariate model for age, sex, hypertension, diabetes, LDL, HDL, triglycerides, and number of diseased vessels in this population. Finally, a significant reduction in MTHFR gene expression was demonstrated in human SV when submitted to an arterial hemodynamic condition (p=0.02).

CONCLUSIONS

There is a dynamic regulation of MTHFR gene expression during the arterialization process of human saphenous vein grafts resulting in lower levels of gene expression when in an arterial hemodynamic condition. In addition, the C677T MTHFR functional variant is associated with a worse outcome in individuals submitted to CABG. Taken together, these data suggest an important role of Hcy metabolism in individuals after CABG.

The organotellurium compound ammonium trichloro(dioxoethylene-o,o')tellurate reacts with homocysteine to form homocystine and decreases homocysteine levels in hyperhomocysteinemic mice

Ammonium trichloro(dioxoethylene-o,o')tellurate (AS101) is an organotellurium compound with pleiotropic functions that has been associated with antitumoral, immunomodulatory and antineurodegenerative activities. Tellurium compounds with a +4 oxidation state, such as AS101, react uniquely with thiols, forming disulfide molecules. In light of this, we tested whether AS101 can react with the amino acid homocysteine both in vitro and in vivo. AS101 conferred protection against homocysteine-induced apoptosis of HL-60 cells. The protective mechanism of AS101 against homocysteine toxicity was directly mediated by its chemical reactivity, whereby AS101 reacted with homocysteine to form homocystine, the less toxic disulfide form of homocysteine. Moreover, AS101 was shown here to reduce the levels of total homocysteine in an in vivo model of hyperhomocysteinemia. As a result, AS101 also prevented sperm cells from undergoing homocysteine-induced DNA fragmentation. Taken together, our results suggest that the organotellurium compound AS101 may be of clinical value in reducing total circulatory homocysteine levels.

Quantification of intracellular homocysteine by stable isotope dilution liquid chromatography/tandem mass spectrometry

A precise and accurate stable isotope dilution liquid chromatography/tandem mass spectrometry method for the analysis of intracellular homocysteine has been developed. An internal standard, [(2)H(8)]-homocystine, was added to cell pellets from EA.hy 926 cells grown in culture under low and high folate concentrations. D,L-dithiothreitol was used to reduce cellular homocystine to homocysteine. Cellular proteins were precipitated by the addition of formic acid in acetonitrile. After centrifugation, a portion of the supernatant was analyzed by liquid chromatography/tandem mass spectrometry. Using a Supelcosil cyano column with an Applied Biosystems API 4000 triple quadrupole mass spectrometer, the SRM transitions for homocysteine (m/z 136 to m/z 90) and [(2)H(4)]-homocysteine (m/z 140 to m/z 94) were monitored. The method was validated by conducting five replicate analyses on three different days at four different concentrations (concentrations at the lower limit of quantitation and expected lower quartile, mid-range and upper quartile). The limit of detection was 2 ng/10(6) EA.hy 926 cells. Using this method, the intracellular homocysteine concentration in EA.hy 926 cells ranged from 10 to 36 ng/10(6) cells.

Mechanisms of homocysteine toxicity in humans

Homocysteine, a non-protein amino acid, is an important risk factor for ischemic heart disease and stroke in humans. This review provides an overview of homocysteine influence on endothelium function as well as on protein metabolism with a special respect to posttranslational modification of protein with homocysteine thiolactone. Homocysteine is a pro-thrombotic factor, vasodilation impairing agent, pro-inflammatory factor and endoplasmatic reticulum-stress inducer. Incorporation of Hcy into protein via disulfide or amide linkages (S-homocysteinylation or N-homocysteinylation) affects protein structure and function. Protein N-homocysteinylation causes cellular toxicity and elicits autoimmune response, which may contribute to atherogenesis.

Oxidative DNA damage and level of thiols as related to polymorphisms of MTHFR, MTR, MTHFD1 in Alzheimer's and Parkinson's diseases

Neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD), are accompanied by increased levels of 8-oxo-2'-deoxyguanosine (8-oxo2dG) and alterations in levels of homocysteine (Hcy), methionine (Met), and cysteine (Cys). Hcy may undergo remethylation due to involvement of MTHFR, MTR and MTHFD1 proteins. Present studies are aimed at determination of 8-oxo2dG, Hcy, Met, and Cys in AD and PD patients as well as in control groups, using HPLC/EC/UV, as well as estimation, by restriction analysis, frequency of following gene polymorphisms: MTHFR (C677T, A1298C, G1793A), MTHFD1 (G1958A), and MTR (A2756G). In AD there were significant differences of the levels of only Cys (GG, MTHFR, G1793A) and Met/Hcy (AA, MTHFD1, G1958A) whereas in PD there were more significant differences of the levels of thiols: Hcy [MTHFR: CT (C677T) and GG (G1793A); MTR, AG (A2756G)], Met [MTR, AA (A2756G)], Cys [MTR, AG (A2756G)], and Met/Hcy [MTHFR: CC, CT (C677T) and AA (A1298C), and GG (G1793A); MTHFD1 AA(G1958A); MTR AA(A2756G)]. Significant differences in the levels of Cys/Hcy, MTHFD1 GA (G1958) were varied between AD and PD groups. The results indicate that of the enzymes studied only polymorphisms of folate-dependent enzyme MTHFD1 have pointed to significant differences in intensity of turnover of circulating thiols between AD and PD patients.

