No net renal extraction of homocysteine in fasting humans

Assessing Plasma Total Homocysteine in Patients with End-Stage Renal Disease

Elevated plasma total homocysteine (tHcy) is a risk factor for cardiovascular disease; however, in light of several recent randomized trials, the issue of causality has been cast into doubt. Patients with end-stage renal disease are particularly interesting as they consistently have elevated tHcy and their leading causes of morbidity and mortality are related to cardiovascular disease. In the present article, we review the early evidence for the homocysteine theory of atherosclerosis, homocysteine metabolism, mechanisms of toxicity, and pertinent available clinical investigations. Where appropriate, the sparse evidence of homocysteine in peritoneal dialysis is reviewed. We conclude by addressing the difficulties associated with lowering plasma tHcy in patients with end-stage renal disease and suggest some novel methods for lowering tHcy in this resistant population. Finally, to address the issue of causality, we recommend that clinicians and scientists await the results of the FAVORIT trial before abandoning homocysteine as a modifiable risk factor for cardiovascular disease, as this study has recruited patients from a population with consistently elevated plasma tHcy who are known to respond to vitamin therapy.

The relationship between the concentration of plasma homocysteine and chronic kidney disease: a cross sectional study of a large cohort

BACKGROUND

High concentrations of homocysteine are considered a risk factor for developing atherosclerosis and coronary artery disease. The aim of this study was to assess the concentrations of homocysteine in subjects with chronic kidney disease (CKD).

METHODS

Data were collected from medical records of individuals examined at a screening center in Israel between the years 2000-2014. Cross sectional analysis was carried out on 17,010 subjects; 67% were men.

RESULTS

Significant differences were observed between four quartiles of homocysteine concentrations and estimated glomerular filtration rate (eGFR)-the higher the homocysteine concentration, the lower the eGFR (p < 0.0001). In subjects with CKD, homocysteine plasma levels were correlated with the stage of renal impairment. Mean (SD) homocysteine concentrations in subjects with eGFR < 60 mL/min per 1.73 m² compared to subjects with eGFR ≥ 60 mL/min per 1.73 m² were: 16.3 (5.9) vs. 11.5 (5.5) μmol/L respectively. These findings remained significant after adjustment for age, smoking status, body mass index, hypertension and diabetes mellitus (p < 0.0001). Compared to subjects with homocysteine concentrations less than 15 μmol/L, those with homocysteine concentrations equal and above 15 μmol/L, had a significantly higher odds ratio (95% CI) of having an eGFR < 60 mL/min per 1.73 m²; non adjusted model, 8.30 (6.17-11.16); adjusted model for age smoking status, body mass index, hypertension and diabetes mellitus, 7.43 (5.41-10.21).

CONCLUSION

Plasma homocysteine concentrations are higher in subjects with CKD. This may contribute to an increased risk for developing atherosclerosis and coronary artery disease in these patients.

In Vivo Stable Isotope Measurements of Methyl Metabolism: Applications in Pathophysiology and Interventions

With intravenous infusion of doubly-labeled [2H3C-1-13C-] methionine and stable isotope enrichments in plasma free methionine and carbon dioxide in breath air, whole body transmethylation, transsulfuration, and remethylation rates can be calculated. This technique demonstrated impaired recycling as the major disturbance to explain hyperhomocysteinemia in patients with end-stage renal failure, and can be used to optimize interventions with folate, B6, and B12 supplementation in this patient group. Intravenous infusion of [2,3,3-2H3] serine has also been applied to demonstrate the appearance of [2H2]- as well as [2H1]-methionine in plasma and protein, suggesting transfer of a one-carbon group from serine via 5,10-methylenetetrahydrofolate in human hepatocyte cytosol and mitochondria, respectively. In sheep, tissue free methionine enrichments after infusion of universally labeled [U-13C] methionine showed the highest remethylation activity in postmortem investigation of jejunum, liver, and kidney tissue samples, but no such activity in muscle and brain samples. Methods to quantitate one-carbon acceptor metabolism pathways and folate metabolism have recently become available.

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

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

Uremic Toxicity: Present State of the Art

The uremic syndrome is a complex mixture of organ dysfunctions, which is attributed to the retention of a myriad of compounds that under normal condition are excreted by the healthy kidneys (uremic toxins). In the area of identification and characterization of uremic toxins and in the knowledge of their pathophysiologic importance, major steps forward have been made during recent years. The present article is a review of several of these steps, especially in the area of information about the compounds that could play a role in the development of cardiovascular complications. It is written by those members of the Uremic Toxins Group, which has been created by the European Society for Artificial Organs (ESAO). Each of the 16 authors has written a state of the art in his/her major area of interest.

Anethole dithiolethione lowers the homocysteine and raises the glutathione levels in solid tissues and plasma of rats: a novel non-vitamin homocysteine-lowering agent

High homocysteine (Hcys) levels are suspected to contribute to the pathogenesis of cardiovascular disease and of other chronic conditions. Failure of B vitamins to reduce the incidence of cardiovascular events while lowering the Hcys levels, has prompted the search for alternative treatments. We tested the ability of anethole dithiolethione (ADT) to lower the Hcys levels in rats and we explored possible underlying mechanisms. Parenteral administration of 10mg/kg ADT to normal rats for 3 days lowered the Hcys levels between 51.4% and 31.5% in kidneys, liver, testis and plasma. Concomitantly, glutathione (GSH) increased between 112% and 28% in kidneys, brain, liver and plasma whereas protein thiolation index decreased 30%. In hyperhomocysteinemic rats, the plasma Hcys levels dropped 70% following a single ip injection of 10mg/kg ADT, while they decreased 55% following oral administration of 2mg/kg/day ADT for one week. Significant additive effects occurred when sub-therapeutic doses of ADT and folic acid were used in combination. To test the possible mechanism(s) of these actions, we perfused isolated rat livers and kidneys with albumin-bound Hcys, the prevalent form of plasma Hcys, and physiological thiols and disulfides at different ratios. In both organ preparations, the elimination rate of albumin-bound Hcys was progressively faster as the amount of reduced thiols was increased in the perfusate. These findings indicate that ADT shifts the redox ratio of GSH and other thiols with their oxidized forms toward the reduced forms, thus favoring the dissociation of albumin-bound Hcys and its transfer to renal and hepatic cells for further processing.

Homocysteine metabolism in diabetes

An increase in the plasma level of Hcy (homocysteine), an intermediate in the catabolism of methionine, has been identified as a risk factor for many diseases including CVD (cardiovascular disease). CVD is the major cause of death in patients with diabetes mellitus. Therefore the study of Hcy metabolism in diabetes mellitus has been a major focus of current research. Studies conducted in our laboratory were able to show that in both Type 1 and Type 2 diabetes with no renal complications, the plasma Hcy levels were lower than in controls. In Type 1 diabetes, increased activities of the trans-sulfuration enzymes were the major cause for the reduction in plasma Hcy. In Type 2 diabetes, BHMT (betaine:homocysteine methyltransferase) was also observed to play a major role in the increased catabolism of Hcy in addition to the trans-sulfuration enzymes. We were also able to demonstrate the direct effect of insulin and the counter-regulatory hormones on the regulation of cystathionine beta-synthase and BHMT, which accounts for the changes in the activities of these two enzymes seen in diabetes mellitus.

