Human homocysteine catabolism: three major pathways and their relevance to development of arterial occlusive disease

Elevated homocysteine level as an indicator for chronic kidney disease in community-dwelling middle-aged and elderly populations in Taiwan: A community-based cross-sectional study

Background

Hyperhomocysteinemia is an important factor for endothelial cell damage and a risk factor for cardiovascular diseases. Chronic kidney disease (CKD) is recognized as a leading burden in Taiwan’s healthcare system. This study aimed to investigate the association between homocysteine levels and CKD in middle-aged and elderly adults from a community in northern Taiwan.

Methods

A total of 396 middle-aged and elderly Taiwanese adults were enrolled and completed the health survey. We divided participants according to tertiles of homocysteine levels as first group (homocysteine level ≤ 11.1 μmol/L), second group (homocysteine level 11.2∼14.3 μmol/L), and third group (homocysteine level > 14.3 μmol/L). CKD was defined as estimated glomerular filtration rate (eGFR) < 60 (mL/min/1.73 m²) or urine albumin to creatinine ratio > 30 (mg/g). Pearson correlation was calculated between eGFR and other related risk factors after adjustment for age. The risk of CKD in the second and third groups compared to that in the first group was assessed by multivariate logistic regression after adjustment for age, sex, smoking, hypertension (HTN), diabetes mellitus (DM), body mass index (BMI), dyslipidemia, and uric acid. The Youden index and receiver operating characteristic (ROC) curve were calculated for the optimized cutoff value.

Results

Elevated plasma homocysteine levels were more likely to lower the eGFR and increase the prevalence of CKD. Pearson correlation showed a positive correlation between eGFR and high-density lipoprotein cholesterol, while a negative correlation was observed between homocysteine levels, waist circumference, systolic blood pressure, uric acid levels and BMI (all p < 0.05). In the logistic regression analysis, the prevalence of CKD increased, as well as the homocysteine level. The odds ratio of CKD under 95% confidence interval was 2.655 (1.284–5.490) for the third group compared with the first group after adjusting for age, sex, smoking, DM, HTN, dyslipidemia, uric acid, and BMI (p = 0.008). The area under the ROC curve was 0.662, and a cutoff value of 15.15 μmol/L for the homocysteine level was obtained for detecting subjects with CKD.

Conclusion

Our study findings revealed that elevated homocysteine levels were significantly associated with CKD and could be used as an indicator of CKD among the middle-aged and elderly populations in Taiwan.

A global map of associations between types of protein posttranslational modifications and human genetic diseases

There are >200 types of protein posttranslational modifications (PTMs) described in eukaryotes, each with unique proteome coverage and functions. We hypothesized that some genetic diseases may be caused by the removal of a specific type of PTMs by genomic variants and the consequent deregulation of particular functions. We collected >320,000 human PTMs representing 59 types and crossed them with >4M nonsynonymous DNA variants annotated with predicted pathogenicity and disease associations. We report >1.74M PTM-variant co-occurrences that an enrichment analysis distributed into 215 pairwise associations between 18 PTM types and 148 genetic diseases. Of them, 42% were not previously described. Removal of lysine acetylation exerts the most pronounced effect, and less studied PTM types such as S-glutathionylation or S-nitrosylation show relevance. Using pathogenicity predictions, we identified PTM sites that may produce particular diseases if prevented. Our results provide evidence of a substantial impact of PTM-specific removal on the pathogenesis of genetic diseases and phenotypes.

•

There is an enrichment of disease-associated nsSNVs preventing certain types of PTMs

•

We report 215 pairwise associations between 18 PTM types and 148 genetic diseases

•

The removal of lysine acetylation exerts the most pronounced effect

•

We report a set of PTM sites that may produce particular diseases if prevented

Protective Effects and Metabolic Regulatory Mechanisms of Shenyan Fangshuai Recipe on Chronic Kidney Disease in Rats

Background

Chronic kidney disease (CKD) is one of the major causes of renal damage. Shenyan Fangshuai Recipe (SFR), a modified prescription of traditional medicine in China, showed potent effects in alleviating edema, proteinuria, and hematuria of CKD in clinical practices. In this study, we aimed to investigate scientific evidence-based efficacy as well as metabolic regulations of SFR in CKD treatment.

Materials and Methods

The effect of SFR on CKD was observed in a rat model which is established with oral administration of adenine-ethambutol mixture for 21 days. Further, metabolites in serum were detected and identified with ultra-performance liquid chromatography-high resolution mass spectrometry (UPLC-HRMS). Metabolomics study was performed using Ingenuity Pathway Analysis (IPA) software.

Results

With H&E staining and Masson's trichrome, the results showed that chronic kidney damage is significantly rescued with SFR treatment and recovered to an approximately normal condition. Along with 44 differential metabolites discovered, the regulation of SFR on CKD was enriched in glycine biosynthesis I, mitochondrial L-carnitine shuttle pathway, phosphatidylethanolamine biosynthesis III, sphingosine-1-phosphate signaling, L-serine degradation, folate transformations I, noradrenaline and adrenaline degradation, salvage pathways of pyrimidine ribonucleotides, cysteine biosynthesis III (Mammalia), glycine betaine degradation, and cysteine biosynthesis/homocysteine degradation. Further, TGFβ-1 and MMP-9 were observed playing roles in this regulatory process by performing immunohistochemical staining.

Conclusion

SFR exerts potent effects of alleviating glomerular sclerosis and interstitial fibrosis in the kidney, mainly via integrated regulations on metabolism and production of homocysteine, L-carnitine, and epinephrine, as well as the expression of TGFβ-1. This study provides evidence for SFR's protective effects on CKD and reveals the metabolic mechanism behind these benefits for the first time.

Polymorphisms C677T and A1298C of MTHFR Gene: Homocysteine Levels and Prothrombotic Biomarkers in Coronary and Pulmonary Thromboembolic Disease

The activity of the enzyme methylenetetrahydrofolate reductase (MTHFR) determines homocysteine (Hcy) levels, and polymorphisms in its gene affect the activity of the enzyme. Changes in the enzyme’s activity may lead to a higher susceptibility to develop arterial and venous thromboembolic disease. The aim was to analyze the relationship between the C677T and A1298C polymorphisms of MTHFR, Hcy levels, and prothrombotic biomarkers in pulmonary embolism (PE) and acute myocardial ischemia (AMI). Clinical files of patients with thromboembolic diseases having complete data and whose doctor had requested an assay to determine the polymorphisms of the MTHFR gene, Hcy levels, and prothrombotic biomarkers were studied to search for the correlation between mutations of the MTHFR gene and Hcy levels in the different diseases. We included 334 files: 158 were from women and 176 from men (51 [19 SD] years). Sixty-three percent have had thrombosis, 8% AMI, and 31% PE. Patients with thrombosis had elevated frequency of the C677T polymorphism. The CC genotype was higher than the TT genotype (P = .003) and CT versus the TT (P = .009). In patients with PE, the CC genotype was higher than the TT genotype (P = .038). Pulmonary embolism with massive and submassive events had predominant genotypes 677 TT (P = .003) and the AA 1298 (P = .017). Elevated Hcy levels in the presence of the T allele in the C677T gene and of the A allele in the A1298C gene are associated with AMI and massive and submassive PE.