Coronary calcification, homocysteine, C-reactive protein and the metabolic syndrome in Type 2 diabetes: the Prospective Evaluation of Diabetic Ischaemic Heart Disease by Coronary Tomography (PREDICT) Study

AIMS

The PREDICT Study aims to determine: (i) the association between cardiovascular risk factors and coronary artery calcification score (CACS) obtained by electron beam tomography and (ii) the predictive value of CACS for coronary heart disease (CHD) events in Type 2 diabetes.

METHODS

Having previously reported relationships between CACS and conventional risk factors, we have now studied the novel risk factors, plasma high-sensitivity C-reactive protein (CRP) and homocysteine, insulin resistance, serum apoprotein A1 and B concentrations, the serum triglyceride/high-density lipoprotein cholesterol ratio and metabolic syndrome (International Diabetes Federation definition) in 573 subjects of the PREDICT Type 2 diabetes cohort.

RESULTS

In univariate analyses, the only significant positive novel correlate of CACS was homocysteine (P = 0.0004). CRP was increased in those with detectable calcification, but decreased with increasing calcification score (P = 0.006). In a multivariate model that included all significant univariate correlates, CACS was independently associated with age (P < 0.0001), waist-hip ratio (P < 0.02), male gender (P < 0.05) and duration of diabetes (P < 0.05), but the association with homocysteine was no longer significant. The negative association between CACS and CRP remained in multivariate analysis, and was independent of statin use.

CONCLUSIONS

Age was the major factor influencing CACS in Type 2 diabetes, with weaker contributions from waist hip-ratio and duration of diabetes. Other novel cardiovascular risk factors appear to have little positive effect.

Conversion from cognitive health to mild cognitive impairment and Alzheimer's disease: prediction by plasma amyloid beta 42, medial temporal lobe atrophy and homocysteine

The changes of plasma amyloid beta (Abeta42) protein, homocysteine and medial temporal lobe atrophy (MTA) were studied by the transition from cognitive health to mild cognitive impairment (MCI) and to Alzheimer's disease (AD) in a prospective cohort of individuals aged 75 years. MTA but not plasma Abeta42 measured at baseline predicted which persons remained cognitively healthy (CH) and who developed AD 2.5 years later. The increase of plasma Abeta42 over time significantly distinguished between persons who remained CH on the one hand and MCI converters and AD converters out of cognitive health on the other (CH-to-MCI and CH-to-AD converters). Although both groups showed similar increase of Abeta42 levels, CH-to-AD converters had a higher increase of homocysteine compared to CH-to-MCI converters or to persons remaining CH. In comparison to all cognitive subgroups, the AD converters from MCI at baseline showed the smallest increase of Abeta42 levels and rather no increase of homocysteine. In logistic regression analysis, the increase of plasma Abeta42 but not change of MTA significantly predicted the conversion from CH to MCI, and changes of MTA and homocysteine but not of plasma Abeta42 predicted the conversion from CH to AD. The increase of plasma Abeta42 correctly allocated CH-to-MCI and CH-to-AD converters with low (63%) specificity (for both) and low (60%) sensitivity (54% for AD group). These results indicate that (1) plasma Abeta42 alone is not suitable as a biomarker for AD, (2) in the course of cognitive deterioration of the AD-type the increase of plasma Abeta42 seems to be an initial event, (3) similar to cerebrospinal fluid, changes of plasma Abeta42 may reflect the transition from cognitive health to AD, and (4) whether persons with MCI develop AD may depend on an accumulation of further toxic metabolites such as homocysteine.

Homocysteine accumulation causes a defect in purine biosynthesis: further characterization of Schizosaccharomyces pombe methionine auxotrophs

Methionine synthase (EC2.1.1.14) catalyses the final step in methionine synthesis, i.e. methylation of homocysteine. A search of the Schizosaccharomyces pombe genomic database revealed a gene designated SPAC9.09, encoding a protein with significant homology to methionine synthase. Disruption of SPAC9.09 caused methionine auxotrophy, and thus the gene was identified as a methionine synthase and designated met26. The met26 mutant was found to exhibit a remarkable growth defect in the absence of adenine even in medium supplemented with methionine. This phenotype was not observed in other methionine auxotrophs. In the budding yeast Saccharomyces cerevisiae, which has been reported to utilize homocysteine in cysteine synthesis, lack of a functional methionine synthase did not cause a requirement for adenine. The introduction of genes from Sac. cerevisiae constituting the cystathionine pathway (CYS4 and CYS3) into Sch. pombe Deltamet26 cells restored growth in the absence of adenine. HPLC analysis showed that total homocysteine content in Deltamet26 cells was higher than in other methionine auxotrophs and that introduction of the Sac. cerevisiae cystathionine pathway decreased total homocysteine levels. These data demonstrate that accumulation of homocysteine causes a defect in purine biosynthesis in the met26 mutant.

