Kinetic basis of hyperhomocysteinemia in patients with chronic renal failure

Correlation between total homocysteine and cyclosporine concentrations in cardiac transplant recipients

Increased circulating total homocysteine (tHcy) has been implicated as an independent risk factor for atherosclerotic disease. In cardiac transplant patients, accelerated coronary atherosclerosis is an important cause of late allograft failure; however, studies of tHcy in this at-risk group are limited. We sampled a cohort of 72 subjects 3.95 ± 3.14 (mean ± SD) years after transplantation and found that all had tHcy concentrations above our upper reference limit (15.0 μmol/L). The mean tHcy in the transplant group (25.4 ± 7.1 μmol/L) was significantly greater than in our reference group (9.0 ± 4.3 μmol/L; n = 457; P <0.001). We also examined the effect of age, gender, time since transplant, serum folate and cobalamin, total protein, urate, creatinine, albumin, and trough whole blood cyclosporine concentrations. In a multiple linear regression model, only creatinine (mean 144 ± 52 μmol/L; P = 0.021) and trough cyclosporine concentrations (191 ± 163 μg/L; P = 0.015) were independent positive predictors of tHcy, whereas serum folate (8.35 ± 7.43 nmol/L; P = 0.018) and time since transplant (P = 0.049) were significant negative predictors. We conclude that hyperhomocysteinemia is a common characteristic of cardiac transplant recipients. Our analysis suggests that folate and renal glomerular dysfunction are important contributory factors; however, whole blood cyclosporine concentrations may also predict the degree of hyperhomocysteinemia in this population and therefore influence interpretation of any apparent response to treatment.

Renal uptake and excretion of homocysteine in rats with acute hyperhomocysteinemia

BACKGROUND

Elevated plasma total homocysteine, an independent risk factor for cardiovascular disease, is commonly observed in renal patients. We have previously shown that the kidney is a major site for the removal of plasma homocysteine in the rat. The present investigation was performed to further characterize the capacity of the kidney to handle acute elevations in plasma homocysteine concentrations.

METHODS

Acute hyperhomocysteinemic conditions (4- to 7-fold > controls) in rats were produced by either a primed-continuous infusion of L-homocysteine or exposure to 80:20% nitrous oxide:oxygen, which results in the inhibition of methionine synthase.

RESULTS

At physiological homocysteine concentrations, approximately 15% of the arterial plasma homocysteine was removed on passage through the kidney. Renal homocysteine uptake was approximately 85% of the filtered load. The urinary excretion of homocysteine was negligible (<2%). During acute hyperhomocysteinemia produced by the infusion of L-homocysteine, renal homocysteine uptake was increased fourfold and was equivalent to 50% of the infused dose, while urinary excretion remained negligible. Renal homocysteine uptake during nitrous oxide-induced hyperhomocysteinemia increased threefold, with urinary excretion remaining negligible.

CONCLUSIONS

These results provide strong evidence that the kidney has a significant capacity for metabolizing acute elevations in plasma homocysteine, and support a very limited role for the re-methylation pathway in renal homocysteine metabolism.

Serum homocysteine level and protein intake are related to risk of microalbuminuria: the Hoorn Study

BACKGROUND

Microalbuminuria (MA) is a strong predictor of cardiovascular disease, but its causes are incompletely understood. Hyperhomocysteinemia is a recently recognized risk factor for cardiovascular disease independent of established risk factors. It is not known whether hyperhomocysteinemia is associated with MA, and thus could be a possible cause of microalbuminuria.

METHODS

We studied an age-, sex- and glucose-tolerance-stratified random sample of a 50- to 75-year old general Caucasian population (N = 680). The urinary albumin-to-creatinine ratio (ACR) was measured in an early morning spot urine sample. MA was defined as an ACR > 3.0 mg/mmol.

RESULTS

The prevalence of MA was 4.3% (13 of 304) in subjects with normal glucose tolerance, 9.2% (17 of 185) in impaired glucose tolerance and 18.3% (30 of 164) in non-insulin-dependent diabetes mellitus (NIDDM); it was 3.7% (15 of 402) in subjects without hypertension and 17.9% (45 of 251) in those with hypertension. After adjusting for age, sex, glucose tolerance category, hypertension, dyslipidemia and smoking, the odds ratio [OR; 95% confidence interval (95%CI)] for MA per 5 mumol/liter total homocysteine increment was 1.33 (1.08 to 1.63). Additional adjustment for HbA1c, waist-hip ratio, protein intake and serum creatinine did not attenuate the association between MA and total homocysteine. A 0.1 g/kg.day increment of protein intake was also associated with an increased risk for MA after adjustment for age, sex, classical risk factors and serum total homocysteine [OR (95% CI); 1.20 (1.08 to 1.32)].