S-Adenosylhomocysteine: a better indicator of vascular disease than homocysteine?

It is widely accepted that elevated plasma total homocysteine is an independent risk factor for vascular disease. The relation is believed to be causal, but there is no generally accepted mechanism for the pathophysiology involved. The metabolic precursor of homocysteine in all tissues is S-adenosylhomocysteine (AdoHcy). AdoHcy is present in normal human plasma at concentrations approximately 1-500th of those of homocysteine, a fact that presents difficulties in measurement. The requirement for specialized equipment, complicated time-consuming methodology, or both is a reason that measurement of plasma AdoHcy has not generally been carried out in large studies. A recently published rapid immunoassay for AdoHcy in human plasma should make measurement of this important metabolite available for general use. Advantages of the measurement of plasma AdoHcy include 1) a smaller overlap of values between control subjects and patients, and thus the possibility of observing significant differences in fewer samples, 2) an accepted mechanism of metabolic activity as an inhibitor of all S-adenosylmethionine-mediated methyltransferases, and 3) evidence (from recent studies) that a higher plasma concentration of AdoHcy is a more sensitive indicator of vascular disease than is a higher plasma concentration of homocysteine.

Plasma creatinine as a determinant of plasma total homocysteine concentrations in the Hordaland Homocysteine Study: Use of statistical modeling to determine reference limits

OBJECTIVES

We established population-based reference limits for plasma total homocysteine (tHcy) according to creatinine.

DESIGN AND METHODS

In 7042 middle-aged and elderly subjects from the Hordaland Homocysteine Study, we used statistical modeling to establish nomograms for tHcy according to creatinine in the whole population and in folate-replete and healthy subgroups.

RESULTS

When plotted against creatinine, tHcy 97.5th percentile almost overlapped in men and women, and, in elderly, increased up to 8 micromol/L from the 2.5th to 97.5th creatinine percentiles. Folate-replete subjects had tHcy upper limits approximately 20% below the whole population at all creatinine levels. Healthy subjects had lower creatinine, but at a given creatinine level, tHcy was the same as in the whole population.

CONCLUSIONS

tHcy difference between men and women is mostly explained by creatinine. The tHcy-reducing effect of folate is independent of creatinine. In elderly people, creatinine should be taken into account when assessing tHcy levels.

Mutations in methylenetetrahydrofolate reductase or cystathionine beta-synthase gene, or a high-methionine diet, increase homocysteine thiolactone levels in humans and mice

Genetic disorders of homocysteine (Hcy) metabolism or a high-methionine diet lead to elevations of plasma Hcy levels. In humans, severe genetic hyperhomocysteinemia results in premature death from vascular complications whereas dietary hyperhomocysteinemia is often used to induce atherosclerosis in animal models. Hcy is mistakenly selected in place of methionine by methionyl-tRNA synthetase during protein biosynthesis, which results in the formation of Hcy-thiolactone and initiates a pathophysiological pathway that has been implicated in human vascular disease. However, whether genetic deficiencies in Hcy metabolism or a high-methionine diet affect Hcy-thiolactone levels in mammals has been unknown. Here we show that plasma Hcy-thiolactone is elevated 59-fold and 72-fold in human patients with hyperhomocysteinemia secondary to mutations in methylenetetrahydrofolate reductase and cystathionine beta-synthase genes, respectively. We also show that mice, like humans, eliminate Hcy-thiolactone by urinary excretion; in contrast to humans, however, mice also eliminate significant amounts of plasma total Hcy (approximately 38%) by urinary excretion. In mice, hyperhomocysteinemia secondary to a high-methionine diet leads to 3.7-fold and 25-fold increases in plasma and urinary Hcy-thiolactone levels, respectively. Thus, we conclude that hyperhomocysteinemia leads to significant increases in the atherogenic metabolite Hcy-thiolactone in humans and mice.

Homocysteine levels are associated with increased risk of congestive heart failure in patients with and without coronary artery disease

BACKGROUND

Increased homocysteine (HCY) is associated with increased risk of vascular disease. Whether HCY affects development of congestive heart failure (CHF) independent of coronary artery disease (CAD) is uncertain. We evaluated whether increased HCY predicts low ejection fraction or clinical CHF.

METHODS

Patients (n = 2,842) undergoing coronary angiography had HCY measured between 1994 and 1999 and were prospectively studied. Left ventricular dysfunction (LVD) was defined as ejection fraction < or =40%. Multivariable regressions assessed predictive strength of HCY for LVD or LVD/CHF.

RESULTS

The average age was 64 +/- 12 years; 69% were men, and 74% had CAD. LVD was present in 12% and the combination of either LVD or clinical CHF was present in 21.9%. Quartiles of HCY were: < or =10.5 (Q1), 10.5-13.2 (Q2), 13.3-17.0 (Q3) and > or =17.1 micromol/l (Q4). LVD and LVD/CHF were more prevalent in Q3 (15, 25%) and Q4 (15, 27%) than in Q1 HCY (8.4, 18%; p < 0.001 vs. Q4). After adjustment, Q3 and Q4 HCY independently predicted LVD (OR = 1.7, 95% CI 1.2-2.5, p = 0.004; OR = 1.8, 95% CI 1.3-2.6, p = 0.002) or LVD/CHF (OR = 1.4, 95% CI 1.04-1.8, p = 0.03; OR = 1.7, 95% CI 1.3-2.2, p < 0.001). Findings did not differ by disease etiology: for Q4 among non-CAD patients, OR = 1.7 for LVD and OR = 1.7 for LVD/CHF. Further, there was no interaction of results with gender.

CONCLUSION

High HCY levels (Q3/4 > or =13.3 micromol/l) are associated with LVD and combined endpoint of LVD/clinical CHF. This relationship is independent of CHF etiology and gender. Further research is indicated to distinguish between a causal or noncausal mechanism for this association.

Relationship between plasma S-adenosylhomocysteine concentration and glomerular filtration rate in children

S-Adenosylhomocysteine (SAH) is the metabolic precursor of all the homocysteine (Hcy) produced in the body. It is formed by the enzyme SAH hydrolase in a reversible reaction. In a previous study we have shown that plasma SAH is a more sensitive indicator of the risk for cardiovascular disease, and in a second study involving patients with renal disease, we also showed that it is a more sensitive indicator of renal insufficiency than plasma Hcy. However, in the latter study, the patients with renal disease were older and had a variety of other diseases such as diabetes and primary hypertension, which are associated with vascular disease and which could reduce renal function by involvement of the kidneys. Our objective was to rule out these complicating factors as the cause of the elevated SAH in renal disease and determine whether renal insufficiency alone was the cause of the elevated SAH. We therefore measured SAH, Hcy, folate, and vitamin B12 in 23 patients between the ages of 1 and 18 years with a wide range of renal function, but who had none of these complicating factors. Glomerular filtration rate (GFR) was calculated using serum creatinine according to the Schwartz formula. None of the children were deficient in folate or vitamin B12. After adjusting for age, folate, and vitamin B12, there was a modest and insignificant decrease of 0.033 micromol/L of Hcy associated with an increase of 1 mL/min of GFR (95% confidence interval, -0.066 to 0.0002). However, there was a strong and statistically significant association between log(SAH) and log(GFR): P < .0005, R2 = 0.76. This result suggests that plasma SAH rather than Hcy is the metabolite primarily affected in renal disease. We suggest that plasma Hcy elevations that have been linked to vascular disease may be due to elevated SAH resulting from renal insufficiency.