Accumulation of homolanthionine and activation of a novel pathway for isoleucine biosynthesis in Corynebacterium glutamicum McbR deletion strains

In the present work, the metabolic consequences of the deletion of the methionine and cysteine biosynthesis repressor protein (McbR) in Corynebacterium glutamicum, which releases almost all enzymes of methionine biosynthesis and sulfate assimilation from transcriptional regulation (D. A. Rey, A. Pühler, and J. Kalinowski, J. Biotechnol. 103:51-65, 2003), were studied. C. glutamicum ATCC 13032 DeltamcbR showed no overproduction of methionine. Metabolome analysis revealed drastic accumulation of a single metabolite, which was not present in the wild type. It was identified by isotopic labeling studies and gas chromatography/mass spectrometry as L-homolanthionine {S-[(3S)-3-amino-3-carboxypropyl]-L-homocysteine}. The accumulation of homolanthionine to an intracellular concentration of 130 mM in the DeltamcbR strain was accompanied by an elevated intracellular homocysteine level. It was shown that cystathionine-gamma-synthase (MetB) produced homolanthionine as a side reaction. MetB showed higher substrate affinity for cysteine (Km = 260 microM) than for homocysteine (Km = 540 microM). The cell is able to cleave homolanthionine at low rates via cystathionine-beta-lyase (MetC). This cleavage opens a novel threonine-independent pathway for isoleucine biosynthesis via 2-oxobutanoate formed by MetC. In fact, the deletion mutant exhibited an increased intracellular isoleucine level. Metabolic flux analysis of C. glutamicum DeltamcbR revealed that only 24% of the O-acetylhomoserine at the entry of the methionine pathway is utilized for methionine biosynthesis; the dominating fraction is either stored as homolanthionine or redirected towards the formation of isoleucine. Deletion of metB completely prevents homolanthionine accumulation, which is regarded as an important step in the development of C. glutamicum strains for biotechnological methionine production.

Schizophrenia and single-carbon metabolism

Schizophrenic patients generally appear to have a disturbed single-carbon metabolism. Methionine and homocysteine are intermediary metabolites in this metabolic system. In a case-control study of the cerebrospinal fluid, a majority of the patients had elevated methionine and a smaller subgroup had elevated homocysteine. Elevated homocysteine is often explained by folate dependency due to mutations in the gene for methylenetetrahydrofolate reductase (MTHFR). A most encouraging feature of single-carbon metabolism is its potential modification by natural means, such as B-vitamins and antioxidants. The findings point to a new area of schizophrenia research: the role of nutrients and antioxidants for rational prevention and treatment.

The central nervous system in animal models of hyperhomocysteinemia

Growing epidemiological evidence of associations between mildly elevated plasma homocysteine with age-related cognitive impairment, neurodegenerative and cerebrovascular disease has stimulated interest in the role of homocysteine in neurological and neuropsychiatric disorders. Homocysteine is an intermediate in the folate, vitamin B12 and B6 dependent pathways of one-carbon and sulfur amino acid metabolism. Impairments of these pathways may cause CNS dysfunction by promoting the intracellular generation of homocysteine, which is postulated to have vasotoxic and neurotoxic properties. It might also inhibit the methylation of myelin basic protein and membrane phospholipids, or disrupt biogenic amine metabolism and many other vital CNS reactions. However, it is unclear which, if any, of these putative mechanisms underlies the epidemiological associations. Genetic mouse models of hyperhomocysteinemia suggest that the primary metabolic disturbances rather than homocysteine per se may be important in determining neurological outcomes. However, severe and early developmental abnormalities in these mice limit their usefulness for understanding the relation of hyperhomocysteinemia to adult CNS disorders. Pharmacologic and dietary studies on homocysteine in rodents have reported heightened neuronal sensitivity to neurotoxic insults, neurochemical abnormalities and cerebrovascular dysfunction. Such studies are consistent with a causal relationship, but they fail to distinguish between effects that might result from a dietary imbalance and those that might be caused by homocysteine per se. Future work should be directed towards refining these models in order to distinguish between the effects of homocysteine and its determinants on neurological and behavioral outcomes that represent different CNS disorders.

Homocysteine and schizophrenia: from prenatal to adult life

Homocysteine is becoming increasingly recognized as an important substance in the pathogenesis and pathophysiology of schizophrenia. In this review, we first present background information supporting a role for homocysteine in schizophrenia. We then discuss our work on the role of hyperhomocystinemia during adulthood and risk of schizophrenia, and present preliminary evidence on a potential relationship between prenatal homocysteine and schizophrenia. Finally, we discuss the implications of these findings for future work on nutritional etiologies of schizophrenia.

Cardiovascular risk in peritoneal dialysis

All patients with CKD have multiple risk factors for CVD and CAD in particular. Some of these risk factors such as age and gender cannot be modified. Others such as diabetes and hypertension are not only CVD risk factors but are also the cause of the patient's CKD. Finally there are a group of risk factors such as disturbances of mineral metabolism and oxidative stress which are present either uniquely in or are exaggerated by renal failure. PD gives patients a more atherogenic lipid and lipoprotein profile, puts them at greater risk for AGE formation and usually causes hyperinsulinemia. All of these contribute to CVD risk. However, they can also achieve excellent blood pressure control, usually easily reach targets for anemia management and have continuous ultrafiltration allowing for the maintenance of good volume status, all of which will reduce risk for CVD. All treatable risk factors should be treated early in the development of CKD and should continue through their time on dialysis and after transplantation.

