An assay for oxalate based on the conductometric measurement of enzyme-liberated carbon dioxide

Abstract

Measurement of oxalate in tissues and fluids poses difficulties, which undoubtedly explains the numerous techniques reported (1–6). Chemical methods with subsequent spectrophotometric measeuremen (3–7) have superseded the classical precipitation and permanganate titration 98), but, for the most part, they are time consuming, tedious, prone to error, and, with reference to urine, require alarge sample volume. Considerable advance in oxalate assay occurred with the availability of oxalate decarboxylase (EC 4.1.1.2) to catalyze the reaction oxalate → CO 2 + formate. The most commonly measured end product has been CO 2, and this gas has been measured by techniques which include Warburg manometry (9), CO 2 electrodes (5), and pH-sensitive dyes (4, 10). Unfortunately, these methods often fall short of the ideal through a lack of sensitivity, endogenous CO 2 contamination, inconvenience with equipment, or through some other cause. The present report describes an apparatus and method for CO 2 detection which is precise, sensitive, relatively rapid, and, in regard to an assay for oxalate, specific by virtue of the enzymic reaction. The determination of CO 2 is made by conductometric measurement in an alkali solution, using principles previously described (11). In addition, a modified procedure for the extraction of oxalate from tissues also is reported.