Homocysteine in Chronic Kidney Disease

Hyperhomocysteinemia occurs in chronic- and end-stage kidney disease at the time when dialysis or transplant becomes indispensable for survival. Excessive accumulation of homocysteine (Hcy) aggravates conditions associated with imbalanced homeostasis and cellular redox thereby resulting in severe oxidative stress leading to oxidation of reduced free and protein-bound thiols. Thiol modifications such as N-homocysteinylation, sulfination, cysteinylation, glutathionylation, and sulfhydration control cellular responses that direct complex metabolic pathways. Although cysteinyl modifications are kept low, under Hcy-induced stress, thiol modifications persist thus surpassing cellular proteostasis. Here, we review mechanisms of redox regulation and show how cysteinyl modifications triggered by excess Hcy contribute development and progression of chronic kidney disease. We discuss different signaling events resulting from aberrant cysteinyl modification with a focus on transsulfuration.

Plasma homocysteine, dietary B vitamins, betaine, and choline and risk of peripheral artery disease

OBJECTIVE

Few studies have examined the roles of homocysteine and related nutrients in the development of peripheral artery disease (PAD). We examined the associations between plasma homocysteine, dietary B vitamins, betaine, choline, and supplemental folic acid use and incidence of PAD.

METHODS

We used two cohort studies of 72,348 women in the Nurses' Health Study (NHS, 1990-2010) and 44,504 men in the Health Professionals Follow-up Study (HPFS, 1986-2010). We measured plasma homocysteine in nested matched case-control studies of clinically recognized PAD within both cohorts, including 143 PAD cases and 424 controls within the NHS (1990-2010) and 143 PAD cases and 428 controls within the HPFS (1994-2008). We examined the association between diet and risk of incident PAD in the cohorts using a food frequency questionnaire and 790 cases of PAD over 3.1 million person-years of follow-up.

RESULTS

Higher homocysteine levels were positively associated with risk of PAD in men (adjusted IRR 2.17; 95% CI, 1.08-4.38 for tertile 3 vs. 1). There was no evidence of an association in women (adjusted IRR 1.14; 95% CI, 0.61-2.12). Similarly, higher folate intake, including supplements, was inversely associated with risk of PAD in men (adjusted HR 0.90; 95% CI, 0.82-0.98 for each 250 μg increase) but not women (HR 1.01, 95% CI, 0.88-1.15). Intakes of the other B vitamins, betaine, and choline were not consistently associated with risk of PAD in men or women.

CONCLUSION

Homocysteine levels were positively associated and dietary folate intake was inversely associated with risk of PAD in men but not in women.

Construction of a fluorescence turn-on probe for highly discriminating detection of cysteine

We developed a cysteine specific probe by utilizing the remarkable difference in reactivity toward discriminating cysteine from homocysteine and glutathione. This probe was also successfully used for detection of Cys in living cells and monitoring cystathionine γ-lyase activityin vitro.

Transsulfuration Is a Significant Source of Sulfur for Glutathione Production in Human Mammary Epithelial Cells

The transsulfuration pathway, through which homocysteine from the methionine cycle provides sulfur for cystathionine formation, which may subsequently be used for glutathione synthesis, has not heretofore been identified as active in mammary cells. Primary human mammary epithelial cells (HMEC's) were labeled with S35-methionine for 24 hours following pretreatment with a vehicle control, the cysteine biosynthesis inhibitor propargylglycine or the gamma-glutamylcysteine synthesis inhibitor buthionine sulfoximine. Cell lysates were prepared and reacted with glutathione-S-transferase and the fluorescent labeling compound monochlorobimane to form a fluorescent glutathione-bimane conjugate. Comparison of fluorographic and autoradiographic images indicated that glutathione had incorporated S35-methionine demonstrating that functional transsulfuration occurs in mammary cells. Pathway inhibitors reduced incorporation by roughly 80%. Measurement of glutathione production in HMEC's treated with and without hydrogen peroxide and/or pathway inhibitors indicates that the transsulfuration pathway plays a significant role in providing cysteine for glutathione production both normally and under conditions of oxidant stress.

Plasma total homocysteine status of vegetarians compared with omnivores: a systematic review and meta-analysis

There is strong evidence indicating that elevated plasma total homocysteine (tHcy) levels are a major independent biomarker and/or a contributor to chronic conditions, such as CVD. A deficiency of vitamin B12can elevate homocysteine. Vegetarians are a group of the population who are potentially at greater risk of vitamin B12deficiency than omnivores. This is the first systematic review and meta-analysis to appraise a range of studies that compared the homocysteine and vitamin B12levels of vegetarians and omnivores. The search methods employed identified 443 entries, from which, by screening using set inclusion and exclusion criteria, six eligible cohort case studies and eleven cross-sectional studies from 1999 to 2010 were revealed, which compared concentrations of plasma tHcy and serum vitamin B12of omnivores, lactovegetarians or lacto-ovovegetarians and vegans. Of the identified seventeen studies (3230 participants), only two studies reported that vegan concentrations of plasma tHcy and serum vitamin B12did not differ from omnivores. The present study confirmed that an inverse relationship exists between plasma tHcy and serum vitamin B12, from which it can be concluded that the usual dietary source of vitamin B12is animal products and those who choose to omit or restrict these products are destined to become vitamin B12deficient. At present, the available supplement, which is usually used for fortification of food, is the unreliable cyanocobalamin. A well-designed study is needed to investigate a reliable and suitable supplement to normalise the elevated plasma tHcy of a high majority of vegetarians. This would fill the gaps in the present nutritional scientific knowledge.

Association of methylenetetrahydrofolate reductase gene 677C>T polymorphism with hypertension in older women in a population of Buenos Aires City

We examined the relationship between the 677C >T polymorphism in the MTHFR gene and tHcy in normotensive (NT) and hypertensive (HT) subjects and the influence of sex and age in a cross-sectional study. Smoking habits, history of vascular disease, diabetes, and tHcy were significantly associated with T allele as hypertension risk factors. The T allele was significantly related with higher tHcy in (i) men versus women (P < .01), (ii) men and women older than 47 years versus the younger ones (P < .05 and P < .001, respectively), (iii) HT women versus NT women (P < .01), and (iv) older HT women versus older NT women (P < .01). We found an association between the 677C>T MTHFR polymorphism and tHcy with hypertension that in women is manifested with age.

