Origin and significance of urinary N-acetyl-beta, D-glucosaminidase (NAG) in renal patients with proteinuria.

In patients with proteinuria, indices of tubular damage are unreliable since filtered plasma enzymes could contribute to tubular enzymuria. Previous work has suggested the existence of various forms of the 'A' isoenzyme of N-acetyl-beta, D-glucosaminidase (NAG), one of which could be kidney specific and thus a useful marker of renal tubular damage. By using fast protein liquid chromatography, two forms of the 'A' isoenzyme, 'A1' and 'A2' were separated in human urine, plasma and kidney tissue. The isoenzyme profile in pathological urine resembled that seen in kidney tissue, the 'A2' isoenzyme predominating. The ratio A2/A1 in the urine of renal patients was significantly greater than in the plasma of renal patients, end-stage renal failure patients and healthy volunteers. There was no difference in the plasma ratios of the three groups studied. The clearances of total NAG, 'A1' and 'A2' isoenzymes were all greater than that of the lower molecular weight protein transferrin. This indicates that the origin of urinary NAG in patients with proteinuria is from the kidney itself. Thus, analysis of urinary NAG and its isoenzymes may be of benefit as an early predictor of renal tubular damage and may also be useful as a non-invasive indicator of disease progression.

Synthesis and physicochemical properties of the furan dicarboxylic acid, 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, an inhibitor of plasma protein binding in uraemia.

The furan dicarboxylic acid, 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (5-propyl FPA) accumulates in the plasma of patients with chronic renal failure and is a major contributor to the drug binding defect of uraemic plasma. This acid has also been implicated in several other aspects of the uraemic syndrome: anaemia, irregularities of thyroid function, neurological symptoms and inhibition of active tubular secretion. The acid is not commercially available and its synthesis, starting with Meldrum's acid and methyl succinyl chloride, is described. The pKa values were measured by titration and values of 3.2 and 3.6 respectively were assigned to the carboxylic acid groups attached directly to the ring at position 3 and at position 2 (on the side-chain). The partition coefficient (log P) between hydrochloric acid and octanol was 1.2 and the distribution coefficient (log D; octanol-phosphate buffer pH 7.4) was -0.59. The pKa values and the degree of hydrophobic character of 5-propyl FPA are consistent with those of other protein-bound acids which undergo active tubular secretion by the kidney and this substance may serve as an endogenous marker for the effects of drugs and disease on this process.

Retention of an albumin-bound furan dicarboxylic acid in patients with chronic renal failure or after a kidney transplant.

BACKGROUND: 3-Carboxy-4-methyl-5-propyl-2-furanpropanoic acid (5-propyl FPA) is a furan dicarboxylic acid which accumulates in the plasma of patients with renal impairment. 5-Propyl FPA is an inhibitor of the binding of drugs to albumin and is also implicated in other aspects of the uraemic syndrome. METHODS: Plasma concentrations of propyl FPA have been measured in non-dialysis-dependent, chronic renal failure patients and in renal transplant patients by high-performance liquid chromatography. Concentrations of haemoglobin, albumin and creatinine were also determined. RESULTS: There was a positive correlation between serum creatinine and 5-propyl FPA and a negative correlation between haemoglobin concentration and 5-propyl FPA in chronic renal failure patients. There was a negative correlation between 5-propyl FPA and duration of transplant only when the serum creatinine was >200 microM. The mean plasma concentration of 5-propyl FPA in chronic renal failure patients with plasma creatinine CONCLUSIONS: This retention of 5-propyl FPA may therefore reflect a specific tubular defect in renal transplant patients treated with cyclosporin and points to the possibility that 5-propyl FPA may serve as a marker of tubular dysfunction.

Plasma clearance in the rat of a furan dicarboxylic acid which accumulates in uremia.

The furan dicarboxylic acid 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (5-propyl FPA) accumulates in the plasma of patients with chronic renal failure and has been implicated in several aspects of the uremic syndrome: the defective binding of organic acids in uremic plasma, inhibition of active tubular secretion, anemia and the severity of neurological symptoms. Evidence from experiments with rat kidney slices suggests that 5-propyl FPA undergoes active tubular secretion, and so its clearance after an intravenous bolus dose (5 mg/kg; 21 mumol/kg) was investigated in anaesthetized female Wistar albino rats in vivo. The effects of intravenous bolus doses of p-aminohippuric acid (PAH) and probenecid on the clearance of this dose of 5-propyl FPA were also studied. The mean values (N = 16) for plasma half-life, plasma clearance and apparent volume of distribution of 5-propyl FPA were 3.6 hours, 2.4 ml . min(-1) . kg(-1) and 0.69 liter . kg(-1), respectively. An equimolar dose of PAH did not affect the clearance of 5-propyl FPA, but a tenfold higher molar dose of PAH (40.4 mg/kg) increased the area under the plasma-concentration time curve of 5-propyl FPA, and there was a trend towards a decrease in the clearance and a prolongation of the half-life. Probenecid at a fivefold higher dose than 5-propyl FPA had a similar effect to PAH and increased the AUC of 5-propyl FPA. PAH and probenecid decreased the plasma clearance of 5-propyl FPA, which is evidence that this uremic metabolite undergoes active tubular secretion. It follows that 5-propyl FPA could therefore inhibit the secretion of other organic acids.

Hypothesis: is accumulation of a furan dicarboxylic acid (3-carboxy-4- methyl-5-propyl-2-furanpropanoic acid) related to the neurological abnormalities in patients with renal failure?

The Plasma concentrations of a lipophilic furan dicarboxylic acid (3- carboxy-4-methyl-5-propyl-2-fluranpropanoic acid; 5-propyl FPA), which is highly bound to albumin and not removed by haemodialysis, have been measured in patients with renal impairment who were not dialysis dependent or who were treated by either haemodialysis or peritoneal dialysis. Neurological abnormalities were assessed as absent, moderate, or severe. A relationship was observed between the increasing severity of abnormalities attributable to the uraemic state and the higher plasma concentrations of 5-propyl FPA. There are theoretical grounds for believing that 5-propyl FPA contributes to these neurological abnormalities because of its structure and also because it inhibits the transport of organic acids in the kidney and could do likewise at the blood-brain barrier.

Effects of haemodialysis and continuous ambulatory peritoneal dialysis on the plasma clearance of an albumin-bound furan dicarboxylic acid.

Organic acids that are strongly bound to albumin are not removed by dialysis and the plasma concentrations of one such substance, a furan dicarboxylic acid (3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid: 5-propyl FPA) have been measured by HPLC in healthy subjects (n = 21), patients on regular haemodialysis (n = 30), and patients treated by continuous ambulatory peritoneal dialysis (n = 21). The mean (+/- SD) concentrations of 5-propyl FPA were significantly higher in haemodialysis (95 +/- 44 microM) compared to CAPD patients (28 +/- 19 microM) and both were higher than in healthy individuals (14 +/- 7 microM). Haemoglobin concentrations in CAPD patients were significantly higher than in those on haemodialysis while these patients had significantly higher albumin concentrations than CAPD patients. The concentration of 5-propyl FPA was positively correlated with the duration of dialysis for haemodialysis patients but not for CAPD patients. The lower concentrations of 5-propyl FPA in CAPD patients may at least partly explain the higher haemoglobin levels found in these patients.