Acute exposure to cyclosporine does not increase plasma homocysteine in rats

There is interest in the postulate that cyclosporine a (CsA) contributes to the elevated homocysteine levels seen in organ transplant recipients, as hyperhomocysteinemia is now considered an independent risk factor for cardiovascular disease (CVD) and may partially explain the increased prevalence of CVD in this population. The main purpose of this investigation was to determine the effect of CsA administration on plasma homocysteine. Eighteen female Sprague Dawley rats (4 months old) were randomly assigned to either a treatment or a control group. For 18 days the treatment group received of CsA (25 mg/kg/d) while the control group received the same volume of the vehicle. Blood samples obtained following sacrifice to measure CsA, total homocysteine, and plasma creatinine. There were no significant differences in plasma homocysteine (mean values +/- SD: treatment = 4.79 +/- 0.63 micromol/L, control = 4.46 +/- 0.75 micromol/L; P = .37). Homocysteine was not significantly correlated with final CsA concentrations (r = .17; P = .69). There was a significant difference in plasma creatinine values between the two groups (treatment = 60.44 +/- 7.68 micromol/L, control = 46.33 +/- 1.66 micromol/L; P < .001). Furthermore, plasma homocysteine and creatinine were positively correlated with the treatment group (r = .73; P < .05) but not the controls (r = -.10; P = .81). In conclusion, CsA does not influence plasma homocysteine concentrations in rats.

Homocysteine and vascular disease in diabetes: a double hit?

Cardiovascular disease is a major problem in diabetes, and risk factors presumably unrelated to diabetes, such as hyperhomocysteinaemia, may be related to the development of cardiovascular complications in diabetic individuals. Plasma homocysteine levels are usually normal in diabetes, although both lower and higher levels have been reported. Homocysteine levels in diabetes are modulated by hyperfiltration and renal dysfunction, as well as low folate status. Insulin resistance does not appear to be a major determinant of plasma homocysteine level. Hyperhomocysteinaemia has been associated with microalbuminuria and retinopathy in type 1 and type 2 diabetes. In patients with type 2 diabetes, plasma homocysteine concentration is a significant predictor of cardiovascular events and death. This relation seems to be stronger in subjects with diabetes than without. The underlying pathophysiological mechanism of this increased vascular risk remains unexplained, but may be related to worsening of endothelial dysfunction and/or structural vessel properties induced by oxidative stress. Because homocysteine and diabetes have apparent synergistic detrimental vascular effects, patients with diabetes are candidates for screening and treatment with folic acid until the results of ongoing clinical trials are available.

Effect of acute hyperhomocysteinemia on methylation potential of erythrocytes and on DNA methylation of lymphocytes in healthy male volunteers

Homocysteine is a precursor of S-adenosylmethionine (AdoMet) and a metabolite of S-adenosylhomocysteine (AdoHcy). The ratio of AdoMet to AdoHcy, defined as the methylation potential (MP), indicates the flow of methyl groups within the cells. Chronic elevations of total homocysteine (tHcy) in plasma correlate with increased AdoHcy concentrations, decreased MP, and impaired DNA methylation. However, the influence of acute hyperhomocysteinemia on MP is unknown. We induced acute hyperhomocysteinemia in 14 healthy volunteers by oral administration of l-homocysteine (65.1 μmol/kg body wt) in an open, randomized, placebo-controlled two-period crossover study. The kinetics of tHcy in blood and urine, MP in blood, and global DNA methylation in lymphocytes were studied systematically during 48 h. Plasma tHcy concentrations reached a peak at 34 ± 11 min after an oral load with l-homocysteine and decreased with a half-life of 257 ± 41 min (means ± SD). Only 2.3% of the homocysteine dose were recovered in urine. AdoHcy concentrations and MP in whole blood and erythrocytes were not affected by the oral homocysteine load. Furthermore, global DNA methylation in lymphocytes did not change under these conditions. We found no difference between the genotypes of 5,10-methylenetetrahydrofolate reductase in response to the homocysteine load. However, AdoMet content in erythrocytes was significantly higher in the C677T carriers (CT; n = 7) compared with the CC genotype ( n = 7). Although chronic elevation of tHcy has been shown to affect MP and DNA methylation, acute elevation of plasma tHcy above 20 μmol/l for 8 h is not sufficient to change MP and to induce DNA hypomethylation in lymphocytes.

Association between Hyperhomocysteinemia and Ultrasonographic Atherosclerotic Indices of Carotid Arteries in Chronic Hemodialysis Patients

BACKGROUND

Atherosclerotic cardiovascular events are a major cause of morbidity and the main cause of mortality in hemodialysis patients. Hyperhomocysteinemia--which is a consistent finding in uremic patients--is considered an independent risk factor for cardiovascular disease (CVD). However, the relationship between plasma homocysteine (Hcy) concentrations and atherosclerotic CVD has not been extensively investigated.

PATIENTS AND METHODS

37 patients undergoing chronic hemodialysis and 30 healthy individuals (control group), sex- and age-matched, were included in this study. Both healthy controls and hemodialysis patients underwent echo-Doppler carotid artery examination. The right and left carotid arteries were assessed separately. Our observation included measurements of the ultrasound images of the intimal wall thickness, the lumen diameter and the atherosclerotic plaques. We determined plasma Hcy, vitamin B12 and folic acid levels and serum cholesterol, triglycerides, HDL, ApoA-I, ApoB-100, Lp(a), CRP, albumin and creatinine levels in blood samples from both studied groups. We also determined the urea reduction ratio in the patient groups. The epidemiological as well as the biochemical data were correlated with the findings of the carotid artery examination.

RESULTS

Plasma Hcy levels were significantly increased in hemodialysis patients compared to controls (33 +/- 12.3 vs. 12.27 +/- 7.47 micromol/l, p < 0.001). Intimal wall thickness, lumen diameter and number of atherosclerotic plaques of both carotid arteries were significantly higher (p < 0.01 or p < 0.001) in patients compared to controls. There was a significant positive correlation between plasma Hcy levels and the number of the atherosclerotic plaques (r = 0.41, p < 0.01 in the right and r = 0.49, p < 0.001 in the left carotid artery). Lumen diameter was significantly (p < 0.01) associated with age, MAP and CRP levels. Significant correlations (p = 0.05-0.01) were also found between the number of the plaques and age as well as the duration of hemodialysis, while folic acid levels were inversely correlated with the number of the plaques.

CONCLUSIONS

Both hyperhomocysteinemia and atherosclerotic indices of the carotid arteries are more prevalent in hemodialysis patients compared to healthy controls. Elevated plasma Hcy levels were associated with the carotid artery atherosclerotic indices in chronic hemodialysis patients.

Maternal plasma homocysteine concentrations are not increased in twin pregnancies

OBJECTIVE(S)

We tested the hypothesis that twin pregnancies would lead to increased maternal plasma homocysteine. We further hypothesized that twin pregnancies complicated by preeclampsia would have increased plasma homocysteine compared to twin pregnancies without preeclampsia and normal singleton pregnancies.

