Plasma total homocysteine concentration in nephrotic patients with idiopathic membranous nephrophathy

Margret Arnadottir,1, Björn Hultberg2 and Anna-Lena Berg3

1 Department of Medicine, National University Hospital, Reykjavik, Iceland, 2 Department of Clinical Chemistry and 3 Department of Nephrology, Lund University Hospital, Lund, Sweden



   Abstract
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
Background. The atherothrombotic risk pattern of the nephrotic syndrome resembles that of hyperhomocysteinemia. However, the effect of nephrotic range proteinuria on homocysteine metabolism has never been studied.

Methods. The study included 11 male nephrotic patients with idiopathic membranous nephropathy who underwent a treatment trial with adrenocorticotrophic hormone and 11 male non-nephrotic, renal function-matched control subjects. The nephrotic patients were studied before and after the treatment, which induced a marked reduction in urinary protein excretion and a moderate improvement in renal function in all cases.

Results. Plasma total homocysteine (tHcy) concentration did not change significantly during treatment, whereas the nephrotic patients had significantly lower tHcy than the non-nephrotic patients (14.2±3.4 µmol/l vs 19.0±5.4 µmol/l). tHcy correlated significantly with serum concentrations of creatinine (r=0.53, P<0.05) and albumin (r=0.43, P<0.05), glomerular filtration rates (GFRs) (iohexol clearances) (r=-0.42, P<0.05) and urinary albumin excretion (r=-0.47, P<0.05).

Conclusion. The expected tHcy-lowering effect of improved renal function may have masked a tHcy-elevating effect due to reduced proteinuria leading to no net change in tHcy during treatment. The notion of an increase in tHcy associated with remission of the nephrotic syndrome is in accordance with the significantly lower tHcy in the nephrotic renal patients compared with the non-nephrotic renal function-matched patients, and the relationships between tHcy and serum albumin concentrations as well as urinary albumin excretion. Thus, the results of this small study suggest that nephrotic range proteinuria directs homocysteine metabolism towards a decrease in tHcy. However, the findings need to be confirmed in larger patient populations and in different varieties of the nephrotic syndrome.

Keywords: homocysteine; idiopathic membranous nephropathy; nephrotic syndrome; proteinuria



   Introduction
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
It is generally recognized that nephrotic patients are at highly increased risk of arterial and venous thrombosis. Moreover, there is evidence of accelerated atherosclerosis in this group of patients [1]. Thus, the atherothrombotic risk pattern of the nephrotic syndrome closely resembles that of hyperhomocysteinemia [2,3]. In spite of this, homocysteine metabolism has never been studied in patients with nephrotic range proteinuria. A few studies have reported an association between increased plasma total homocysteine (tHcy) concentrations and lesser degrees of proteinuria [46]. This applied to microalbuminuria, the linkage probably being mediated by endothelial dysfunction [4,5]. A direct correlation between tHcy and moderate urinary albumin excretion has also been shown, but this relationship disappeared upon correction for glomerular filtration rate (GFR) [6] probably reflecting the inability of the diseased kidney to prevent protein leakage.

We analysed tHcy in nephrotic patients before and after a protocol-based treatment trial with adrenocorticotrophic hormone [7] and in renal function-matched controls.



   Subjects and methods
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 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
Patients
The subjects consisted of 22 male patients. Eleven patients, aged 33–79 years (mean age 60 years), had nephrotic range proteinuria and eleven patients, aged 36–74 years (mean 64 years), had normal or slightly increased urinary albumin excretion. Each non-nephrotic patient was chosen in order to match a nephrotic patient with regard to the GFR by ±5 ml/min/1.73 m2. All the nephrotic patients suffered from idiopathic membranous nephropathy. The non-nephrotic patients were not diagnosed by renal biopsy but, on clinical grounds, were supposed to have nephrosclerosis or interstitial nephritis. No patient was taking vitamin supplementation.

Procedure
Urine was collected for 24 h for albumin measurement (8.00 a.m.-8.00 a.m.). Then, fasting blood samples were drawn for analysis of plasma tHcy concentration and the serum concentrations of albumin, creatinine, folate and vitamin B12. The sample intended for homocysteine analysis was immediately put on ice, centrifuged within 15 min, and the plasma stored at -20°C. Directly after this, 5 ml iohexol (Omnipaque, 300 mg iodine/ml, Nycomed, Herlev, Denmark) was injected intravenously, and after 4–24 h, depending on the estimated GFR, a blood sample was collected for iohexol analysis [8].

