Liquid chromatography-tandem mass spectrometry determination of oxalate in spot urine

BACKGROUND

For assessment of total oxalic acid (OX) status, reliable quantification of OX in both urine and plasma is important. For urine, but not plasma, a commercial kit is available. We have recently described a LC-MSMS method for OX in plasma. The aim of the present study was to evaluate the usefulness of this assay for urine. We also wanted to evaluate if 24 h urine collection could be substituted by OX/creatinine-ratio (U-OX/crea) in spot-urine, and establish precursory reference intervals for U-OX/crea in children and adults.

METHODS

Acidified urines were analysed and relevant validation parameters assessed. Diurnal excretion patterns were investigated in nine healthy volunteers on self-chosen diets. For method comparison, 29 urine samples were analysed with both the present method and a commercial urine-oxalate kit. Precursory reference values for U-OX/crea in children and adults (N=103, 1 month-76 years) were calculated.

RESULTS

The within-batch coefficient of variation (CV) was 2.5% and a relative recovery of 97% in urine spiked with 5-200 micromol/L OX was found. The LC-MSMS method gave 7.9% higher OX values compared to the kit. No significant diurnal pattern of U-OX/crea was observed. U-OX/crea in children decreases with age, with no gender dependency. In adults no age variation was found, but females had somewhat higher U-OX/Crea compared to males.

CONCLUSION

The LC-MSMS method has proven useful for urinary OX quantification. Random spot-urine samples can be used. Age-dependent reference limits for U-OX/crea must be applied in children, in contrast to adults.

Rapid and convenient determination of oxalic acid employing a novel oxalate biosensor based on oxalate oxidase and SIRE technology

A new method for rapid determination of oxalic acid was developed using oxalate oxidase and a biosensor based on SIRE (sensors based on injection of the recognition element) technology. The method was selective, simple, fast, and cheap compared with other present detection systems for oxalate. The total analysis time for each assay was 2-9 min. A linear range was observed between 0 and 5 mM when the reaction conditions were 30 degrees C and 60 s. The linear range and upper limit for concentration determination could be increased to 25 mM by shortening the reaction time. The lower limit of detection in standard solutions, 20 microM, could be achieved by means of modification of the reaction conditions, namely increasing the temperature and the reaction time. The biosensor method was compared with a conventional commercially available colorimetric method with respect to the determination of oxalic acid in urine samples. The urine oxalic acid concentrations determined with the biosensor method correlated well (R=0.952) with the colorimetric method.

Improved determination of urinary oxalate with alkylamine glass bound barley oxalate oxidase

The measurement of oxalate in urine was improved by employing barley oxalate oxidase immobilized on alkylamine glass beads affixed in a glass beaker. The minimum detection limit was 3.6 mg l(-1) urine. The recovery of added oxalate was 88.9+/-9.2%. Within- and between-assay coefficients of variation (CV) were <4.0 and <9.4%, respectively. The urinary oxalate values were obtained by a commercial kit method and the present method showed a good correlation (0.999). The method is free from tedious handling of glass beads and Cl- interference.

Discrete analysis of plasma oxalate with alkylamine glass bound sorghum oxalate oxidase and horseradish peroxidase

We have reported a simple method of determination of plasma oxalate using a Cl(-) and NO(3)(-) insensitive oxalate oxidase purified from grain sorghum leaf and commercially available peroxidase from horseradish [Pundir et al., Ind. J. Biochem. Biophys., 35 (1998) 120-122]. The present report describes the immobilization of both the enzymes onto alkylamine glass, their kinetic properties and application for discrete analysis of plasma oxalate. In the analytic method, H(2)O(2) generated from plasma oxalate by immobilized oxalate oxidase is measured colorimetrically at 520 nm by oxidative coupling with 4-aminophenazone, and phenol catalyzed by immobilized peroxidase. The minimum detection limit of the method is 2.5 micromol/l. Analytic recovery of added oxalate in plasma was 89. 5+/-4.1% (mean+/-S.D.). The within and between day CV for plasma oxalate measurement were <9.37 and <11.0%, respectively. The normal range of plasma oxalate as measured by the present method was 3.6 to 5.7 micromol/l. The method is not only free from interference by plasma Cl(-) and NO(3)(-) but also provides the reuse of glass beads and thus reduces the cost of analysis for routine.