Choline supplementation and measures of choline and betaine status: a randomised, controlled trial in postmenopausal women

Choline is an essential nutrient and can also be obtained by de novo synthesis via an oestrogen responsive pathway. Choline can be oxidised to the methyl donor betaine, with short-term supplementation reported to lower plasma total homocysteine (tHcy); however, the effects of longer-term choline supplementation are less clear. We investigated the effect of choline supplementation on plasma concentrations of free choline, betaine and tHcy and B-vitamin status in postmenopausal women, a group more susceptible to low choline status. We also assessed whether supplementation altered plasma lipid profiles. In this randomised, double-blinded, placebo-controlled study, forty-two healthy postmenopausal women received 1 g choline per d (as choline bitartrate), or an identical placebo supplement with their habitual diet. Fasting blood samples were collected at baseline, week 6 and week 12. Administration of choline increased median choline and betaine concentrations in plasma, with significant effects evident after 6 weeks of supplementation (P<0·001) and remaining significant at 12 weeks (P<0·001); no effect was observed on folate status or on plasma lipids. Choline supplementation induced a median (25th, 75th percentile) change in plasma tHcy concentration at week 6 of -0·9 (-1·6, 0·2) μmol, a change which, when compared to that observed in the placebo group 0·6 (-0·4, 1·9) μmol, approached statistical significance (P=0·058). Choline supplementation at a dose of 1 g/d significantly increases the circulating concentration of free choline, and can also significantly increase the concentration of the methyl donor, betaine, thereby potentially enhancing the betaine-homocysteine methyltransferase-mediated remethylation of tHcy.

Small changes huge impact: the role of protein posttranslational modifications in cellular homeostasis and disease

Posttranslational modifications (PTMs) modulate protein function in most eukaryotes and have a ubiquitous role in diverse range of cellular functions. Identification, characterization, and mapping of these modifications to specific amino acid residues on proteins are critical towards understanding their functional significance in a biological context. The interpretation of proteome data obtained from the high-throughput methods cannot be deciphered unambiguously without a priori knowledge of protein modifications. An in-depth understanding of protein PTMs is important not only for gaining a perception of a wide array of cellular functions but also towards developing drug therapies for many life-threatening diseases like cancer and neurodegenerative disorders. Many of the protein modifications like ubiquitination play a decisive role in various drug response(s) and eventually in disease prognosis. Thus, many commonly observed PTMs are routinely tracked as disease markers while many others are used as molecular targets for developing target-specific therapies. In this paper, we summarize some of the major, well-studied protein alterations and highlight their importance in various chronic diseases and normal development. In addition, other promising minor modifications such as SUMOylation, observed to impact cellular dynamics as well as disease pathology, are mentioned briefly.

Levels of circulating homocysteine, vitamin B6, vitamin B12, and folate in different types of open-angle glaucoma

OBJECTIVE

To compare the levels of plasma homocysteine (Hcy), vitamin B6 (vit-B6), serum vitamin B12 (vit-B12), and folate in healthy individuals and in patients with normal tension glaucoma (NTG), pseudoexfoliative glaucoma (PXG), or primary open-angle glaucoma (POAG).

STUDY DESIGN

A prospective controlled trial.

PARTICIPANTS AND METHODS

Forty healthy subjects, 48 patients with NTG, 38 patients with PXG, and 34 patients with POAG were included in the study. Those who used vitamin supplements or medications affecting Hcy and vitamin levels were excluded from the study. The levels of Hcy and vit-B6 were measured by High Performance Liquid Chromatography (HPLC). The levels of serum vit-B12 and folic acid were measured by competitive chemiluminescent enzyme immunoassay (CEI). One-way analysis if variance (ANOVA), analysis of covariance (ANCOVA), and the Tukey honestly significant difference test were used for statistical analysis.

RESULTS

The mean Hcy level of the PXG group was 15.46+/-9.27 micromol/L which was significantly higher (P=0.03) than that of the control group. There were no statistical differences in serum vit-B12 and folate levels among control subjects and NTG, PXG and POAG groups (P>0.05). It was found that the mean plasma vit-B6 level was significantly higher in subjects with NTG (P=0.03) and POAG (P=0.025) versus controls. Mean vit-B6 levels in NTG and POAG were 30.50+/-11.29 microg/L and 30+/-12.15 microg/L, respectively.

CONCLUSIONS

The plasma level of Hcy was found to be increased only in PXG patients and the plasma levels of vit-B6 were found to increase in the NTG and POAG sample groups. Using homocysteine and vit-B6 levels as the determinants of hyperhomocysteinemia still needs further research.

Study on homocysteine levels and methylenetetrahydrofolate reductase gene variant (C677T) in a population of Buenos Aires City

The substitution of cytosine (C) by thymine (T) at nucleotide 677 of the methylenetetrahydrofolate reductase (MTHFR) gene, which converts an alanine to a valine residue, is a frequent polymorphism with reduced specific activity, associated with moderate increase in plasma homocysteine levels (tHcy) and risk of vascular diseases.

OBJECTIVES

This study was designed to investigate an association of this polymorphism with tHcy and vascular risk factors.

METHODS

We used a cross-sectional study on subjects affiliated to three health centers from Buenos Aires city. The diagnosis of hypertension was ascertained by patients' clinical history. Only subjects under long-term antihypertensive treatment were included.

RESULTS

Samples from 138 physically active individuals (44 men and 94 women) randomly selected were included. The mean tHcy was significantly higher amongst hypertensives (HT) than normotensives (NT). The risk of hypertension was compared in subjects with CC genotype and the combined number of subjects with at least one T allele (CT/TT). There was no significant difference regarding the risk of hypertension between NT and HT groups in the overall sample. However, as obesity is considered a risk factor for hypertension development, when only HT (n = 29) and NT (n = 66) subjects with body mass index below 30 kg/m(2) (BMI<30) were compared, subjects bearing CT/TT presented a significantly higher risk of hypertension than those bearing the CC genotype and significantly higher concentration of tHcy.

CONCLUSIONS

Our results indicate an association of hyper-tHcy and MTHFR C677T mutation with hypertension. MTHFR C677T mutation may contribute to hypertension or affect the development of hypertension through hyperhomocysteinemia.

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.

Hyperhomocysteinemia associated with decreased renal transsulfuration activity in Dahl S rats

Elevated plasma homocysteine (Hcys) has been reported to participate in the development of arterial and glomerular sclerosis in Dahl salt-sensitive hypertensive (SS) rats. The mechanism resulting in hyperhomocysteinemia in these animals remains unknown. Disposal of Hcys in the kidneys plays an important role in regulating the plasma Hcys level. We, therefore, examined the activities and expressions of the enzymes involved in the metabolism of Hcys in the kidneys of SS rats, compared with that of Brown Norway rats and SSBN13 rats, a consomic subcolony of SS rats that carries a substituted chromosome 13 from Brown Norway rats. High-performance liquid chromatography analysis demonstrated that plasma Hcys levels were significantly higher in SS rats. The conversion of S-adenosylhomocysteine into Hcys via S-adenosylhomocysteine hydrolase by renal tissue was not different among these 3 rat strains. However, the metabolic rate of Hcys into cysteine was markedly reduced in the SS rat kidneys. The mRNA and protein levels of cystathionine beta-synthase (CBS), one of the key enzymes in the transsulfuration pathway in the kidneys, were significantly lower in SS rats. In microdissected nephron segments, CBS mRNA was shown to be mainly present in renal proximal tubules (PTs). The mRNA levels of CBS in the PTs were also significantly decreased in SS rats, accompanied by a reduced CBS activity in PTs. We conclude that hyperhomocysteinemia is associated with a decreased activity and expression of CBS in renal PTs because of the defect of chromosome 13 in SS rats.