METHODS

Plasma was collected at delivery from 127 nulliparous subjects: 57 women with normal singleton pregnancies, 39 women with singleton and preeclampsia, 17 women with uncomplicated twin pregnancies, and 14 women with twins and preeclampsia. Subjects were group matched for prepregnancy body mass index (BMI) and race. Plasma homocysteine was analyzed by high pressure liquid chromatography (HPLC) with fluorescence detection, and plasma folic acid was measured by radio immunoassay (RIA).

RESULTS

The mean plasma concentration of homocysteine was significantly increased in all women with preeclampsia (7.4+/-2.9 microM) compared to all normal pregnant women (5.9+/-2.1 microM, p=0.002). However, homocysteine was not significantly increased in all women with twins (6.7+/-2.1 microM) compared to all women with singleton pregnancies (6.5+/-2.7 microM, p=0.61). In addition, women with twins and preeclampsia did not have increased homocysteine (6.8+/-2.1 microM) compared to women with twins and normal pregnancy (6.7+/-2.1 microM, p=0.72). As expected, because of extra supplementation, plasma folic acid was significantly increased in women with twins (27.9+/-11.6 ng/mL) compared to women with singleton pregnancies (20.8+/-8.5 ng/mL, p=0.0003). However, folic acid was not different between preeclamptics and controls (23.5+/-10.8 vs. 21.9+/-9.2 ng/mL respectively, p=0.36). Lastly, there was a significant inverse correlation between homocysteine and folic acid among all the subjects (r2=- 0.053, p< 0.01), and this correlation persisted in the women with singleton pregnancies (r2=- 0.078, p< 0.01), but was lost in the twins (r2=- 0.073, p=0.14).

CONCLUSIONS

With contemporary management including increased folic acid supplementation, plasma homocysteine is not increased in twin pregnancies with or without preeclampsia.

Homocysteine, renal disease and cardiovascular disease in a remote Australian Aboriginal community

BACKGROUND

Rates of renal and cardiovascular disease are high among Aboriginal Australians living in remote communities. Nutritional problems, in particular low folate levels, are also common. This suggests that increased homocysteine concentrations might be widespread, and a possible contributor to the high rates of cardiovascular disease.

AIMS

To examine homocysteine concentrations, and their relationships to folate levels, and to markers of renal disease and cardiovascular disease in a remote Aboriginal Australian community

METHODS

As part of a cross-sectional survey among adults in one community, homocysteine concentrations, concentrations of the crucial determinants (red blood cell (RBC) folate, vitamin B(12) and the C677T methylene tetrahydrofolate reductase polymorphism) and cardiovascular risk factors were examined.

RESULTS

Among 221 people, geometric mean homocysteine concentration was 11.8 micromol/L (range: 11.1-12.5 micromol/L), with 57/221 (26%) values > or =15.0 micromol/L. Higher concentrations were associated with older age, male gender, lower RBC folate and lower vitamin B(12) concentrations and homozygosity for C677T. Homocysteine concentrations were not related to the presence of albuminuria, other than over the overt albuminuria range. Homocysteine concentrations were inversely correlated with calculated glomerular filtration rate (GFR). Carotid intima-media thickness, however, was not related to homocysteine concentration. In multivariate analyses, age, male gender, lower RBC folate concentrations, lower vitamin B(12) concentrations, lower calculated GFR and the C677T polymorphism were all associated with homocysteine concentrations.

CONCLUSIONS

Homocysteine concentrations were consistent with previous limited reports in Aboriginal communities. Although superficially they are similar to reports from non-Aboriginal settings, the younger age of this cohort and the association of homocysteine concentrations with age suggest that age-specific concentrations are higher among Aboriginal Australians. In addition to dietary determinants, the high prevalence of apparently reduced renal function renal disease appears to be an important determinant of homocysteine concentrations in remote Aboriginal communities. The role of homocysteine concentrations as a potential mediator of the high rates of cardiovascular disease remains to be determined.

Folic acid supplementation to prevent adverse events in individuals with chronic kidney disease and end stage renal disease

PURPOSE OF REVIEW

This review summarizes our current understanding of the role of folate in the treatment of hyperhomocysteindemia and the prevention of cardiovascular disease in patients with chronic kidney disease and end stage renal disease. Relevant papers published between 2003 and 2004 are referenced.

RECENT FINDINGS

With the exception of one paper, recent therapeutic studies supported previous findings that folate therapy achieves only a modest reduction in plasma homocysteine and seldom normalizes homocysteine. Large prospective studies are under way to evaluate the causal relationship between homocysteine and cardiovascular risk. Recent work supports earlier data that suggested that homocysteine inflicts its damage by oxidative stress. A newly described consequence of hyperhomocysteindemia is DNA hypomethylation and alteration of gene expression. A recent study in the general population suggested that while folate may lower homocysteine it does not improve endothelial function in individuals without cardiovascular disease.

SUMMARY

The causes of hyperhomocysteindemia in renal failure remain obscure. The possibilities include impairment of both renal and extrarenal metabolic pathways by uraemia. Hyperhomocysteindemia is associated in some but not all studies with an increased risk for cardiovascular disease. A low homocysteine may reflect malnutrition and predict a poor outcome. Folate achieves modest reductions of homocysteine in some but not all studies. There are no data to support therapy with very high-dose folic acid. Hyperhomocysteindemia impairs endothelial function which is not adequately reversed by folate.

Reduced serum homocysteine levels in type 2 diabetes

OBJECTIVE

To assess the contribution of fasting blood glucose and methylene-tetrahydrofolate reductase (MTHFR) gene polymorphism on fasting serum homocysteine (tHcy) levels in patients with uncomplicated type 2 diabetes compared with healthy subjects.

METHODS AND RESULTS

We studied 105 type 2 diabetic patients without cardiovascular complications or diabetic nephropathy (55 males, 50 females, mean age 53+/-10 years, mean duration of diabetes 11.4+/-8 years) and 120 age- and sex-matched control subjects (65 males, 55 females, mean age 52+/-8 years). tHcy and other biochemical variables were measured. The C677T MTHFR gene polymorphism was determined by analysis of HinfI restriction fragment length polymorphism tHcy levels were significantly lower in diabetic patients compared with control subjects (7.7 +/- 2.2 vs. 11.8 +/- 4.5 micromol/l, P < 0.0001). In both patients and control subjects, homocysteinemia was higher in men than in women (8.4+/-2.6 vs. 7.3+/-2.0 micromol/l, P < 0.03, and 13.0+/-5.3 vs. 10.4+/-2.6 micromol/l, P < 0.0001, respectively). Levels were slightly higher in subjects with the mutated Val/Val genotype compared with the Ala/Val plus Ala/Ala genotypes in both diabetic patients (P < 0.02) and control subjects (P < 0.003). On simple regression analysis, tHcy was inversely related with blood glucose levels (P < 0.02) and directly with sex (P < 0.04) in diabetic patients, and with sex (P < 0.0001), age (P < 0.02), BMI (P < 0.03), systolic and diastolic blood pressure (P < 0.0004 and P < 0.0002), uric acid and creatinine (P < 0.0001 and P < 0.0003) in control subjects. On multiple regression, tHcy levels were associated with sex (P < 0.03) and glucose levels (P < 0.04) in diabetic patients, and with uric acid (P < 0.002) and MTHFR genotype (P < 0.03) in control subjects.