Idiopathic calcium oxalate urolithiasis and endogenous oxalate production

Despite the great effort that has gone into investigating urolithiasis, this condition still persists as one of the major ailments of the urinary tract. Calcium oxalate urolithiasis is the most common form, accounting for some 60 to 80% of total stones. This review examines the elements (i.e., urine volume and pH and urinary excretion of calcium, oxalate, citrate, urate, magnesium, pyrophosphate, and glycosaminoglycans) that give rise to idiopathic calcium oxalate urolithiasis. Treatment strategies for idiopathic calcium oxalate urolithiasis, including lithotripsy, also are discussed. Urinary oxalate excretion is a major risk factor for calcium oxalate urolithiasis, with 85 to 95% of the urinary load derived endogenously. The factors controlling endogenous oxalate production are reviewed, including pathways for the diversion of glyoxylate from oxalate production. The use of beta-aminothiols and other substances to reduce endogenous oxalate production in subjects with idiopathic calcium oxalate urolithiasis is also discussed. A review of current methodologies for the determination of urinary oxalate is also included.

Amperometric determination of oxalate in plasma and urine by liquid chromatography with immobilized oxalate oxidase

A detection system for plasma and urinary oxalate involving a postcolumn enzymatic reaction and electrochemical detection is described. Oxalate oxidase was immobilized on AF-Tresyl Toyopearl 650 gel. The immobilized oxalate oxidase was packed into a stainless-steel column (10 x 4 mm I.D.) and used on-line as an immobilized enzyme reactor (IMER). The hydrogen peroxide produced by enzymatic reaction was detected amperometrically at a platinum electrode maintained at +0.5 V vs. Ag/AgCl. The HPLC separation of oxalate was carried out using a Capcell Pak C8 column and an isocratic mobile phase containing 80 mM KH2PO4 and 5 mM tetra-n-butylmammonium phosphate as an ion-pair reagent. The IMER was active and stable in the mobile phase employed. The plot of peak height against concentration of oxalate was linear in the range 0.1-1.6 mumol/ml with a correlation coefficient of 0.9989. The detection limit was 10 nmol/ml at a signal-to-noise ratio of 3. The within-day and between-day coefficients of variation were 5.3 and 7.9%, respectively.

Comparative study of two commercial enzymatic kits for determining oxalate concentrations in urine

We performed a comparative study of two commercial kits for determining oxalate in urine. These were: (a) an oxalate decarboxylase-based assay (Boehringer Mannheim); (b) an oxalate oxidase-based assay (Sigma). The within-run and between-run imprecision were found to be similar in both methods. The recovery was 94% with the oxalate decarboxylase method. The pH of the specimen had a major effect on the recovery obtained by the oxalate oxidase method (66-86% at pH = 2.5 and 37-68% at pH = 1.5). The decarboxylase method was linear up to at least 2224 mumol/L and the oxidase method was linear up to at least 890 mumol/L. We also studied the interference of ascorbic acid in both techniques and found a positive bias with the oxidase method and a negative bias using the decarboxylase method. The correlation coefficient was 0.592.

The inhibition of metabolic oxalate production by sulfhydryl compounds

A number of sulfhydryl compounds were shown to inhibit CO2 and oxalate formation from glyoxylate by rat liver homogenates and hepatocytes. The most significant inhibition occurred with cysteine and this inhibition was concentration-dependent. In rats made hyperoxaluric by administering ethylene glycol in their drinking water, daily intraperitoneal injections of cysteine caused a rapid and marked decrease in urinary oxalate excretion which was maintained over the duration of the treatment (28 days). Over this time period, the level of urinary oxalate excretion in these ethylene glycol-treated rats was reduced to that of the controls. It is postulated that the decrease is due to the formation of a cysteine-glyoxylate adduct, 2-carboxy-4-thiazolidine carboxylate, which prevents glyoxylate being further oxidized to oxalate. Cysteine or similar sulphydryl compounds may therefore have potential as therapeutic agents in the prevention of renal stones.

The effect of decreasing the concentration of urinary urate on the crystallization of calcium oxalate in undiluted human urine

The effect of lowering the urate concentration, using uricase immobilized on to nylon tubing, on the nucleation, growth and aggregation of calcium oxalate crystals in undiluted human urine was examined. The median urate concentration was significantly (p less than 0.01) reduced from 2.8 to 0.55 mmol/l., but this had no reproducible effect on the metastable limits, the volume of crystalline calcium oxalate deposited and the size of the crystals and aggregates produced from the 10 urine samples examined. It was concluded that further studies aimed at testing the effects of a raised urinary urate concentration on the crystallization of calcium oxalate should be made a matter of high priority.