Supplemental Conditionally Essential Nutrients in Cardiovascular Disease Therapy

Conditionally essential nutrients (CENs) are organic compounds that are ordinarily produced by the body in amounts sufficient to meet its physiological requirements. However, in disorders, such as cardiovascular disease (CVD), and in other physiologically stressful conditions, their biosynthesis may be inadequate. Under these circumstances, CENs become essential nutrients, comparable to vitamins. The CENs of primary importance in CVD, based on the quantity and quality of human clinical studies, are l-arginine, l-carnitine, propionyl-l-carnitine, and coenzyme Q10. Controlled studies of these CENs are reviewed in depth. Taurine is a CEN of secondary importance caused by a limited human database. Other putative CENs include alpha-lipoic acid, betaine, chondroitin sulfate, glutamine, and d-ribose, each of which is mentioned in passing. Collectively, CENs have demonstrated favorable clinical effects in CVDs, including chronic heart failure, myocardial infarction, angina pectoris, and in CVD risk factors, such as hypertension, hyperlipidemia, and lipoprotein(a). Limited research has pointed to possible benefits in CVD therapy accruing from supplementation with several CENs in combination. Additional controlled clinical studies of CENs in CVD are urgently needed. In view of the efficacy and safety of appropriate supplementation with CENs, it is strongly suggested that healthcare professionals become knowledgeable of these potentially important additions to the CVD therapeutic armamentarium.

Is there a possible relation between atrophic gastritis and premature atherosclerosis?

BACKGROUND

In this study, we have aimed to show the possible relation between atrophic gastritis and premature atherosclerosis via hyperhomocysteinemia.

MATERIALS AND METHODS

Thirty-four patients with atrophic gastritis were enrolled to the study. The control group consisted of 35 patients with non-atrophic gastritis. Classical cardiovascular disease risk factors did not significantly differ between atrophic gastritis and control subjects. The presence and degree of atrophic gastritis were assessed histologically and Helicobacter pylori infection was determined by both histologic and serologic methods. Body mass index was measured by standard technique blood fasting glucose, serum creatinine, total cholesterol, triglyceride, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, vitamin B12, folic acid, and homocysteine levels were measured by biochemical methods. Carotid intima-media thickness was measured by B-mode ultrasonography to examine the premature atherosclerosis.

RESULTS

Plasma vitamin B12 levels were significantly lower (p = .00) and homocysteine levels were significantly higher (p = .01) in the atrophic gastritis group. There was no statistically significant difference in plasma folic acid levels between the two groups (p = .728). Carotid intima-media thickness was higher in the atrophic gastritis group than in the control group (0.516 mm versus 0.465 mm), but this difference did not show any statistical significance (p = .062).

CONCLUSION

Our results showed that atrophic gastritis may cause hyperhomocysteinemia, which is an independent risk factor for atherosclerosis and cardiovascular diseases. However, when compared with controls, carotid intima-media thickness of the atrophic gastritis patients was found to be higher but did not reach statistically significant levels.

Effects of insulin on methionine and homocysteine kinetics in type 2 diabetes with nephropathy

Although hyperhomocysteinemia, an independent cardiovascular risk factor, is common in type 2 diabetes with nephropathy, the mechanism(s) of this alteration is not known. In healthy humans, hyperinsulinemia increases methionine transmethylation, homocysteine transsulfuration, and clearance. No such data exist in type 2 diabetes either in the fasting state or in response to hyperinsulinemia. To this purpose, seven male type 2 diabetic patients with albuminuria (1.2 +/- 0.4 g/day, three with mild to moderate renal insufficiency) and seven matched control subjects were infused for 6 h with L-[methyl-(2)H(3), 1-(13)C]methionine. Methionine flux, transmethylation, and disposal into proteins as well as homocysteine remethylation, transsulfuration, and clearance were determined before and after euglycemic hyperinsulinemia (approximately 1,000 pmol/l). In type 2 diabetic subjects, homocysteine concentration was twofold greater (P < 0.01) and methionine transmethylation and homocysteine clearance lower (from approximately 15 to >50% and from approximately 40 to >100%, respectively; P < 0.05) than in control subjects. The insulin-induced increments of methionine transmethylation, homocysteine transsulfuration, and clearance were markedly reduced in type 2 diabetic subjects (by more than threefold, P < 0.05 or less vs. control subjects). In contrast, methionine methyl and carbon flux were not increased in the patients. In conclusion, pathways of homocysteine disposal are impaired in type 2 diabetes with nephropathy, both in postabsorptive and insulin-stimulated states, possibly accounting for the hyperhomocysteinemia of this condition.

Handling of the HomocysteineS-Conjugate of Methylmercury by Renal Epithelial Cells: Role of Organic Anion Transporter 1 and Amino Acid Transporters

Recently, the activity of the organic anion transporter 1 (OAT1) protein has been implicated in the basolateral uptake of inorganic mercuric species in renal proximal tubular cells. Unfortunately, very little is known about the role of OAT1 in the renal epithelial transport of organic forms of mercury, such as methylmercury (CH(3)Hg(+)). Homocysteine (Hcy) S-conjugates of methylmercury [(S)-(3-amino-3-carboxypropylthio)(methyl)mercury (CH(3)Hg-Hcy)] have been identified recently as being potentially important biologically relevant forms of mercury. Thus, the present study was designed to characterize the transport of CH(3)Hg-Hcy in Madin-Darby canine kidney (MDCK) cells (which are derived from the distal nephron) that were transfected stably with the human isoform of OAT1 (hOAT1). Data on saturation kinetics, time dependence, substrate specificity, and temperature dependence demonstrated that CH(3)Hg-Hcy is a transportable substrate of hOAT1. However, substrate-specificity data from the control MDCK cells also showed that CH(3)Hg-Hcy is a substrate of one or more transporter(s) that is/are not hOAT1. Additional findings indicated that at least one amino acid transport system was probably responsible for this transport. It is noteworthy that the activity of amino acid transporters accounted for the greatest level of uptake of CH(3)Hg-Hcy in the hOAT1-expressing cells. Furthermore, rates of survival of the hOAT1-transfected MDCK cells were significantly lower than those of corresponding control MDCK cells when they were exposed to cytotoxic concentrations of CH(3)Hg-Hcy. Collectively, the present data indicate that CH(3)Hg-Hcy is a transportable substrate of OAT1 and amino acid transporters and, thus, is probably a transportable mercuric species taken up in vivo by proximal tubular epithelial cells.

Hyperhomocysteinemia in acute Plasmodium falciparum malaria: an effect of host-parasite interaction

BACKGROUND

Plasmodium falciparum utilises the polyamine pathway, essential in proliferation and differentiation, and imposes an oxidative stress on host cell, enhancing the loss of glutathione.

