Enzymic determination of inorganic phosphates, organic phosphates and phosphate-liberating enzymes by use of nucleoside phosphorylase-xanthine oxidase (dehydrogenase)-coupled reactions.

Coupled enzyme assays are described for measuring inorganic phosphates, organic phosphates and phosphate-liberating enzymes in biological material. The assays all determine Pi by its reaction with inosine, catalysed by nucleoside phosphorylase; this yields ribose 1-phosphate and hypoxanthine. The hypoxanthine is oxidized to uric acid by xanthine oxidase, and may be measured either by the absorbance of the uric acid, or by the formazan formed when a tetrazolium salt is used as the oxidant. The coupled enzyme assays are characterized by high sensitivity, quantitative utilization of phosphates and stoichiometric formation of the measurable products, measurement at pH 6.0-8.5, determination of phosphates within a single analytical step, and continuous measurement of phosphohydrolase activity in a corresponding rate assay. Examples include determinations of substrates such as Pi, PPi and AMP, and of enzymes such as 5'-nucleotidase, inorganic pyrophosphatase and glucose-6-phosphatase. Directions for further examples are given.