CONCLUSION

In type 2 diabetic patients without nephropathy, basal levels of tHcy were 35% lower compared with healthy controls. Chronic hyperglycemia may control tHcy by affecting its renal excretion, or accelerate hepatic trans-sulfuration secondary to insulin disorders.

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.

Effect of immunosuppressive agents on long-term survival of renal transplant recipients: focus on the cardiovascular risk

In the control of acute rejection, attention is being focused more and more on the long-term adverse effects of the immunosuppressive agents used. Since cardiovascular disease is the main cause of death in renal transplant recipients, optimal control of cardiovascular risk factors is essential in the long-term management of these patients. Unfortunately, several commonly used immunosuppressive drugs interfere with the cardiovascular system. In this review, the cardiovascular adverse effects of the immunosuppressive agents currently used for maintenance immunosuppression are thoroughly discussed. Optimising immunosuppression means finding a balance between efficacy and safety. Corticosteroids induce endothelial dysfunction, hypertension, hyperlipidaemia and diabetes mellitus, and impair fibrinolysis. The use of corticosteroids in transplant recipients is undesirable, not only because of their cardiovascular effects, but also because they induce such adverse effects as osteoporosis, obesity, and atrophy of the skin and vessel wall. Calcineurin inhibitors are the most powerful agents for maintenance immunosuppression. The calcineurin inhibitor ciclosporin (cyclosporine) not only induces these same adverse effects as corticosteroids but is also nephrotoxic. Tacrolimus has a more favourable cardiovascular risk profile than ciclosporin and is also less nephrotoxic. It has little or no effect on blood pressure and serum lipids; however, its diabetogenic effect is more prominent in the period immediately following transplantation, although at maintenance dosages, the diabetogenic effect appears to be comparable to that of ciclosporin. The diabetogenic effect of tacrolimus can be managed by reducing the dose of tacrolimus and early corticosteroid withdrawal. The effect of tacrolimus on endothelial function has not been completely elucidated. The proliferation inhibitors azathioprine and mycophenolate mofetil (MMF) have little effect on the cardiovascular system. Yet, indirectly, by inducing anaemia, they may lead to left ventricular hypertrophy. MMF is an attractive alternative to azathioprine because of its higher potency and possibly lower risk of malignancies. Sirolimus also induces anaemia, but may be promising because of its antiproliferative features. Whether the hyperlipidaemia induced by sirolimus counteracts its beneficial effects is, as yet, unknown. It may be combined with MMF, however, initial attempts resulted in severe mouth ulcers.

Creatine supplementation does not decrease total plasma homocysteine in chronic hemodialysis patients

BACKGROUND

Hyperhomocysteinemia is present in the majority of chronic hemodialysis patients. Treatment with folic acid, vitamin B12, and vitamin B6 cannot fully normalize plasma homocysteine concentrations (tHcy). Previously we have demonstrated the tHcy-lowering effect of creatine supplementation in an animal model of uremia (Kidney Int 64:1331-1337, 2003). The present study investigates the effects of creatine supplementation on tHcy in a vitamin-repleted chronic hemodialysis population.

METHODS

Forty-five hemodialysis patients receiving folic acid and vitamin B6 and B12 were included. Patients were treated with creatine (2 g/day) or placebo during 2 treatment periods of 4 weeks, separated by a washout of 4 weeks. Plasma tHcy, creatine, Kt/V(urea), folic acid, vitamin B12, and routine biochemistry were determined, as well as the prognostic inflammatory and nutritional index.

RESULTS

All patients had elevated tHcy concentrations (21.2 +/- 5.6 micromol/L). Creatine treatment resulted in increased plasma and red blood cell creatine levels, documenting uptake of creatine. Creatine did not affect tHcy concentrations. There was no relationship between plasma creatine concentrations and tHcy concentrations. No changes in body weight, routine biochemistry, nutritional status, folic acid, or vitamin B12 were observed during the study.

CONCLUSION

Creatine supplementation at a rate of 2 g/day does not further decrease tHcy concentrations in chronic dialysis patients already treated with high dose folic acid, vitamin B6, and B12 supplementation.

Hyperhomocysteinemia and cardiovascular disease in uremia: The newest evidence in epidemiology and mechanisms of action

In the general population, hyperhomocysteinemia is an independent risk factor for cardiovascular disease (ischemic disease, such as stroke and myocardial infarction, and arterial and venous thrombosis). We can presume that the association is causal, based on the example of homocystinuria, and on the evidence put forward by several basic science and epidemiologic studies. However, the results of large intervention trials, which may grant further support to this hypothesis, are not yet available. In chronic renal failure and in uremia, the evidence that is offered by carefully performed prospective studies also indicate the presence of an association, although some studies suggest reverse epidemiology. The mechanisms underlying the association, and able to explain the several toxic effects of homocysteine, related or not to cardiovascular disease, are unclear. Oxidation, nitrosylation, and hypomethylation are among the postulated mechanisms. In uremia, protein hypomethylation interferes with protein repair; DNA hypomethylation impairs regulation of gene expression, whereas folate treatment reverts such alterations. Acylation, another structural modification able to impair protein function, is a possible mediator of homocysteine toxicity.

Homocysteine clearance and methylation flux rates in health and end-stage renal disease: association withS-adenosylhomocysteine

Hyperhomocysteinemia is a risk factor for cardiovascular disease and occurs frequently in end-stage renal disease (ESRD), but its pathogenesis is poorly understood. We aimed to evaluate one-carbon flux rates of methionine and homocysteine (Hcy) in ESRD patients and healthy controls. Transmethylation (TM), remethylation (RM), and transsulfuration (TS), as well as Hcy clearance by TS (i.e., TS/plasma total Hcy concentration) and by RM (i.e., RM/plasma total Hcy concentration) were evaluated in relation to body composition, vitamins, and S-adenosylhomocysteine (AdoHcy) and S-adenosylmethionine (AdoMet) levels. After a fixed protein diet for 3 days, primed-continuous infusion of [2H3-methyl-1-13C]methionine was performed in the postabsorptive state in 12 hemodialysis patients and 16 healthy volunteers. Hcy clearance by TS (−80%, P < 0.001) and by RM (−77%, P < 0.001) in ESRD patients was decreased compared with healthy controls. The absolute flux rates of TM (−27%, P < 0.01) and RM (−28%, P = 0.02) were lower in the ESRD patients. After adjustment for age, TS was not significantly reduced. Whole blood AdoHcy was significantly elevated in ESRD and was a significant determinant of TM (standardized β = −1.24, P = 0.01) and RM (standardized β = −1.43, P = 0.03). In conclusion, patients with ESRD have impaired Hcy clearance by TS and RM. Elevated whole blood AdoHcy levels are associated with impaired RM and TM flux rates in these patients, and AdoHcy may be a key regulatory compound in one-carbon flux.