METHODS

Standard hematological parameters were determined in 40 black African subjects with acute P. falciparum malaria, 30 aged 5-24 months, 5 aged 4-10 years and 5 aged 19-35 years. Plasma homocysteine, cysteine, glutathione and cysteinylglycine levels were measured by HPLC method. Twenty-eight healthy black children (15 aged 6-24 months and 13 aged 3-10 years) and 20 healthy black adults (aged 20-40 years) were also included as controls.

RESULTS

Plasma homocysteine levels were higher in all subjects with P. falciparum malaria and correlated positively with the disease severity and number of parasites, but negatively with Hb levels and patient ages. Cysteine level was found higher in all patients and markedly higher in 4-10 year old patients. Cysteinylglycine level was found lower particularly in 19-35 year old patients. Glutathione level was significantly lower in all patients.

CONCLUSIONS

The elevated level of homocysteine during acute P. falciparum infection suggests an imbalance in the folate cycle, which could be a consequence of the reduced availability of NADPH and Vit B12, caused by increased oxidative stress. This may suggest a selection for the C677T MTHFR allele, driven by P. falciparum in sub-Saharan regions. Hence Hcy level could be useful as a predictive parameter of severity, as well as of treatment efficacy.

Mild hyperhomocysteinemia promotes renal hemodynamic dysfunction without histopathologic changes in adult rats

BACKGROUND

Hyperhomocysteinemia is able to promote glomerular damage and generate tubulointerstitial lesions. These findings were reported in rats with unilateral nephrectomy or in weanling rats with normal function, two experimental models that are exposed to other concomitant vascular risk factors. The aim of this work is to study whether mild hyperhomocisteinemia per se can induce renal histopathologic changes in adults rats with normal renal function at either 10 or 44 weeks of hyperhomocysteinemia.

METHODS

Two months old male Wistar rats (N= 52) were randomly allocated to either a normal control (N= 26) or hyperhomocysteinemic (N= 26) group. Control and hyperhomocysteinemic groups had free access to either tap water or homocysteine thiolactone 50 mg/kg/day, during 10 or 44 weeks. Plasma homocysteine levels were determined by a high-performance liquid chromatography (HPLC) method. Glomerular filtration rate (GFR) and renal plasma flow (RPF) were calculated from inulin and sodium para-aminohippurate (PAH) clearance determinations. Structural renal changes were investigated in kidneys fixed by perfusion. Histopathologic and morphometric analysis were carried out by standard methods.

RESULTS

Plasma total homocysteine levels were 53% (10 weeks) and 56% (44 weeks) higher in hyperhomocysteinemic group compared to the control group. GFR and RPF were significantly lower in hyperhomocysteinemic than in control group. The histopathologic and morphometric studies did not show any differences between the control and hyperhomocysteinemic rats at 10 or 44 weeks.

CONCLUSION

The present results show that mild hyperhomocysteinemia is able to induce renal functional and biochemical alterations in male adult rats that are not associated with renal histopathologic changes.

Reduced plasma total homocysteine concentrations in Type 1 diabetes mellitus is determined by increased renal clearance

INTRODUCTION

Elevated plasma levels of total homocysteine are related to the development of vascular complications. Patients with diabetes mellitus are particularly at risk for the development of these complications. Several factors determine plasma total homocysteine including renal function.

AIMS

As early Type 1 diabetes is characterized by a relative glomerular hyperfiltration, increased renal clearance could contribute to decreased levels of homocysteine as observed in Type 1 diabetes mellitus. Therefore we investigated the relationship between plasma total homocysteine and the glomerular filtration rate (GFR).

METHODS

In 92 Type 1 diabetes patients and 44 control subjects, we measured GFR and effective renal plasma flow (ERPF) by means of continuous infusion of inulin and p-aminohippurate. Fasting plasma total homocysteine was measured using high performance liquid chromatography.

RESULTS

GFR (121 +/- 21 resp. 104 +/- 14 ml/min; P < 0.001) and ERPF (563 +/- 127 resp. 516 +/- 121 ml/min; P = 0.05) were significantly higher in Type 1 diabetes patients as compared with control subjects. Plasma total homocysteine was reduced in Type 1 diabetes patients as compared with control subjects (11.0 +/- 4.5 resp. 13.4 +/- 7 micromol/l; P = 0.01). Plasma total homocysteine was strongly correlated with GFR (Type 1 diabetes patients: r = -0.43, P < 0.001; control subjects: r = -0.39, P = 0.01).

CONCLUSION

GFR is a major determinant of plasma total homocysteine levels in Type 1 diabetes patients as well as control subjects. The reduced plasma total homocysteine levels in diabetes patients can be explained by an increased GFR.

Homocysteine, glycine betaine, and N,N-dimethylglycine in patients attending a lipid clinic

We recruited nondiabetic subjects (n = 158) attending a lipid disorders clinic, a subset of whom (n = 46) had established cardiovascular disease. Glycine betaine, N,N-dimethylglycine, and carnitine were measured in fasting plasma and urine samples. The concentrations and excretions were related to known cardiovascular risk factors in multivariate regression models. The relationships between homocysteine and plasma and urinary glycine betaine were highly significant (P < .002), comparable with the known relationships with folate and plasma creatinine. The regression coefficient for plasma glycine betaine was consistently approximately -0.1 in 5 different regression models (3 best-subsets and forward and backward stepwise regression models) for predicting homocysteine using 23 variables. Plasma glycine betaine was higher in males than in females, and the difference was associated with a difference in percentage of body fat. Its concentration included a constant factor of approximately 20 micromol/L that was independent of any of the variables investigated here. In the total group, body fat, homocysteine, and carnitine were significant predictors of plasma glycine betaine. Carnitine, an important betaine that is involved in lipid metabolism positively correlated with both homocysteine and glycine betaine. In our sample, the urinary excretion of glycine betaine was outside the reference range in 14 of the 158 subjects and the betaine fractional clearances were above the reference range in 23 subjects. Fractional clearance correlated strongly with plasma homocysteine (r = 0.50), and this relationship may be stronger in patients with known vascular disease. Urinary loss of glycine betaine may contribute to hyperhomocysteinemia and the development of cardiovascular disease.

Cystathionine beta-synthase deficiency: effects of betaine supplementation after methionine restriction in B6-nonresponsive homocystinuria

PURPOSE

For treatment of cystathionine beta-synthase (CbetaS) deficiency, we determined the effect of betaine (N,N,N-trimethylglycine) therapy and examined the genotype-phenotype relationships to betaine.

METHODS

In five patients with B6-nonresponsive homocystinuria, we defined the CbetaS genotypes and determined metabolic responses to betaine as an additive to traditional dietary methionine restriction.

RESULTS

After betaine therapy, tHcy declined (mean 47.4 micromol/L; range: -21.2 to -104.0 micromol/L; P=0.02), whereas total plasma cysteine and methionine did not change. Plasma methionine/tHcy ratios increased by 5.45 (range: +1.5 to 15.3; P=0.05) inpatients with B6-nonresponsive alleles.

CONCLUSION

Betaine improves metabolic control in B6-nonresponsive patients with homocystinuria after optimum dietary control.