A simple, rapid assay for plasma oxalate in uraemic patients using oxalate oxidase, which is free from vitamin C interference

An enzymatic assay for the determination of oxalate in plasma was developed which is specific, simple, rapid and requires no specialised equipment; interference from vitamin C was removed by incubation of acidified plasma ultrafiltrate with ascorbate oxidase prior to oxalate estimation. Recoveries were 93 +/- 11% and the inter-batch coefficient of variation for 31 determinations at an oxalate level of 24 mumol/l was 10%. The assay is linear up to 300 mumol/l with a detection limit of 2 mumol/l. The reference range, based on results from 25 healthy volunteers, was defined as less than 2-5 mumol/l which is similar to levels established for the in vivo isotope dilution technique. The assay has an added advantage over the latter method, which requires a urine collection, in that it can be applied to plasma from anuric patients. A linear correlation (r = 0.68, p less than 0.001) was found between plasma oxalate and serum creatinine in individuals with varying degrees of renal failure.

The determination of plasma oxalate concentrations using an enzyme/bioluminescent assay. 2. Co-immobilisation of bioluminescent enzymes and studies of in vitro oxalogenesis

An inexpensive, continuous flow assay for the determination of oxalate in plasma is described. The assay is based on the bioluminescent determination of NADH, a product of the degradation of oxalate by oxalate decarboxylase and formate dehydrogenase, using bioluminescent enzymes immobilized on cyanogen bromide-activated sepharose. The detection limit of the assay is 0.8 mumol/l. Intra-batch CV values of 5.2 and 3.8% were obtained at oxalate concentrations of 18 and 60 mumol/l. Recovery of added oxalate averaged 100.7%. Plasma oxalate ranged from less than 0.8 to 2 mumol/l in 14 healthy subjects, and from 6 to 134 mumol/l in 125 patients with renal disease treated by continuous ambulatory peritoneal dialysis. Ascorbic and dehydroascorbic acid did not directly interfere in the assay. In vitro oxalogenesis was observed in blood from 12 healthy subjects, but only after samples had stood at room temperature for more than 6 h. No significant oxalate generation occurred in blood from 24 patients with impaired renal function, even after standing at room temperature for 24 h. Oxalate generation was inhibited by the addition of oxalate to plasma, but the addition of urea and creatinine was without effect.

Dichloroacetate derivatives. Metabolic effects and pharmacodynamics in normal rats

Dichloroacetate (DCA) reduces blood glucose, lactate and lipids in diabetes or during fasting. Chronic use of DCA, however, is limited by toxicity, probably due in part to its rapid conversion to oxalate in vivo. In theory, therefore, DCA's efficacy may be retained and its toxicity minimized by controlling its rate of metabolism. We attempted to alter DCA pharmacokinetics and bioavailability by synthesizing various derivatives comprising DCA esters with polyols and DCA ionic complexes. Twenty-four hour fasted, nondiabetic rats received single, orogastric doses of saline (control) sodium DCA (100mg/kg) or the following derivatives (D1-4): the esters D1-D3: potassium tetra (dichloroacetyl) glucuronate (D1), inositol-monophosphate-tetradichloroacetate (D2), inositol-hexadichloroacetate (D3) and inositol-hexa [N-methylnicotinate] hexadichloroacetate salt (D4). Each derivative was administered at a dose that would ultimately provide 100 mg/kg DCA as the anion. All derivatives were orally effective in significantly decreasing blood glucose and lactate. D4 exerted the most potent and long-lasting glucose- and lactate-lowering effects, yet increased plasma DCA concentrations less than an equivalent dose of the sodium salt. When administered to reverse light-cycled rats, D4 markedly inhibited the incorporation of tritiated water into cholesterol and triglycerides. We conclude that derivatives of DCA retain the biological activity of the parent compound, but may exhibit different pharmacokinetics. They may eventually prove useful in the treatment of diabetes mellitus, hyperlipidemia and lactic acidosis in man.

