Advances in Analysis of Urinary Oxalate: The Ascorbate Problem Solved
Urinary oxalate is more critical than urinary calcium for calcium urolithiasis1, but still not measured in most laboratories. Despite many new recent methods (Table 1), none has found general acceptance2−5. One particular difficulty concerns ascorbate. In 1933, it was reported that ascorbic acid could be oxidised to oxalate6. Stage 1, thought to be oxidation of ascorbate to dehydroascorbate, is catalysed by nitrite and by Fe3+. In steps 2 and 3 (alkaline pH only), the latter converts to 2,3-diketogulonate and then to oxalate plus threonate. This important fact, forgotten for 50 years, was rediscovered simultaneously elsewhere7 and here. Since several promising methods have used alkaline conditions, serious errors could be introduced by ascorbate in urine, particularly in the summer time when several mmols per day may be present. We have therefore investigated this conversion in a number of methods and found ways of preventing it. Fortunately, the oxalate decarboxylase method2, operates at pH 3.5, where physiological ascorbate in urine does not yield detectable oxalate and it can be used as a reference method for studying the conversion. Figs. 1 and 2 show the conversion of ascorbate to oxalate under various conditions of time and pH. Fortunately, methods of preventing these conversions have recently been found. In the presence of oxygen, nitrite catalyses the oxidation of ascorbate to dehydroascorbate8, which is stable in acid, but not in alkaline conditions. Fe3+ forms an unstable complex with ascorbate which quickly converts to Fe2+ and dehydroascorbate9. Rather surprisingly, and for reasons unknown, ferric chloride prevents conversion of ascorbate to oxalate. Perhaps the iron forms a stable complex with the intermediate diketogulonate.
KeywordsUrinary Oxalate Oxalate Concentration Oxalate Oxidase Oxalate Oxalate Unstable Complex
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