Homocysteine, system b0,+ and the renal epithelial transport and toxicity of inorganic mercury

Proximal tubular epithelial cells are major sites of homocysteine (Hcy) metabolism and are the primary sites for the accumulation and intoxication of inorganic mercury (Hg(2+)). Previous in vivo data from our laboratory have demonstrated that mercuric conjugates of Hcy are transported into these cells by unknown mechanisms. Recently, we established that the mercuric conjugate of cysteine [2-amino-3-(2-amino-2-carboxy-ethylsulfanylmercuricsulfanyl)propionic acid; Cys-S-Hg-S-Cys], is transported by the luminal, amino acid transporter, system b(0,+). As Cys-S-Hg-S-Cys and the mercuric conjugate of Hcy (2-amino-4-(3-amino-3-carboxy-propylsulfanylmercuricsulfanyl)butyric acid; Hcy-S-Hg-S-Hcy) are similar structurally, we hypothesized that Hcy-S-Hg-S-Hcy is a substrate for system b(0,+). To test this hypothesis, we analyzed the saturation kinetics, time dependence, temperature dependence, and substrate specificity of Hcy-S-Hg-S-Hcy transport in Madin-Darby canine kidney (MDCK) cells stably transfected with system b(0,+). MDCK cells are good models in which to study this transport because they do not express system b(0,+). Uptake of Hg(2+) was twofold greater in the transfectants than in wild-type cells. Moreover, the transfectants were more susceptible to the toxic effects of Hcy-S-Hg-S-Hcy than wild-type cells. Accordingly, our data indicate that Hcy-S-Hg-S-Hcy is transported by system b(0,+) and that this transporter likely plays a role in the nephropathy induced after exposure to Hg(2+). These data are the first to implicate a specific, luminal membrane transporter in the uptake and toxicity of mercuric conjugates of Hcy in any epithelial cell.

Betaine in human nutrition

Betaine is distributed widely in animals, plants, and microorganisms, and rich dietary sources include seafood, especially marine invertebrates ( approximately 1%); wheat germ or bran ( approximately 1%); and spinach ( approximately 0.7%). The principal physiologic role of betaine is as an osmolyte and methyl donor (transmethylation). As an osmolyte, betaine protects cells, proteins, and enzymes from environmental stress (eg, low water, high salinity, or extreme temperature). As a methyl donor, betaine participates in the methionine cycle-primarily in the human liver and kidneys. Inadequate dietary intake of methyl groups leads to hypomethylation in many important pathways, including 1) disturbed hepatic protein (methionine) metabolism as determined by elevated plasma homocysteine concentrations and decreased S-adenosylmethionine concentrations, and 2) inadequate hepatic fat metabolism, which leads to steatosis (fatty accumulation) and subsequent plasma dyslipidemia. This alteration in liver metabolism may contribute to various diseases, including coronary, cerebral, hepatic, and vascular diseases. Betaine has been shown to protect internal organs, improve vascular risk factors, and enhance performance. Databases of betaine content in food are being developed for correlation with population health studies. The growing body of evidence shows that betaine is an important nutrient for the prevention of chronic disease.

Elevated plasma homocysteine is positively associated with age independent of C677T mutation of the methylenetetrahydrofolate reductase gene in selected Egyptian subjects

This study aimed to evaluate the plasma homocysteine (tHcy) and folate levels as well as the methylenetetrahydrofolate reductase (MTHFR) C677T mutation in Egyptian subjects. Fasting total homocysteine (tHcy) and the (MTHFR) C677T mutation were evaluated in 50 healthy young control males (age 35-50 years, Gp1), 50 elderly males age ranged between 50-75 years without any cardiovascular diseases (Gp2) and 50 age matched elderly male patients (Gp3) with myocardial infarction. There was a significant elevation of plasma tHcy in the patients group and Gp2 compared to the young control group (Gp1). The total plasma homocysteine (tHcy) in the control group, Gp2 and the patients group were 17.99 +/- 9.76, 39.9 +/- 20.06 and 43.8 +/- 13.13 mumol/L respectively. The frequency of the TT genotype was 12% in the patient group compared with 8 % in the young healthy controls and elderly subjects (Gp2). The CT genotype constituted 36%, 48% and 44% in the control group, Gp2 and the patients group respectively. There was no significant difference in the occurrence of the TT genotype between the studied groups. Plasma tHcy correlated positively with age, total cholesterol, urea, creatinine, glucose levels and carotid intimal thickness (CIT). Conclusion: The MTHFR mutation does not seem to be associated with either high tHcy or the occurrence of cardiovascular diseases in the studied patients. However, elevated plasma tHcy level positively correlates with age in the studied subjects.

Determinants of enzymatic specificity in the Cys-Met-metabolism PLP-dependent enzymes family: crystal structure of cystathionine gamma-lyase from yeast and intrafamiliar structure comparison

The crystal structure of cystathionine gamma-lyase (CGL) from yeast has been solved by molecular replacement at a resolution of 2.6 A. The molecule consists of 393 amino acid residues and one PLP moiety and is arranged in the crystal as a tetramer with D2 symmetry as in other related enzymes of the Cys-Met-metabolism PLP-dependent family like cystathionine beta-lyase (CBL). A structure comparison with other family members revealed surprising insights into the tuning of enzymatic specificity between the different family members. CGLs from yeast or human are virtually identical at their active sites to cystathionine gamma-synthase (CGS) from E. coli. Both CGLs and bacterial CGSs exhibit gamma-synthase and gamma-lyase activities depending on their position in the metabolic pathway and the available substrates. This group of enzymes has a glutamate (E333 in yeast CGL) which binds to the distal group of cystathionine (CTT) or the amino group of cysteine. Plant CGSs use homoserine phosphate instead of O-succinyl-homoserine as one substrate. This is reflected by a partially different active site structure in plant CGSs. In CGL and CBL the pseudosymmetric substrate must dock at the active site in different orientations, with S in gamma-position (CBL) or in delta-position (CGL). The conserved glutamate steers the substrate as seen in other CGLs. In CBLs this position is occupied by either tyrosine or hydrophobic residues directing binding of CTT such that S is in the in gamma-position. In methionine gamma-lyase a hydrophic patch operates as recognition site for the methyl group of the methionine substrate.

Relation of blood homocysteine and its nutritional determinants to age-related maculopathy in the third National Health and Nutrition Examination Survey

BACKGROUND

Blood homocysteine and its nutritional determinants folate and cyanocobalamin (vitamin B-12) have been shown to affect the risk of vascular disease. The pathogenesis of age-related maculopathy (ARM) is related to adverse vascular changes.

OBJECTIVE

The objective was to evaluate the associations between homocysteine, its nutritional determinants, and ARM in persons aged >or= 40 y participating in the third National Health and Nutrition Examination Survey.

DESIGN

A nonmydriatic fundus photograph of one eye, taken in a mobile examination center, was used to ascertain ARM status. Phlebotomy was performed for measurement of homocysteine, cyanocobalamin, and erythrocyte folate in participants of phase 2 of the survey (n = 3828). Logistic regressions were used to compute odds ratios and 95% CIs by quintile of serum analyte by using sample weights and jackknife replication methods to adjust for the complex survey design. The final analyses were adjusted for potential risk factors that influenced odds ratios.