The nephrotic patients were studied both before and after 2 months of treatment with adrenocorticotrophic hormone, which in all cases resulted in markedly reduced urinary albumin excretion and moderately improved GFR (Table 1Go). This treatment is described in Berg et al. [7].


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Table 1. Plasma total homocysteine (tHcy) concentrations as well as as indicators of renal function, B-vitamin status and albumin metabolism in 11 nephrotic patients before and after treatment, and in 11 renal function-matched control subjects

 

Analytical methods
After reduction of disulfide bonds by dithiothreitol, the homocysteine concentration was analysed by high performance liquid chromatography (HPLC). The intraassay variation of the method was 1.5%, while the interassay variation was 2.5% [9]. The creatinine concentration was determined by an enzymatic method and the albumin concentration in serum and urine by immunoturbidimetry with reagents from Roche (Basel, Switzerland). The folate and vitamin B12 concentrations were analysed by radioassays using purified intrinsic factor and purified folate binding protein (vitamin B12/folate dual RIA kit, Amersham International, Amersham, UK). The concentration of iohexol was analysed by HPLC [8].

Statistical methods
Data are presented as mean±SD. The t-test was applied for comparison between the groups, after log transformation of skewed data. Relationships between variables were analysed by Spearman's rank correlation test. A P-value of <0.05 was considered to reflect statistical significance.



   Results
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 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
In the nephrotic patients, tHcy did not change significantly during treatment, despite clearly improved renal function, whereas tHcy was significantly higher in the non-nephrotic group than in the nephrotic group (Table 1Go). The plasma tHcy concentrations, serum concentrations of albumin, creatinine, folate as well as vitamin B12, GFRs and urinary albumin excretion of both groups are shown in Table 1Go.

In the whole data set there were significant correlations between tHcy and the serum concentrations of creatinine (r=0.53, P<0.05) and albumin (r=0.43, P<0.05), GFRs (r=-0.42, P<0.05) and urinary albumin excretion (r=-0.47, P< 0.05). There were no correlations between tHcy and the serum concentrations of folate and vitamin B12.



   Discussion
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 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
The previously described inverse relationship between GFRs and tHcy [6,10] was seen in the present study. Thus, the fact that there was no change in tHcy during treatment in spite of clearly improved renal function in all cases suggests that remission of the nephrotic syndrome is associated with a tHcy-elevating effect. This could not be demonstrated statistically because the material was too small to allow correction for renal function, but is in accordance with the significantly higher tHcy in the non-nephrotic renal function-matched group as compared with the nephrotic group. The direct relationship between tHcy and serum albumin concentrations, as well as the inverse relationship between tHcy and urinary albumin excretion, further support the notion that the nephrotic syndrome leads to a decrease in tHcy.

Due to the increased atherothrombotic risk, hyperhomocysteinemia would be expected in nephrotic patients. However, as described above, the present results suggest the opposite. There are several potential explanations for these findings. First, there is probably some direct urinary loss of homocysteine due to albumin binding; however, not more than a few micromoles every 24 h. Secondly, the decrease in serum albumin concentration resulting in fewer binding sites may be of importance [9,11]. Thirdly, in seriously ill patients, protein-malnutrion probably lowers tHcy. Fourthly, the urinary loss and/or hepatic overproduction of relevant proteins such as enzymes or vitamin-binding proteins possibly lowers tHcy.

The present study suffers from limitations: the patient number is small, there is no information available about the C677T mutation of the methylenetetrahydrofolate reductase gene and adrenocorticotrophic hormone may influence homocysteine metabolism. These flaws are partly compensated by the double approach giving results that point in same direction. Even though the results must be interpreted with caution, they indicate that nephrotic range proteinuria as such is not associated with hyperhomocysteinemia. Therefore, homocysteine does not seem to add to the cardiovascular risk induced by nephrotic range proteinuria. However, as seen in Table 1Go, mild hyperhomocysteinemia (tHcy >16 µmol/l) is not uncommon in nephrotic patients, probably due to a reduced GFR caused by the underlying renal disease.



   Notes
 
Correspondence and offprint requests to: Margret Arnadottir, Department of Internal Medicine, National University Hospital, IS-101 Reykjavik, Iceland. Back



   References
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 

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Received for publication: 23.11.99
Revision received 17. 8.00.