2 minute non-invasive screening for cardio-vascular diseases: relative limitation of C-Reactive Protein compared with more sensitive L-Homocystine as cardio-vascular risk factors; safe and effective treatment using the selective drug uptake enhancement method

Contrary to the present practice of measurement of cardio-vascular risk factors or inflammatory risk factors such as C-Reactive Protein (CRP) from a blood sample from the vein of one arm, by using the Bi-Digital O-Ring Test Resonance Phenomena between 2 identical substances, one can non-invasively detect the approximate location on the body of abnormally increased risk factors in just 2 minutes, by detecting the resonance with L-Homocystine, even when blood CRP failed to detect any abnormality. This is performed by projecting a 0.5 to approximately 5mW red spectral laser beam with 560-670nm wavelength, to at least 6 standard parts of the body, when one of the control risk markers placed next to the laser beam also exists in the part of the body tested. It is generally believed that CRP is increased in the presence of acute myocardial infarct, chronic rheumatoid arthritis, ulcerative colitis, metabolic abnormalities such as often detected in diabetes, inflammation and underlying infection of the cardio-vascular system, and in some cancers. However, in our study, when the clinical significance of CRP and L-Homocystine was compared, we found that CRP often was not increased when there was extensive infection of Mycobacterium Tuberculosis as well as asymptomatic infection by Cytomegalovirus, Herpes Simplex Virus Type I, Human Herpes Virus Type 6, Borrelia Burgdorferi, or Chlamydia Trachomatis in the heart (and other parts of the body), particularly when there was liver cell dysfunction such as an increase in ALT. In contrast, L-Homocystine was often increased in the presence of localized infections of the heart and other parts of the body. For screening of Cardio-Vascular diseases by this method, 0.5mg of L-Homocystine as a control marker was found to be the most sensitive and reliable, compared with most effective amount of CRP, 0.5ng, for detecting early Cardio-Vascular problems due to various localized infections. About 0.5ng of cardiac Troponin T and cardiac Troponin I were also useful for detecting early stages of heart disease but they are not as sensitive as L-Homocystine. Once the pathogenic factors were identified, the effective medication was given, and the Selective Drug Uptake Enhancement Method (originally discovered by the first author in 1990) was applied after the effective drug was administered, to selectively deliver the medication to the pathological area, while reducing drug uptake to the normal parts of the body. As a result, the therapeutic effect was markedly accelerated.

Hyperhomocysteinemia in cerebral vein thrombosis

High plasma levels of total homocysteine (tHcy) are a risk factor for deep vein thrombosis. Because no information on the relationship between cerebral vein thrombosis and hyperhomocysteinemia is available, a case-control study of 121 patients with a first episode of cerebral vein thrombosis and 242 healthy control subjects was carried out. Fasting plasma levels of tHcy and their postmethionine load (PML) increments, together with other laboratory markers of thrombophilia, were measured in plasma or DNA. Hyperhomocysteinemia (high fasting tHcy and/or PML increments) was diagnosed in 33 patients (27%) and 20 control subjects (8%) (odds ratio, 4.2; 95% confidence interval [CI], 2.3-7.6). Low levels of serum folate and the 677TT methylene tetrahydrofolate reductase were associated with hyperhomocysteinemia, but in a multivariate model hyperhomocysteinemia only was associated with an increased risk of cerebral vein thrombosis. Oral contraceptive intake was associated with the disease with an odds ratio of 6.1 (95% CI, 3.3-11.0). The combined presence of the latter and hyperhomocysteinemia increased the risk of the disease with an odds ratio of 19.5 (95% CI, 5.7-67.3). In conclusion, hyperhomocysteinemia is associated with a 4-fold increased risk of cerebral vein thrombosis; whether or not its correction with vitamins reduces the risk of the disease remains to be demonstrated.

[Homocysteine and hyperhomocysteinemia]

This paper reviews the biochemical and physiological data underlying hyperhomocysteinemia. Elevation of plasma homocysteine arises from disrupted of its metabolism (remethylation to methionine and transsulfuration to cystathionine) and is a function of a complex interaction between multiple genetic and environmental factors. Hyperhomocysteinemia, a conditions that recent epidemiological studies have shown to be associated with an increased risk of vascular diseases, may be a equal importance to hypercholesterolemia, hypertension and smoking in etiology of atherosclerosis. Based on the role of folic acid, vitamin B12 and B6 in homocysteine metabolism supplementation with these vitamins in treatment of hyperhomocysteinemia both in cardiovascular diseases and neural tube defects is reviewed.

Homocyst(e)ine, oxidative stress, and endothelium function in uremic patients

Moderate hyperhomocyst(e)inemia and impaired endothelium-dependent vasodilatation are present in uremic patients. However, the precise mechanism(s) underlying the link between moderate hyperhomocyst(e)inemia and endothelium dysfunction in uremic patients remains to be determined. Experimental and clinical evidence have led to the suggestion that moderate hyperhomocyst(e)inemia may predispose to endothelium dysfunction through a mechanism that involves generation of reactive oxygen species and a decrease in nitric oxide bioavailability. Recent preliminary findings in uremic patients provide support for some aspects of this suggestion. These data must be confirmed in additional studies. Moreover, the relative importance of homocysteine-induced oxidant stress versus other potential mechanisms of endothelium dysfunction in these patients remains to be determined.