RESULTS

Total serum homocysteine, red blood cell folate, and serum cyanocobalamin were unrelated to ARM in the overall sample. However, red blood cell folate was inversely related to one type of early ARM lesion (soft drusen) in non-Hispanic blacks.

CONCLUSIONS

ARM does not appear to be associated with homocysteine or its dietary determinants in this survey. There is a need for further investigation to rule out potential associations in subgroups with low folate status that may not have been detected because of the cross-sectional survey design.

Quantitation of choline and its metabolites in tissues and foods by liquid chromatography/electrospray ionization-isotope dilution mass spectrometry

Choline is important for normal membrane function, acetylcholine synthesis, lipid transport, and methyl metabolism. The U.S. National Academy of Sciences recently set requirements for choline in the human diet. In tissues and foods, there are multiple choline compounds that contribute to choline content. Betaine, a derivative of choline, is also important because of its role in donation of methyl groups to homocysteine to form methionine. Radioisotopic, high-pressure liquid chromatography, and gas chromatography/isotope dilution mass spectrometry (GC/IDMS) methods are available for measurement of choline. However, these existing methods are cumbersome and time-consuming, and none measures all of the compounds of interest. In this study, we describe a new method for quantitation of choline, betaine, acetylcholine, glycerophosphocholine, cytidine diphosphocholine, phosphocholine, phosphatidylcholine, and sphingomyelin in liver, plasma, various foods, and brain using liquid chromatography/electrospray ionization-isotope dilution mass spectrometry (LC/ESI-IDMS). Choline compounds were extracted by and partitioned into organic and aqueous phases using methanol and chloroform and analyzed directly by LC/ESI-IDMS without the need for isolation and derivatization of each compound separately as was required by the GC/IDMS method. The new LC/ESI-IDMS method was validated using the existing published GC/IDMS method.

Selenium deficiency in Fisher-344 rats decreases plasma and tissue homocysteine concentrations and alters plasma homocysteine and cysteine redox status

The purpose of the present study was to determine the effect of graded amounts of dietary selenium on plasma and tissue parameters of methionine metabolism including homocysteine. Male weanling Fisher-344 rats (n = 7-8/group) were fed a selenium-deficient, torula yeast-based diet, supplemented with 0 (selenium deficient), 0.02, 0.05 or 0.1 microg (adequate) selenium (as selenite)/g diet. After 61 d, plasma total homocysteine and cysteine were decreased (P < 0.0001) and glutathione increased (P < 0.0001) by selenium deficiency. The concentrations of homocysteine in kidney and heart were decreased (P = 0.02) by selenium deficiency. The activities of liver betaine homocysteine methyltransferase, methionine synthase, S-adenosylmethionine synthase, cystathionine synthase and cystathionase were determined; selenium deficiency affected only betaine homocysteine methyltransferase, which was decreased (P < 0.0001). The ratios of plasma free reduced homocysteine (or cysteine) to free oxidized homocysteine (or cysteine) or to total homocysteine (or cysteine) were increased by selenium deficiency, suggesting that selenium status affects the normally tightly controlled redox status of these thiols. Most differences due to dietary selenium were between rats fed 0 or 0.02 microg selenium/g diet and those fed 0.05 or 0.1 microg selenium/g diet. The metabolic consequences of a marked decrease in plasma homocysteine and smaller but significant decreases in tissue homocysteine are not known.

Homocysteine and cardiovascular disease in diabetes mellitus

BACKGROUND

Patients with diabetes mellitus (DM) have 2- to 6-fold increase in the prevalence of cardiovascular disease (CVD) compared to non-DM subjects. Epidemiological data show that DM is synergic with other conventional risk factors. Total plasma homocysteine (tHcy) is an emerging CVD risk factor. We reviewed the literature to explore the relation between tHcy and CVD in patients with DM.

METHODS

We searched the MEDLINE database for articles on homocysteine, DM and CVD published from January 1991 to October 2000.

RESULTS

The mean plasma tHcy level is usually low or normal in DM patients, except when nephropathy is present. Levels in that case tend to be higher than in non-DM patients. An independent association with tHcy and CVD was shown in retrospective studies, for DM patients. Prospective studies showed an association between elevated tHcy and all cause mortality in DM patients. In general, the association between elevated levels of tHcy and the outcome was stronger than in non-DM individuals, for all types of study.

DISCUSSION

To date, there are no prospective work that specifically examined the relationship between levels of tHcy and the presence of CVD in the DM population. Nor are there studies to show that treating elevated tHcy results in a reduction of CVD events. Such studies are ongoing. Nevertheless, since hyperhomocysteinemia is potentially reversible with vitamin therapy, interaction of DM with high levels tHcy on the risk of CVD may have consequences with regard to management of primary and secondary prevention in DM patients who are at particularly high risk of CVD events.

Homocysteine and endogenous markers of renal function in type 2 diabetic patients without coronary heart disease

The aim of this study was to assess parameters of renal function and other determinants of plasma homocysteine in type 2 diabetic patients without coronary heart disease (CHD). Fasting plasma homocysteine, serum cystatin C and serum creatinine were determined in 183 (75 men, 108 women) Type 2 diabetic patients without clinical evidence of CHD. Creatinine clearance was calculated and parameters such as blood pressure, body mass index (BMI), and glycated haemoglobin (HbA(1c)) were assessed. The urine albumin:creatinine ratio was used to classify patients as normo-, micro- or macroalbuminuric. One hundred and ten patients were normoalbuminuric, 67 patients were microalbuminuric and six patients were macroalbuminuric. There was no statistically significant difference in plasma homocysteine concentration between patients with normoalbuminuria and microalbuminuria. There was a trend towards increasing plasma homocysteine with decreasing glomerular filtration rate (GFR) (r=-0.46; P<0.0001). There was statistically significant correlation between plasma homocysteine and age (r=0.37), serum cystatin C (r=0.47), and serum creatinine (r=0.56). Plasma homocysteine concentration was significantly higher in patients with BMI<30 kg/m(2) and showed significant inverse correlation with weight (r=-0.16; P=0.03) and body mass index (r=-0.24; P=0.001). Homocysteine and serum creatinine were significantly higher in males than females and higher in smokers than non smokers but was not associated with glycemic control and duration of diabetes. In conclusion, elevated homocysteine concentration in patients with type 2 DM without CHD is related to age, gender, smoking, BMI and GFR. Follow up studies will provide further information on the association between hyperhomocysteinemia and the development of cardiovascular disease.