CONCLUSION

Both hyperhomocysteinemia and protein intake are related to microalbuminuria independent of NIDDM and hypertension. Hyperhomocysteinemia may partly explain the link between MA and increased risk of cardiovascular disease.

No net renal extraction of homocysteine in fasting humans

BACKGROUND

The pathophysiological mechanism of hyperhomocysteinemia in chronic renal failure in humans is unknown. The loss of a putative renal homocysteine extraction in chronic renal failure has been hypothesized as significant homocysteine uptake has been demonstrated in the normal rat kidney. We studied homocysteine extraction in the normal human kidney.

METHODS

We measured plasma total (free and protein-bound) and free homocysteine (tHcy and fHcy, respectively) in arterial and renal venous blood sampled from the aorta and right-side renal vein during cardiac catheterization in 20 fasting patients with normal renal function. Renal homocysteine extraction was calculated as the arteriovenous difference divided by the arterial levels times 100%.

RESULTS

No significant renal extraction was demonstrated either for tHcy: 0.9% (SD 5.8; 95% CI -1.8 to +3.6) or for fHcy: -0.2% (11.0; -5.4 to +4.9).

CONCLUSIONS

We conclude that no significant net renal uptake of homocysteine occurs in fasting humans with normal renal function. The loss of such uptake, therefore, cannot cause hyperhomocysteinemia in patients with renal failure.

Homocysteine, lipoprotein(a) and fibrinogen: metabolic risk factors for cardiovascular complications of chronic renal disease

High plasma concentrations of homocysteine, lipoprotein(a) and fibrinogen are accompanied by an increased risk for cardiovascular complications in the general population. All three parameters are markedly elevated in patients with renal disease, a group with a high prevalence and incidence of cardiovascular complications. This review discusses these parameters in such patients in relation to the occurrence of atherosclerotic complications.

Homocysteine and cardiovascular disease

An elevated level of total homocysteine (tHcy) in blood, denoted hyperhomocysteinemia, is emerging as a prevalent and strong risk factor for atherosclerotic vascular disease in the coronary, cerebral, and peripheral vessels, and for arterial and venous thromboembolism. The basis for these conclusions is data from about 80 clinical and epidemiological studies including more than 10,000 patients. Elevated tHcy confers a graded risk with no threshold, is independent of but may enhance the effect of the conventional risk factors, and seems to be a particularly strong predictor of cardiovascular mortality. Hyperhomocysteinemia is attributed to commonly occurring genetic and acquired factors including deficiencies of folate and vitamin B12. Supplementation with B-vitamins, in particular with folic acid, is an efficient, safe, and inexpensive means to reduce an elevated tHcy level. Studies are now in progress to establish whether such therapy will reduce cardiovascular risk.

Plasma total homocysteine levels in hyperthyroid and hypothyroid patients

We found a higher plasma concentration of total homocysteine (tHcy), an independent risk factor for cardiovascular disease, in patients with hypothyroidism (mean, 16.3 micromol/L; 95% confidence interval [CI], 14.7 to 17.9 micromol/L) than in healthy controls (mean, 10.5 micromol/L; 95% CI, 10.1 to 10.9 micromol/L). The tHcy level of hyperthyroid patients did not differ significantly from that of the controls. Serum creatinine was higher in hypothyroid patients and lower in hyperthyroid patients than in controls, whereas serum folate was higher in hyperthyroid patients compared with the two other groups. In multivariate analysis, these differences did not explain the higher tHcy concentration in hypothyroidism. We confirmed the observation of elevated serum cholesterol in hypothyroidism, which together with the hyperhomocysteinemia may contribute to an accelerated atherogenesis in these patients.

Mass spectrometry in the search for uremic toxins

This article reviews the literature on the mass spectrometry (MS) that has been used in the research of uremic toxins. Gas chromatography/mass spectrometry (GC/MS) has been most often used for the analysis of low-molecular-weight compounds in uremic blood such as organic acids, phenols, and polyols. However, it cannot be used for the analysis of middle- to high-molecular-weight substances or for involatile compounds. The development of fast atom bombardment (FAB) and liquid secondary ion mass spectrometry (LSIMS) has made possible the analysis of middle-molecules and involatile low-molecular-weight substances such as peptides and nucleosides. The development of atmospheric pressure chemical ionization (APCI) has also lead to the analysis of involatile low-molecular-weight substances. The recent advances in ionization methods, such as electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI), have permitted the MS analysis of high-molecular-weight substances such as beta 2-microglobulin, a major component of dialysis amyloid. Liquid chromatography/mass spectrometry (LC/MS), using ESI, APCI, or FAB as an ionization method, is currently the preferred method for the analysis of low- to high-molecular-weight substances in uremic blood. ESI-LC/MS and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOFMS) are useful for elucidating the structure of post-translationally modified proteins obtained from the blood and tissues of uremic patients. Post-translational modification such as the formation of advanced glycation end-products and carbamoylation is enhanced in uremic patients, and is considered to be responsible for some uremic symptoms. Laser microprobe MS is unique in its capability for the two-dimensional detection of atoms such as aluminum in a tissue section obtained from uremic patients. This review focuses on the mainstream research for discovering uremic toxins, specific uremic toxins identified or quantified using MS, and the MS analysis of post-translationally modified proteins in uremia. These studies have provided ample evidence that MS has played an important role in the search for uremic toxins.