[Genetic diagnosis and cardiovascular diseases]

Many of cardiovascular diseases are multifactorial and both environmental and genetic factors are associated with their onset and/or progression. Recently, the development of genetic analysis has enabled us to understand genetic backgrounds regulating the susceptibility to the specific environmental factors. These progress may bring us new strategies for diagnosis and/or prevention of diseases. In this review, we mainly discuss the current knowledge of these genetic analysis and perspectives in cardiovascular diseases. We also present briefly genetic mutations in cardiomyopathy, long QT syndrome, congenital cardiovascular diseases and so on. Additionally, we describe considerable problems about the interpretation and notification of the results of genetic diagnosis.

Homocyst(e)ine, atherosclerosis, and thrombosis

Ample clinical and epidemiologic evidence exists to implicate homocyst(e)ine as a risk factor for atherosclerotic vascular disease and thrombosis. The precise mechanisms by which this occurs are uncertain but probably involve injury to endothelium, impairment of endothelial function, lipid peroxidation, oxidation of low-density lipoprotein, and creation of a prothrombotic environment in areas of endothelial injury. Plasma homocyst(e)ine concentration (PHC) can be effectively reduced with oral administration of folic acid. Whether vitamins B6 and B12 are also required in the absence of vitamin deficiency remains uncertain. Studies currently in progress may help to determine whether reduction of PHC will translate into a decrease in clinical vascular events.

Post- and prenatal diagnostic methods for the homocystinurias

Diagnosis of the homozygous homocystinurias can be performed by investigations at the metabolite, enzyme and DNA level. The existence of variant forms due to the wide range of genetic variation may result in only small differences in various parameters between controls and affected subjects. 1. Sulphur amino acid concentrations in plasma, especially total homocysteine, are useful in first line diagnostic investigations. 2. Cystathionine-beta-synthase (CBS), methylenetetrahydrofolate reductase (MTHFR) and methylfolate homocysteine methyltransferase (MFMT) can be directly assayed in many tissues including fibroblasts (each) and blood cells (except CBS). Indirect whole cell assays which measure pathway activity dependent on a particular enzyme can provide useful diagnostic information. 3. Direct analysis of mutations is available for CBS, MTHFR and recently also for MFMT deficiencies. However the existence of a larger number of very rare, often private, mutations limits the usefulness of this approach in routine diagnosis. The above diagnostic approaches can generally be applied to prenatal diagnosis. Measurement of methylmalonic acid and other metabolites in amniotic fluid by stable isotope dilution / gas chromatography-mass spectrometry is well established for the methylmalonic acidurias. This method has also been applied to combined homocystinuria/methylmalonic aciduria supported by enzyme assays in cultured cells. Total homocysteine measurement in cell free amniotic fluid is also possible, performed so far in 14 cases with two affected fetuses. The indirect assay of methionine formation from [14C] labelled formate in intact cultured amniotic fluid cells has been for prenatal diagnosis of the remethylation defects.

Reduction-oxidation (Redox) and vascular tissue level of homocyst(e)ine in human coronary atherosclerotic lesions and role in extracellular matrix remodeling and vascular tone

Hyperhomocyst(e)inemia in patients with coronary and peripheral arterial occlusion has been demonstrated by others. Redox-state of homocyst(e)ine causes dysfunction of endothelial cells and promote growth of vascular smooth muscle cells. The role of tissue, protein bound and unbound, oxidative mixed disulfides in the development of fibrous plaque in atherosclerotic lesion is not known. Redox-state around the fibroblasts and vascular smooth muscle cells modulates the expression of extracellular matrix (ECM) components (Tyagi et al. 1996, J Cell Biochem, 61: 139-151). To determine the role of tissue homocystine in fibrotic atherosclerotic plaque development, coronary arteries were isolated from ischemic explanted hearts (n = 10). Apparently normal vascular tissue was obtained from idiopathic cardiomyopathic explanted hearts (n = 10). Tissue extract were prepared from atherosclerotic lesions and from normal arteries devoid of adventitia. Interaction of homocystine with Ellman's reagent (5, 5'-dithio-bis-2-nitro benzoic acid) catalyzed by limiting amount of reducing agent (catalyst) generated change in optical density (OD) at 412 nm in dose dependent fashion. We have generated a standard curve between change at 412 nm and amount of homocystine. The change in OD at 412 nm with increasing amount (0-25 microg) of homocystine demonstrated linearity. The protein-bound oxidized disulfides were precipitated by trichloroacetic acid (TCA) and free-oxidative disulfides in the supernatant were collected. The pathophysiological amount of protein-bound disulfide in atherosclerotic tissue (1.0 +/- 0.2 microg/mg total protein) was 10 times that in normal tissue (0.1 +/- 0.01 microg/mg, p < 0.001). The amount of free oxidative disulfide in atherosclerotic tissue (1.5 +/- 0.3 microg/mg) was 15 times that in normal tissue (0.12 +/- 0.02 microg/mg, p < 0.001). To determine the role of homocystine in ECM expression, ECM collagenase activity in the presence and absence of homocystine was measured by zymography. The effect of homocysteine on collagenase activity was biphasic, increased at < [0.01 mM] and inhibited at > [0.1 mM]. To determine whether homocystine regulates vascular tone, isometric measurements were carried out using normal coronary rings. Results suggested that homocystine induced endothelial-modulated vasoconstriction in coronary vessels. Tissue oxidative disulfides and the homocystine may contribute to the development of fibrotic atherosclerotic lesions and vascular dysfunction.