Plasma homocysteine levels in living kidney donors before and after uninephrectomy

An increased prevalence of hyperhomocysteinemia has been observed among patients with end-stage renal disease, and numerous studies have demonstrated that kidney function is one of the most important determinants of plasma total homocysteine (tHcy) concentration. In an effort to understand the mechanism of hyperhomocysteinemia in renal disease, we chose, as our model, living kidney donors who had undergone uninephrectomy. We studied 10 living kidney donors and measured fasting plasma tHcy, plasma creatinine, folate, vitamins B(12) and B(6), and high-sensitivity C-reactive protein (hsCRP) 24 hours before nephrectomy and 2 days, 6 weeks, and 6 months after nephrectomy compared to the values 24 hours before nephrectomy. Mean fasting tHcy and creatinine concentrations were significantly higher in donors 2 days, 6 weeks and 6 months after nephrectomy they were 24 hours before nephrectomy. Both the increases in tHcy levels 2 days after nephrectomy and subsequent decreases 6 weeks and 6 months after are paralleled by the changes in plasma creatinine values, although neither returned to its presurgery value. Decreases in tHcy are significantly correlated with decreases in creatinine values. The B vitamins were unchanged, and the hsCRP level was increased 2 days after surgery but had returned to the baseline level after 6 weeks. We conclude that tHcy and creatinine levels parallel each other after uninephrectomy and that the gradual decrease in tHcy is accounted for by hypertrophy of the remaining kidney. Our results, the first to be obtained from living kidney donors, support the hypothesis that renal metabolism of tHcy is the mechanism responsible for the correlation between renal function and plasma tHcy level.

Hyperhomocysteinemia in pediatric and young adult renal transplant recipients

Hyperhomocysteinemia (HHcy) has been recently identified as an important and reversible cardiovascular risk factor in adult and pediatric renal transplant recipients. A retrospective cross-sectional analysis of 70 pediatric and young adult renal transplant recipients was performed to determine the prevalence, and important clinical and laboratory correlates of HHcy. Total homocysteine concentration, free and protein bound, was determined by fluorescence polarization immunoassay using an IMX analyzer. Hyperhomocysteinemia was defined as a serum homocysteine (Hcy) level above the 95th percentile for age. Fifty-four of 70 patients (77%) had HHcy. Comparison of patients with HHcy with patients without HHcy demonstrated no statistical difference in age (p = 0.35), gender (p = 0.76) or donor type (p = 0.20). Patients with HHcy had significantly lower calculated creatinine clearance values (Ccr) (p = 0.02), 67.3 +/- 21.2 mL/min/1.73 m(2) vs. 90.7 +/- 32.3 mL/min/1.73 m(2) for patients without HHcy. Immunosuppression did not correlate with the diagnosis of HHcy. Stepwise logistic regression identified patient age (0.18, p = 0.013) and Ccr (-0.04, p = 0.011) as significant variables. In conclusion, HHcy is more common than expected in pediatric renal transplant recipients. Patients with Ccr <80 mL/min/1.73 m(2) were statistically more likely to have a diagnosis of HHcy. We recommend that Hcy levels should be evaluated in this high risk population.

Hyperhomocysteinemia in patients with heart failure referred for cardiac transplantation: preliminary observations

BACKGROUND

Hyperhomocysteinemia is becoming recognized as a risk factor for cardiovascular disease, yet there are limited data on the prevalence of hyperhomocysteinemia in patients with heart failure.

HYPOTHESIS

The purpose of this study was to examine the prevalence of hyperhomocysteinemia in patients with severe heart failure and to correlate serum homocysteine levels with factors that may affect homocysteine metabolism.

METHODS

Serum homocysteine levels were measured at the time of cardiac transplant evaluation in 89 consecutive patients with severe heart failure. Homocysteine levels for patients with ischemic cardiomyopathy (ICM) were compared with levels obtained in patients with nonischemic cardiomyopathy (NICM), and homocysteine levels were correlated with demographic and hemodynamic parameters as well as functional status.

RESULTS

The mean plasma homocysteine level was increased (14.3 +/- 5.3 micromol/l, normal <9.0 micromol/l) and was equivalent between patients with ICM versus NICM (14.7 +/- 5.8 micromol/l vs. 13.8 +/- 4.5 micromol/l, p = 0.44). Elevated homocysteine levels were seen in a large proportion (89%) of patients and were equally common to patients with NICM (94%) and ICM (85%). Serum homocysteine levels correlated with serum creatinine (r = 0.51, p < 0.001), with a history of diabetes (p = 0.028), and with a history of peripheral vascular disease (p = 0.045). Only 6% of patients were receiving folic acid therapy at the time of transplant referral.

CONCLUSION

Hyperhomocysteinemia is common in patients with severe heart failure, and plasma homocysteine levels are uniformly elevated regardless of the etiology of heart failure. Elevated plasma homocysteine levels are likely a consequence of heart failure-related renal insufficiency.

Homocysteine metabolism in renal disease

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

Renal metabolism of amino acids: its role in interorgan amino acid exchange

The kidneys play a role in the synthesis and interorgan exchange of several amino acids. The quantitative importance of renal amino acid metabolism in the body is not, however, clear. We review here the role of the kidney in the interorgan exchange of amino acids, with emphasis on quantitative aspects. We reviewed relevant literature by using a computerized literature search (PubMed) and checking relevant references from the identified articles. Our own data are discussed in the context of the literature. The kidney takes up glutamine and metabolizes it to ammonia. This process is sensitive to pH and serves to maintain acid-base homeostasis and to excrete nitrogen. In this way, the metabolism of renal glutamine and ammonia is complementary to hepatic urea synthesis. Citrulline, derived from intestinal glutamine breakdown, is converted to arginine by the kidney. Renal phenylalanine uptake is followed by stoichiometric tyrosine release, and glycine uptake is accompanied by serine release. Certain administered oligopeptides (eg, glutamine dipeptides) are converted by the kidneys to their constituent components before they can be used in metabolic processes. The kidneys play an important role in the interorgan exchange of amino acids. Quantitatively, for several important amino acids, the kidneys are as important as the gut in intermediary metabolism. The kidneys may be crucial "mediators" of the beneficial effects of specialized, disease-specific feeding solutions such as those enriched in glutamine dipeptides.

Homocysteine and cardiovascular disease in renal disease. Review Article

Elevated homocysteine (hyperhomocysteinaemia) in renal patients is a major concern for physicians. Although cause and effect between homocysteine and cardiovascular disease (CVD) has not been established in either the general population or renal patients, there is much evidence that this relationship does exist. Purported mechanisms that may explain this effect include increases in endothelial injury, smooth muscle cell proliferation, low-density lipoprotein oxidation and changes in haemostatic balance. Renal patients have a much greater incidence of hyperhomocysteinaemia and this may be explained by decreases in either the renal or extrarenal metabolism of the compound. We conclude that data from long-term placebo-controlled trials are urgently required to determine whether hyperhomocysteinaemia in renal patients is a cause of CVD events and requires therapeutic targeting.

Effect of dialyser membrane pore size on plasma homocysteine levels in haemodialysis patients

BACKGROUND

Hyperhomocysteinaemia is a putative risk factor for atherothrombotic cardiovascular disease in the haemodialysis population. High-dose vitamin B therapy does not entirely normalize elevated plasma total homocysteine (tHcy) levels in haemodialysis patients. Alternative therapies to reduce tHcy further are therefore required. Modifications of the dialysis regimen may result in a better removal of Hcy. We examined the effect of dialyser membrane pore size on tHcy levels in vitamin-replete chronic haemodialysis patients.