Co-administration of equimolar doses of betaine may alleviate the hepatotoxic risk associated with niacin therapy

High-dose niacin has versatile and substantial efficacy for the treatment of hyperlipidemias, but its utility is compromised by various side effects, the most serious of which is liver damage. It is proposed that this hepatotoxicity reflects the high demand for methyl groups imposed by niacin catabolism, leading to a reduction in hepatic levels of S-adenosylmethionine (SAM). Depletion of the hepatic SAM pool has likewise been shown to mediate, at least in part, the hepatotoxic effects of ethanol, methotrexate, and niacinamide. If niacin does indeed decrease SAM, a likely consequence would be a counterproductive elevation of plasma homocysteine. Conceivably, methyl group deficiency, by altering membrane properties of skeletal muscle, also contributes to niacin-induced insulin resistance. Concurrent betaine supplementation - preferably administered as a complex with equimolar amounts of niacin - may represent the most cost-effective way to prevent niacin-mediated depletion of SAM and thus avoid hepatotoxicity (and possibly other adverse effects) while controlling homocysteine. Betaine also merits evaluation as an adjuvant to methotrexate and niacinamide therapies.

Hyperhomocyst(e)inemia in chronic stable renal transplant patients

PURPOSE

Hyperhomocyst(e)inaemia is an important risk factor for atherosclerosis, which is currently a major cause of death in renal transplant patients. The aim of this study was to assess the influence of immunosuppressive therapy on homocyst(e)inemia in renal transplant recipients.

METHODS

Total serum homocysteine (by high performance liquid chromatography), creatinine, lipid profile, folic acid (by radioimmunoassay-RIA) and vitamin B12 (by RIA) concentrations were measured in 3 groups. Group I patients (n=20) were under treatment with cyclosporine, azathioprine, and prednisone; group II (n=9) were under treatment with azathioprine and prednisone; and group III (n=7) were composed of renal graft donors for groups I and II. Creatinine, estimated creatinine clearance, cyclosporine trough level, lipid profile, folic acid, and vitamin B12 concentrations and clinical characteristics of patients were assessed with the aim of ascertaining determinants of hyperhomocyst(e)inemia.

RESULTS

Patient ages were 48.8 +/- 15.1 yr (group I), 43.3 +/- 11.3 yr (group II); and 46.5 +/- 14.8 yr (group III). Mean serum homocyst(e)ine (tHcy) concentrations were 18.07 +/- 8.29 mmol/l in renal transplant recipients; 16.55 +/- 5.6 mmol/l and 21.44 +/- 12.1 mmol/l respectively for group I (with cyclosporine) and group II (without cyclosporine) (NS). In renal donors, tHcy was significantly lower (9.07 +/- 3.06 mmol/l; group I + group II vs. group III, p<0.008). There was an unadjusted correlation (p<0.10) between age (r=0.427; p<0.005) body weight (r=0.412; p<0.05), serum creatinine (r=0.427; p<0.05), estimated creatinine clearance (r=0.316; p<0.10), and tHcy in renal recipients (group I +II). Independent regressors (r2=0.46) identified in the multiple regression model were age (coefficient= 0.253; p=0.009) and serum creatinine (coefficient=8.07; p=0.045). We found no cases of hyperhomocyst(e)inemia in the control group. In contrast, 38% of renal recipients had hyperhomocyst(e)inemia: 7 cases (35%) on cyclosporine and 4 (45%) without cyclosporine, based on serum normal levels.

CONCLUSIONS

Renal transplant recipients frequently have hyperhomocyst(e)inemia. Hyperhomocyst(e)inemia in renal transplant patients is independent of the scheme of immunosuppression they are taking. The older the patients are and the higher are their serum creatinine levels, the more susceptible they are to hyperhomocyst(e)inemia following renal transplantation.

Homocysteine and risk of cardiovascular disease

This pictorial introduction to homocysteine illustrates at a glance the nature of homocysteine and its role in cardiovascular disease by means of eight simple figures and an essential bibliography. Homocysteine is a sulfur-containing metabolite of methionine. Conversion back to methionine or transsulfuration to cysteine are the two major metabolic pathways that reduce total homocysteine (tHcy) concentrations in cells and blood. B vitamins are essential cofactors in homocysteine metabolism. Median fasting total homocysteine levels in adult males are approximately 10 micromol/L. Increased plasma tHcy concentrations are found with methionine-rich diets, low vitamin B intake, male gender, age, impaired renal function, and genetically determined defects of the enzymes involved in homocysteine metabolism. An inverse relation exists between plasma tHcy and circulating folate or vitamin B(6) concentrations, and folic acid supplements of 0.5 mg/d can reduce tHcy levels by approximately 25%. Homocystinuric patients, who have severe hyperhomocysteinemia, die prematurely of atherothrombotic disease. Many (but not all) cross-sectional and prospective studies indicate, on average, that plasma tHcy levels <.10 micromol/L are associated with, or predict the development of, coronary, cerebral, and peripheral vascular disease. The risk conferred by hyperhomocysteinemia is graded and is independent of traditional risk factors, with an estimated odds ratio for ischemic heart disease of 1.4 for every 5 micromol/L increase in plasma tHcy. In vitro and in vivo, tHcy has been found to impair endothelial function. It is now well established that tHcy represents a marker of current or subsequent ischemic vascular disease. However, irrefutable proof that hyperhomocysteinemia actually causes atherothrombosis will come only if interventions to lower plasma tHcy will produce concomitant reductions in cardiovascular events.

Betaine in wine: answer to the French paradox?

In France, a diet high in saturated fat and cholesterol is associated with low coronary artery disease mortality and it may be that drinking wine is protective against ischemic heart disease. Recent studies suggest that high plasma homocysteine concentrations are an independent risk factor for coronary, cerebral and peripheral arterial occlusive diseases. One of several routes for metabolism of homocysteine involves methylation using betaine as the methyl donor. Betaine is often added to less expensive wine when beet sugar is used to increase alcohol content. We found that many commercial wines contain betaine; an average glass of wine contains approximately 3 mg betaine. This small amount is less than the dose used to lower homocysteine in patients with genetic forms of hyperhomocysteinemia, but we do not know whether humans with modest elevations of homocysteine would be influenced by this dose.

Kinetics and inhibition of recombinant human cystathionine gamma-lyase. Toward the rational control of transsulfuration

The gene encoding human cystathionine gamma-lyase was cloned from total cellular Hep G2 RNA. Fusion to a T7 promoter allowed expression in Escherichia coli, representing the first mammalian cystathionine gamma-lyase overproduced in a bacterial system. About 90% of the heterologous gene product was insoluble, and renaturation experiments from purified inclusion bodies met with limited success. About 5 mg/liter culture of human cystathionine gamma-lyase could also be extracted from the soluble lysis fraction, employing a three-step native procedure. While the enzyme showed high gamma-lyase activity toward L-cystathionine (Km = 0.5 mM, Vmax = 2.5 units/mg) with an optimum pH of 8.2, no residual cystathionine beta-lyase behavior and only marginal reactivity toward L-cystine and L-cysteine were detected. Inhibition studies were performed with the mechanism-based inactivators propargylglycine, trifluoroalanine, and aminoethoxyvinylglycine. Propargylglycine inactivated human cystathionine gamma-lyase much more strongly than trifluoroalanine, in agreement with the enzyme's preference for C-gamma-S bonds. Aminoethoxyvinylglycine showed slow and tight binding characteristics with a Ki of 10.5 microM, comparable with its effect on cystathionine beta-lyase. The results have important implications for the design of specific inhibitors for transsulfuration components.