[Homocystinuria--biochemical, clinical and genetic aspects]

Homocystinuria is a rare autosomal recessive disease characterized by homocystinuria and multisystemic clinical disorders. The term denotes a biochemical abnormality of methionine metabolism caused both by transsulfuration pathway disorders and remethylation of homocysteine into methionine, and as such it can be a result of numerous specific and different genetic lesions. Homocystinuria is most commonly caused by deficiency of cystathionine beta-synthase (CBS) activity (EC 4.2.1.22). In this lesion, depending on specific characteristics of mutant enzyme molecules, in regard to existence of residual activity, responsive and nonresponsive homocystinuria can be differed regarding clinical response to high doses of pyridoxine. Although there are numerous different clinical abnormalities, changes on four organ systems are dominant. The most common symptoms of homocystinuria include lens dislocation, vascular disorders, skeletal abnormalities and mental retardation. Laboratory findings are the first diagnostic procedure, while determination of enzymatic activity is a direct parameter for making diagnosis. Prenatal diagnosis and early detection are extremely important for the course and prognosis of the disease as they enable application of currently available therapy as soon as possible. The presently available therapy can, only in such cases, prevent occurrence of serious clinical symptoms, prevent their advancement to some extent or improve reversible clinical manifestations.

Measurement of homocyst(e)ine in the prediction of arteriosclerosis

Homocyst(e)ine [H(e)], the sum of homocysteine, homocystine, and the homocysteine-cysteine mixed disulfide, free and protein-bound, has been shown to be associated in retrospective case control studies, and in one prospective study, with vascular disease, including coronary artery disease (CAD), cerebrovascular disease, and peripheral vascular disease. Elevated levels of homocyst(e)ine severe enough to cause homocystinuria are seen in severe nutritional deficiencies of vitamin B12, folic acid and vitamin B6. Rare genetic disorders of vitamin B12 synthesis of 5'-10'-methylene tetrahydrofolate reductase, or the pyridoxal phosphate-dependent enzyme cystathionine beta-synthase may cause severe hyperhomocyst(e)inemia and homocystinuria. The clinical manifestation of these disorders are mental retardation, neurological disorders, and widespread thromboembolic phenomena. The measurement of H(e) is currently performed using high-pressure liquid chromatography with fluorescence detection. Other methods, especially mass spectroscopy, are also used. Internal standards using increasing concentrations of homocystine and acetylcysteine and several external standards are used to ensure accuracy of the assay. Milder elevations of H(e) have recently been associated with vascular disease, in both men and women. The strength of this association appears to be stronger for peripheral and cerebrovascular disease than for CAD. Nevertheless, several case control studies in Europe, Canada, and the United States have shown that H(e) levels are elevated in CAD patients compared with controls, and H(e) levels are independent of the conventional cardiovascular risk factors (age, gender, lipid and lipoprotein cholesterol levels, hypertension, or cigarette smoking). One prospective study, the Physicians' Health Study, has shown that H(e) levels are slightly but significantly higher in CAD cases vs controls in a population of US physicians.(ABSTRACT TRUNCATED AT 250 WORDS)

Prevention of neural tube defects by and toxicity of L-homocysteine in cultured postimplantation rat embryos

Mild hyperhomocysteinemia is frequently observed in mothers who gave birth to a child with a neural tube defect (NTD). In a previous study we showed L-homocysteine was embryotoxic to gestational day 10 (GD10) rat embryos in culture, however, no NTDs were observed. We therefore investigated the effect of L-homocysteine on the development of neural plate stage (GD9.5) rat embryos. Other objectives of this study were investigation into whether the embryotoxicity of L-homocysteine could be attenuated by compounds related to its metabolism and clarification of the mechanism of L-homocysteine embryotoxicity. In GD9.5 rat embryos L-homocysteine was not toxic at 1- and 2-mM concentrations. Rather at these concentrations it promoted development of the rat embryos in serum that without supplementation caused NTDs in the embryos. L-Methionine had the same preventive effect at even lower concentrations, but folinic acid (1 mM) did not improve embryonic development. N5-Methyltetrahydrofolate (5-CH3-THF) (100 microM), L-serine (6 mM), and L-methionine (6 and 12 mM) attenuated the embryotoxicity of L-homocysteine (6 mM) in GD10 rat embryos. Vitamin B12 (10 microM) completely abolished the embryotoxicity of L-homocysteine, which was shown to be mediated by catalysis of the spontaneous oxidation of L-homocysteine to the less toxic L-homocystine. In GD11 rat embryos, both L- and D-homocysteine were readily taken up when added to the culture (3 mM) and increased embryonic S-adenosylhomocysteine (SAH) levels 14- and 3-fold, respectively. This difference was shown to be caused by the stereospecific preference of SAH hydrolase. We propose the basis for L-homocysteine embryotoxicity is an inhibition of transmethylation reactions by increased embryonic SAH levels.