METHODS

Forty-five haemodialysis patients were dialysed during 4 weeks with a low-flux, a high-flux and a super-flux membrane, in random order. Pre-dialysis tHcy was determined at baseline and every 4 weeks. In 18 patients, plasma tHcy before and after dialysis and dialysate tHcy concentrations were measured.

RESULTS

Pre-dialysis tHcy decreased significantly during 4 weeks super-flux dialysis (-14.6 +/- 2.8%), whereas it remained stable during high-flux (+0.5 +/- 2.4%) and low-flux dialysis (+1.7 +/- 3.2%). The homocysteine reduction ratio was not different for the three membranes: 0.39 +/- 0.03 for the super-flux, 0.47 +/- 0.02 for the high-flux and 0.39 +/- 0.02 for the low-flux dialyser. The amount of Hcy recovered in the dialysate during a single dialysis session was also similar: 117.5 +/- 3.6 micro mol during super-flux, 95.3 +/- 11.5 micro mol during high-flux and 116.5 +/- 11.6 micro mol during low-flux dialysis.

CONCLUSION

Super-flux dialysis significantly lowers tHcy in chronic haemodialysis patients. Improved removal of middle-molecule uraemic toxins with inhibitory effects on Hcy-metabolizing enzymes, rather than better dialytic clearance of Hcy itself, may explain the beneficial effect of the super-flux membrane.

Oral and transdermal estrogens both lower plasma total homocysteine in male-to-female transsexuals

Plasma total homocysteine (tHcy) levels are on average lower in women versus men, indicating an estrogenic effect. Oral estrogens (absorbed via the liver) may be hypothesized to have stronger effects on hepatic homocysteine metabolism than transdermal estrogens. We randomly assigned 30 male-to-female transsexuals (20-44 years old) to 4 months' administration of oral ethinyl estradiol (n=15) or transdermal 17beta-estradiol (n=15), both with the antiandrogen cyproterone acetate (CA). Ten other male controls were treated with CA only. At baseline and after 2 and 4 months, plasma tHcy was analyzed in conjunction with plasma folate. Oral ethinyl estradiol and transdermal 17beta-estradiol similarly reduced plasma tHcy (geometric mean 10.6 micromol/l [95% CI 8.2-13.9] to 7.5 [6.5; 8.8], and 11.3 [8.1; 16.4] to 8.4 [6.5; 11.1]; P<0.001 for both), whereas CA had no effects. No effects were found on folate levels. Thus, oral and transdermal estrogens decrease plasma tHcy to a similar degree (by geometric mean -26%), which suggests that a hepatic mechanism is unlikely to play an important role in the decline of tHcy levels.

Interorgan exchange of aminothiols in humans

In the present study, we used organ balance across the kidney, splanchnic organs, and lower limb in subjects undergoing diagnostic central venous catheterizations to gain insight into the renal and extrarenal exchange of aminothiols in humans. Although Hcy was released only in low amounts from leg tissues, Cys-Gly (a peptide derived from GSH hydrolysis) was released by both the leg and splanchnic organs, whereas Cys was released by the kidney and taken up by splanchnic organs. The kidney removed approximately 90% of the Cys-Gly released into the circulation. Removal of Cys-Gly by the kidney depended on Cys-Gly arterial levels and showed a high fractional extraction ( approximately 26%), with clearance rates slightly higher than the glomerular filtration rate (GFR). Although the average kidney removal of Hcy was not statistically significant, the fractional extraction of Hcy across the kidney varied directly with renal plasma flow. Our data show that thiol metabolism in humans is a compartmentalized interorgan process involving fluxes of individual aminothiols that are parallel and of opposite sign among peripheral tissues, splanchnic organs, and kidney. Cys-Gly is released by peripheral tissue and splanchnic organs from GSH hydrolysis and is taken up by the kidney by GFR; the kidney returns Cys to the circulation to preserve substrate availability for GSH synthesis. On the other hand, Hcy is released by peripheral tissues in low amounts, and its removal by the kidney seems to depend on blood supply. These findings may help explain several alterations in aminothiol metabolism observed in patients with chronic diseases.

Homocysteine, folate, vitamin B12 levels, and C677T MTHFR mutation in children with renal failure

Hyperhomocysteinemia is well documented in chronic renal failure (CRF) and premature and progressive occlusive vascular disease is common in CRF. The combined effects of renal failure, folate and vitamin B(12) levels, and a common mutation (C677T) in the methylenetetrahydrofolate reductase (MTHFR) gene that leads to total plasma homocysteine (tHcy) elevation in CRF children were investigated. Forty-two children (15 females) with CRF, mean age 10.3+/-4.7 years, were included. The mean glomerular filtration rate (GFR) was 37.3+/-16.9 ml/min per 1.73 m(2). The control group comprised 33 children (18 females) with a mean age of 8.6+/-3.4 years. There were 40% of CRF patients with hyperhomocysteinemia. Folate and vitamin B(12) deficiencies were identified in 14% (n=6) and 5% (n=2), respectively, of all patients. On univariate analysis, the tHcy serum concentration was negatively correlated with the plasma folate concentration (P<0.05) in controls, and with GFR (P<0.05) in patients. On multiple regression analysis for the predictors of tHcy serum concentrations, folic and vitamin B(12 )were significant in controls, whereas only GFR was significant in CRF children. In our patients no effect of the MTHFR polymorphism on tHcy levels was seen This result, in addition to the limited number of patients, may partially be explained by the low prevalence of folate deficiency in our patients.

Uremia-related metabolic cardiac risk factors in chronic kidney disease

Growing evidence has been gathered over the last 15 years regarding the role of nontraditional or uremia-related risk factors in the pathogenesis of atherosclerosis in subjects with renal failure. Among those factors, dyslipidemia, inflammation, hyperhomocysteinemia, and oxidant stress have been extensively studied. However, the clinical significance of many of these factors remains controversial in light of reported studies. In this article, the existing evidence regarding the role of uremia-related risk factors in the pathogenesis of atherosclerosis is reviewed, with special emphasis on prevalence, cardiac risk, and management in patients with chronic kidney disease (CKD). Consensus treatment recommendations are provided for risk factors for which there is evidence to support preventive or therapeutic interventions.

Association between serum aspartate transaminase and homocysteine levels in hemodialysis patients

BACKGROUND

Hyperhomocysteinemia is a common metabolic abnormality in patients undergoing hemodialysis (HD). An impairment of remethylation of homocysteine (Hcy) is seen in these patients but cannot account completely for hyperhomocysteinemia. Homocysteine is derived from transmethylation of methionine that can be metabolized through transamination pathway alternatively. However, the significance of transamination in the metabolism of Hcy in HD patients is not studied.

METHODS

A total of 145 patients undergoing HD for more than 3 months were enrolled in the study. Vitamins B were not prescribed routinely to these patients. Among them, 49 patients had positive test results for hepatitis B surface antigen or antihepatitis C virus antibody. Serum Hcy, folic acid, vitamin B12, pyridoxal 5' -phosphate, methionine, and transaminase were measured, and parameters of dialysis adequacy were calculated. Multiple linear regression model was used to analyze the factors determining Hcy levels.