The effect of dietary refined sugars and sugar alcohols on renal calcium oxalate deposition in ethylene glycol-treated rats

The effect of administering refined carbohydrates in the diet on calcium oxalate deposition in the kidneys of rats given 1% (v/v) ethylene glycol in their drinking-water was investigated. The rats were given 0, 2.5, 10, 30 or 60% sucrose in the feed (w/w) and/or drinking-water (w/v) or 20% (w/w) starch, glucose, sucrose, fructose, galactose, xylitol or sorbitol in the feed for 3 wk. All of the animals remained healthy over the test period as far as could be assessed by the measurement of 19 plasma biochemical parameters. The inclusion of 30 or 60% (w/w) sucrose in the diet resulted in a more than tenfold increase in the deposition of calcium oxalate in the kidneys. However, this deposition could not be predicted from data on urinary pH and urinary excretion of calcium, oxalate and urate, which have been reported to be risk factors for stone formation. There was no evidence of increased rates of oxalate production from ethylene glycol. The administration of fructose, xylitol or sorbitol was associated with the greatest renal deposition of calcium oxalate, and glucose was associated with by far the least.

The long-term effect of dietary administration of refined sugars and sugar alcohols on plasma biochemistry, urine biochemistry and tissue histology in mice given a limited degree of dietary self-selection

A prototype animal feeding model is described in which mice were meal-fed a balanced diet but were given free access to water (controls) or 20% (w/v) solutions of glucose, sucrose, fructose, xylitol or sorbitol. Under these conditions it was found that the provision of an alternative energy source, in the form of a refined carbohydrate, produced marked effects on total energy intake, mouse cube (i.e. balanced energy) intake and body weight. There were also changes in the metabolic states of the animals as assessed by serum levels of glucose, urea and cholesterol, plasma levels of lactate and D-3-hydroxybutyrate, and urinary excretion of urea and oxalate. Histological examinations of tissue indicated that the sucrose-fed mice had a tendency to suffer from acute congestion of the lungs and liver steatosis. Given a limited degree of dietary self-selection it appears that mice are more likely to be at risk of excessive food consumption and obesity when given glucose- or sucrose-containing diets than they are when fructose-, xylitol- or sorbitol-containing diets are given.

Comparison of urinary oxalate excretion in urolithiasis patients with and without hypercalciuria

The relationships between urinary oxalate, calcium and magnesium were investigated in 81 patients with idiopathic calcium oxalate urolithiasis on their regular diets. A significant relationship was established between calcium and oxalate excretion in the analysis of recurrent stone-formers (n = 44, P less than 0.01), though there was no significant difference between the two in the analysis of the patients overall or in single stone-formers. This suggests that recurrent stone-formers may have some abnormality of oxalate absorption in relation to calcium absorption. The role of calcium-oxalate interaction in the gut as a cause of mild hyperoxaluria is discussed.

Continuous-flow assay for urinary oxalate using immobilised oxalate oxidase

A continuous-flow assay for measuring oxalate in urine is described. Covalently attached oxalate oxidase (EC 1.2.3.4) is used to oxidize the oxalate anion to carbon dioxide and hydrogen peroxide. The formed hydrogen peroxide is measured colorimetrically (A580) with an established reaction using horseradish peroxidase (EC 1.11.17), 3-methyl-2-benzothiazolinone hydrazone (MBTH) and 3-dimethylaminobenzoic acid (DMAB). Ascorbate interference is eliminated by treating the urine sample with sodium nitrite prior to assaying. The assay is accurate (mean recovery of added oxalate in spiked urine sample is 93 +/- 11%), sensitive (detection limit 1.0 mumol/L), reproducible (within-batch CV 3.5%; between-batch CV 5%) and relatively rapid (15 samples/h). This assay correlates well (R = 0.99) with another established enzymatic method (using oxalate decarboxylase).

Simplified urinary oxalate determination using an enzyme electrode

An enzyme electrode for urine oxalate measurement has been produced using acrylamide gel-entrapped oxalate decarboxylase retained over a CO2 sensor. Urine required pre-treatment with EDTA, but oxalate extraction was not necessary and inhibition by phosphate and sulphate was not apparent until after 10-14 days. The linear range was 0-0.2 mmol/l. Analysis of 50 urine specimens diluted 1 in 10 in pH 3.0 glycine buffer showed good correlation with a widely-used colorimetric method (y = 1.101x-0.018, r = 0.955).