Elevated fasting total plasma homocysteine levels and cardiovascular disease outcomes in maintenance dialysis patients. A prospective study

There is an excess prevalence of hyperhomocysteinemia in dialysis-dependent end-stage renal disease (ESRD) patients. Cross-sectional studies of the relationship between elevated total homocysteine (tHcy) levels and prevalent cardiovascular disease (CVD) in this patient population suffer from severe methodologic limitations. No prospective investigations examining the association between tHcy levels and the subsequent development of arteriosclerotic CVD outcomes among maintenance dialysis patients have been reported. To assess whether elevated plasma tHcy is an independent risk factor for incident CVD in dialysis-dependent ESRD patients, we studied 73 maintenance peritoneal dialysis or hemodialysis patients who received a baseline examination between March and December 1994, with follow-up through April 1, 1996. We determined the incidence of nonfatal and fatal CVD events, which included all validated coronary heart disease, cerebrovascular disease, and abdominal aortic/lower-extremity arterial disease outcomes. After a median follow-up of 17.0 months, 16 individuals experienced at least one arteriosclerotic CVD event. Cox proportional-hazards regression analyses, unadjusted and individually adjusted for creatinine, albumin, and total cholesterol levels, total/HDL cholesterol ratio, dialysis adequacy/residual renal function, baseline CVD, and the established CVD risk factors (ie, age, sex, smoking, hypertension, diabetes/glucose intolerance, and dyslipidemia) revealed that tHcy levels in the upper quartile (> or = 27.0 mumol/L) versus the lower three quartiles (< 27.0 mumol/L) were associated with relative risk estimates (hazards ratios, with 95% confidence intervals for the occurrence of (pooled) nonfatal and fatal CVD ranging from 3.0 to 4.4; 95% confidence intervals (1.1-8.1) to (1.6-12.2). We conclude that the markedly elevated fasting tHcy levels found in dialysis-dependent ESRD patients may contribute independently to their excess incidence of fatal and nonfatal CVD outcomes.

Excess prevalence of fasting and postmethionine-loading hyperhomocysteinemia in stable renal transplant recipients

Hyperhomocysteinemia, either fasting or after methionine loading, may contribute to the increased incidence of cardiovascular disease events experienced by renal transplant recipients. Limited data are available on fasting homocysteine (Hcy) levels, and none on postmethionine-loading Hcy levels, in these patients. We assessed the prevalence and potential determinants of fasting and postmethionine-loading hyperhomocysteinemia in 29 stable renal transplant recipients and 58 age- and sex-matched, population-based controls free of renal disease with serum creatinine levels of 1.5 mg/dL or less. Total (t) plasma Hcy was determined fasting and 2 hours after methionine loading, along with fasting determinations of the B-vitamin cofactors/substrates for Hcy metabolism, ie, pyridoxal 5'-phosphate, B-12, and folate and serum creatinine. Geometric mean fasting (18.1 versus 9.8 microM, P < .001) and postmethionine-loading increase (22.0 versus 15.2, P = .001) in tHcy levels were significantly greater in the renal transplant recipients, as were the prevalence odds (with 95% confidence intervals) for fasting [14.8 (3.4-64.7)], postmethionine loading [6.9 (1.5-32.8)], combined fasting and postmethionine-loading [18.0 (2.3-142.1)] hyperhomocysteinemia, and inadequate circulating folate [4.2 (1.1-16.5)] or pyridoxal 5'-phosphate [3.2 (0.9-11.0) status. Correlation analyses suggested important potential relationships between creatinine and both fasting (+0.64, P < .001) and postmethionine-load increase (+0.38, P = .045) in tHcy, folate and fasting (-0.41, P = .025) tHcy, and pyridoxal 5'-phosphate and postmethionine-loading increase (-0.33, P = .091) in tHcy. We conclude that there is an excess prevalence of fasting and postmethionine-loading hyperhomocysteinemia in stable renal transplant recipients. Renal function is related to both fasting and postmethionine loading-hyperhomocysteinemia, inadequate folate status is associated with fasting hyperhomocysteinemia, and inadequate vitamin B-6 status may be related to postmethionine-loading hyperhomocysteinemia in this patient population.