Symptomatic and asymptomatic methylenetetrahydrofolate reductase deficiency in two adult brothers

We describe two brothers with 5,10-methylene tetrahydrofolate reductase (MTHFR) deficiency. The younger patient first developed limb weakness, incoordination, paresthesiae, and memory lapses at age 15 years, and by his early twenties he was wheelchair bound. His older brother remains asymptomatic at age 37 years. Both had homocystinuria and homocystinemia and low plasma levels of methionine. MTHFR activities in cultured skin fibroblasts of both patients were < 10% control and residual enzyme activities were markedly reduced on heating. The parents had intermediate enzyme activities and the reductase in the father (who had unexplained paraparesis and homocystinemia), but not in the mother, was also thermolabile. Both patients were treated with oral folate and betaine which improved, but did not totally correct, their biochemical abnormality. MTHFR deficiency should be considered in the differential diagnosis of unexplained neurologic disease in adolescents and adults.

Regulation of methionine synthesis in Escherichia coli

The biosynthesis of methionine in Escherichia coli is under complex regulation. The repression of the biosynthetic pathway by methionine is mediated by a repressor protein (MetJ protein) and S-adenosyl-methionine which functions as a corepressor for the MetJ protein. Recently, a new regulatory locus, metR, has been identified. The MetR protein is required for both metE and metH gene expression, and functions as a transactivator of transcription of these genes. MetR is a unique prokaryotic transcription activator in that it possesses a leucine zipper motif, first described for eukaryotic DNA-binding proteins. The transcriptional activity of MetR is modulated by homocysteine, the metabolic precursor of methionine. Finally, it is known that vitamin B12 can repress expression of the metE gene. This effect is mediated by the MetH holoenzyme, which contains a cobamide prosthetic group.

Homocysteine and myocardial infarction

Five (24%) subjects out of a group of 21 men, 48-58 years old (mean 54), who had suffered their first myocardial infarction (MI) before the age of 55 and with a low risk profile vis-à-vis conventional risk factors in a health screening preceding the MI, had abnormally high total plasma homocysteine values in the fasting state when investigated within 1-7 years (mean 3) after their MI. The patient group was exactly matched with 36 control subjects for sex, age, diastolic blood pressure, smoking, and serum concentrations of cholesterol and triglycerides. Total plasma homocysteine was negatively correlated to both erythrocyte folate and serum vitamin B12, and vitamin concentrations below the median of the normal distribution were found in the five with high plasma homocysteine content, indicating a possible involvement of reduced remethylation of plasma homocysteine to methionine. After methionine loading, in 3 of the patient group (14%) homocysteine levels exceeded mean +2 SD for controls, which may indicate heterozygosity for homocystinuria. Results are consistent with the hypothesis that a high plasma homocysteine content may be a risk factor for MI.

Metabolism of sulfur-containing amino acids in the dermatophyte Microsporum gypseum. I. Neutral amino acids

The dermatophyte Microsporum gypseum was cultivated on a glucose-arginine medium to which on out of six sulfur-containing amino acids was added (L-cystine, L-djenkolic acid, DL-lanthionine DL-homocystine, L-methionine, or L-methionine-sulfone at a concentration of 5 mM with respect to sulfur content). The addition of these substances did not stimulate the growth and some amino acids (djenkolic acid and particularly methionine and methionine-sulfone) were inhibitory. All tested compounds were utilized during the growth not only as sulfur source but as a source of carbon and nitrogen as well. In four substrates excess sulfur was excreted after oxidation into the medium in the form of sulfate. Small amounts of sulfite were also observed. It usually reacted with remaining disulfides in the medium forming S-sulfo compounds (R-S.SO3H). Cystine and djenkolic acid were oxidized rapidly and completely. In contrast, lanthionine and particularly homocystine were oxidized slowly and only after a longer adaptation. To some extent lanthionine was already oxidized extracellularly in the medium. With methionine and its sulfone, excess sulfur was not removed by oxidation but by demethiolation to methane thiol and further volatile products.

Environmental validation of the homocystine theory of arteriosclerosis

It has been proposed that elevated concentrations of homocystine in vascular tissue could cause arterial damage leading to arteriosclerosis. This theory is indirectly supported by research in the area of environmental toxicology, which has revealed that carbon monoxide and carbon disulfide, agents whose prolonged exposure is known to result in the development of arteriosclerotic changes, induced vitamin B6 deficiency states which predictably lead to a homocystinuria-like state. Such information provides strong indirect support of the controversial homocystine theory of arteriosclerosis.

Factors affecting the accumulation of homocyst(e)ine in rats deficient in vitamin B-6

Factors that may affect the concentration of accumulated homocyst(e)ine in rats deficient in vitamin B-6 were studied. Hepatic cystathionine synthase activity in vitamin B-6 deficient rats was 41.5% of control and plasma protein-bound and free homocyst(e)ine concentrations were significantly increased; however, among deficient rats there was no correlation between the concentration of homocyst(e)ine and the activity of hepatic cystathionine synthase. The plasma protein-bound homocysteine concentration did not fluctuate with time of day, but a decrease in bound homocysteine was found when blood was sampled repeatedly from the same animal during a 24-hour period. A 24-hour fast resulted in a reduction in plasma homocyst(e)ine to concentrations not different from those of control animals. The protein content of the diet affected growth but not food intake of rats. Feeding a 10% versus a 60% casein diet did not change the concentration of accumulated homocyst(e)ine. Of the factors examined, only food restriction significantly affected plasma homocyst(e)ine concentration in rats deficient in vitamin B-6. It is speculated that decreased and sporadic food intake in rats deficient in vitamin B-6 may be responsible for the large fluctuations in homocyst(e)ine concentration observed in these animals over time.