RESULTS

All patients had higher Hcy levels (40.3 +/- 28.3 micromol/L) than the upper limit of reference range 15 micromole/L. The levels of vitamin B(12) were all higher than 160 pg/mL (118 pmol/L). Only 9 patients had serum folic acid lower than 3 ng/mL (6.8 nmol/L). The predialysis Hcy levels were correlated with age, HD duration, folic acid, vitamin B12, and aspartate transaminase (AST) levels among all patients or the subgroup of hepatitis noncarriers with linear multiple regression analysis. In hepatitis carriers, AST levels were not associated with Hcy. A cutoff value of AST less than 14 U/L predicted a predialysis Hcy level higher than 27 micromol/L in noncarriers, with a sensitivity of 83.9% and a specificity of 50.2%.

CONCLUSION

In addition to vitamin B12 and folic acid, the serum AST levels correlated inversely with predialytic Hcy levels independently in hepatitis noncarrier HD patients. The results suggest that transamination may play an important role in the development of hyperhomocysteinemia when impaired transmethylation is encountered in uremic patients.

Relationship of renal function to homocysteine and lipoprotein(a) levels: the frequency of the combination of both risk factors in chronic renal impairment

BACKGROUND

Total homocysteine (tHcy) and lipoprotein(a) [Lp(a)] levels have been recognized as risk factors for vascular disease. The combination of elevated tHcy and Lp(a) levels may be particularly atherogenic, although no study has examined the prevalence of the combination of both risk factors in patients with chronic renal impairment.

METHODS

One hundred ninety-seven patients with renal impairment were studied. Patients had glomerular filtration rate (GFR) measured by clearance of chromium 51-labeled EDTA. Blood was obtained for the determination of tHcy, Lp(a), and apolipoprotein(a) [apo(a)] isoform levels.

RESULTS

Patients were divided into five groups according to GFR. Mean tHcy levels in the five groups were as follows: GFR less than 10 mL/min, 30.2 +/- 9.8 (SD) micromol/L; GFR of 10 to 20 mL/min, 26.6 +/- 10.5 micromol/L; GFR of 20 to 30 mL/min, 23.9 +/- 8.6 micromol/L; GFR of 30 to 45 mL/min, 22.2 +/- 8.6 micromol/L; and GFR of 45 to 75 mL/min, 18.2 +/- 9.1 micromol/L compared with control levels of 12.7 +/- 4.6 micromol/L. There was a progressive increase in median Lp(a) levels with declining renal function: median Lp(a) levels for those with a GFR less than 10 mL/min were 37.1 mg/dL (range, 0.6 to 156.0 mg/dL); GFR of 10 to 20 mL/min, 30.3 mg/dL (range, 2.6 to 163.7 mg/dL); GFR of 20 to 30 mL/min, 26.1 mg/dL (range, 0.0 to 164.0 mg/dL); GFR of 30 to 45 mL/min, 20.9 mg/dL (range, 0.0 to 99.8 mg/dL), and GFR of 45 to 75 mL/min, 16.8 mg/dL (range, 2.1 to 81.0 mg/dL) compared with control values of 12.5 mg/dL (range, 0.0 to 88.7 mg/dL).

CONCLUSION

Defining hyperhomocysteinemia as tHcy levels greater than the 90th percentile of controls and elevated Lp(a) level as greater than 30 mg/dL, the frequency of the combination increased with declining renal function. Fifty-eight percent of patients with a GFR less than 10 mL/min had both hyperhomocysteinemia and elevated Lp(a) levels, and even in patients with mild renal impairment, 20% of patients had both risk factors present.

Association of hyperhomocysteinemia with plasma sulfate and urine sulfate excretion in patients with progressive renal disease

BACKGROUND

Plasma total homocysteine (tHcy) level is increased in patients with renal disease, parallel to serum creatinine concentration. In renal failure, the final product of sulfated amino acid metabolism, sulfate, also accumulates as renal function declines. We hypothesized that the elevation in sulfate level could cause hyperhomocysteinemia and tested the relation between tHcy level and both urinary excretion and plasma levels of sulfate.

METHODS

Forty patients with renal disease were divided into three groups: patients without renal failure (nRF; creatinine clearance [CCr] > or = 80 mL/min/1.73 m2 [> or =1.33 mL/s/1.73 m2]), patients with mild renal failure (mRF; 80 > CCr > or = 25 mL/min/1.73 m2 [1.33 > CCr >/ or 0.42 mL/s/1.73 m2]), and patients with severe renal failure (sRF; CCr < 25 mL/min/1.73 m2 [<0.42 mL/s/1.73 m2]). Daily urinary excretion and plasma levels of tHcy, total cysteine (tCys), and sulfate were measured. A healthy control (HC) group also was tested. Serum methionine, taurine, vitamin B12, and folate levels also were determined in patients with renal disease.

RESULTS

Plasma tHcy and sulfate concentrations in the groups with mRF and sRF were greater than in the HC group. Plasma tCys concentrations in the mRF and sRF groups were greater than in the nRF group. Daily urinary Hcy and Cys excretion did not differ among the four groups. Daily urine sulfate and urea nitrogen excretion in the sRF group were significantly less than in the HC and nRF groups. Multiple regression analyses showed that plasma creatinine (beta = 0.40) and sulfate (beta = 0.43) levels were independently associated with plasma Hcy level; among urine parameters, only daily urine sulfate excretion (beta = -0.52) was independently associated with plasma Hcy level.

CONCLUSION

The elevated plasma sulfate level, in accordance with renal function, is associated with plasma tHcy level. Decreased sulfate excretion, which might parallel the intake of sulfated amino acid or protein, may increase tHcy levels.

Methods for measuring sulfur amino acid metabolism

PURPOSE OF REVIEW

The importance of sulfur amino acid metabolism has become increasingly apparent in recent years. Methionine and cysteine are precursors of glutathione, which plays an important role in intracellular antioxidant/free radical defenses. Homocysteine is a non-protein-bound sulfur amino acid strongly implicated in the pathogenesis of several diseases. Both glutathione and homocysteine are affected by abnormalities in sulfur amino acid metabolism that occur in the clinical setting.

RECENT FINDINGS

The Storch-Young model, which determines methionine turnover and homocysteine remethylation by means of a tracer methionine infusion, has been improved by using plasma homocysteine (rather than methionine) enrichment in the model. A complex new tracer method involving the use of tracer serine, methionine, and leucine has been described to determine the effects of folate or pyridoxine deficiency on sulfur amino acid-methyl transfer reactions in humans. The etiology of hyperhomocysteinemia in chronic renal failure is controversial; new concepts in this area are described. There is new interest in the subspecies of homocysteine in the circulation. A new method is described for measuring the extremely low plasma concentrations of reduced homocysteine, using gas chromatography-mass spectrometry. Plasma S-adenosylhomocysteine, measured by fluorescence high-performance liquid chromatography, has been suggested as being superior to homocysteine as a predictor of the risk of vascular disease.

SUMMARY

This review highlights and critiques the above recent developments, and points out some of the complexities and pitfalls in designing and interpreting human metabolic studies involving the sulfur amino acids.