The nitrogen-sparing effect of methionine sulphoxide and some other sulphur-containing amino acids

The nitrogen-sparing effect of methionine, methionine sulphoxide, homocystine, cystine and choline was studied in rats by determining dialy N excretions during 5 d after changing from a high-protein diet to a protein-free diet, L-glutamic acid was used as a negative control. 2. L-, D- and DL-methionine were equally active in sparing N. L-methionine sulphoxide, DL-homocystine and L-cystine were as active as L-methionine. D-methionine sulphoxide was slightly less active than L-methionine sulphoxide. Choline hydrogen tartrate was not different from the negative control. 3. It is concluded that in short-term experiments cystine is the key substance in the N-sparing effect.

Homocystine uptake in isolated rat renal cortical tubules

Isolated rat renal cortical tubules were used to study the nature of homocystine entry into the tubule cell and its transport interactions with cystine and the dibasic amino acids. The uptake of homocystine with time was progressive, reaching a steady state after 60 min. of incubation. Analysis of the intracellular pool after 5 and 30 min. of incubation revealed that virtually all of the transported homocystine had been converted to other metabolites of the transsulfuration pathway. The major metabolite was cystathionine with a somewhat lesser, but still significant amount as S-adenosylhomocysteine. A kinetic analysis showed that two systems for cellular entry of homocysteine existed with a Km1 of 0.17 mM and a Km2 of 7.65 mM. Arginine and lysine inhibited homocystine uptake via the low Km, high affinity system, but appeared not to inhibit the high Km, low affinity system. Cystine inhibited the low Km, high affinity system, but had an indeterminate effect on the high Km, low affinity system. Homocystine inhibited the uptake of cystine, lysine and arginine by isolated rat renal cortical tubules. The inhibition of homocystine on cystine uptake appeared to occur on both the high and low Km system for tubule cell entry of cystine. The data suggest that the low Km system for homocystine transport is shared with cystine and the dibasic amino acids. These data extend the knowledge of homocystine metabolism and provide a rational basis for new approaches to the treatment of homocystinuria.

Effect of nutritional and enzymatic methionine deprivation upon human normal and malignant cells in tissue culture

Human embryonic lung fibroblasts (F-136-35-56) capable of growing in medium containing DL-homocysteine instead of L-methionine and human acute lymphoblastic leukemia cells (CCRF-HSB-2) with absolute methionine requirement exhibited dose-dependent growth inhibition when semipurified L-methionine-degrading enzyme (L-methioninase, EC 4.4.1.11) was added to the tissue cultures. When D-homocystine was added to the cultures together with L-methioninase (0.1 units/ml, which completely degraded the available L-methionine in tissue culture), the F-136-35-56 cells continued to grow whereas the CCRF-HSB-2 cells were completely inhibited. In mixed cultures of the two cell lines with added L-methioninase + D-homocystine or L-methioninase + L-homocysteine thiolactone, the normal fibroblasts grew and synthesized DNA vigorously, whereas the lymphocytic malignant cells lost their viability completely and died within 3 to 4 days.

Protein-bound homocyst(e)ine in normal subjects and in patients with homocystinuria

A method was developed to quantitate protein-bound homocyst(e)ine using 2-mercaptoethanol. Protein-bound homocyst(e)ine was discovered in the plasma from normal individuals, ranging from 0.5--2.2 nmole/ml. In two obligatory heterozygotes for classical homocystinuria, plasma protein-bound homocyt(e)ine was 3.5 and 4.8 nmole/ml, respectively. Untreated homozygotes showed approximately a 40-fold increase of plasma protein-bound homocyst(e)ine. Furthermore, using conventional methods, no free homocystine was detectable in the supernatant of plasma precipitate from two classical homocystinuric patients treated with pyridoxine, but plasma protein-bound homocyst(e)ine showed a 10-fold increase. Protein-bound homocyst(e)ine was also demonstrated in the liver, kidney, and brain tissues from a patient with methylenetetrahydrofolate reductase deficiency.

[Biosynthesis of thiazole from thiamine in Escherichia coli]

The incorporation into the thiazole moiety of thiamine of several labeled compounds has been studied on short time incubations of washed-cells suspensions. No incorporation of radioactivity from [G-14C] methionine was found in a mutant auxotrophic for methionine. No radioactivity was incorporated from [U-14C] aspartate or from [U-14C] serine. The incorporation of 35S from sulphate was lowered by cysteine or glutathione but was unaffected by methionine or homocystine. Although the synthesis of thiazole is dependent on methionine, neither the sulphur atom nor the carbon chain of thiazole originate from methonine in E. coli. No carbon originates from cysteine which is the likely direct donor of sulphur.