New assay for enzymatic phosphate release: application to aspartate transcarbamylase and other enzymes

The colorimetric method for phosphate determination described in the preceding paper is adapted for the assay of orthophosphate liberated in the aspartate transcarbamylase reaction. The method provides for simple, accurate, and sensitive measurement of enzyme activity. The assay uses ammonium molybdate and zinc acetate to form a colored complex with the enzymatically released phosphate; mild conditions which minimize the nonenzymatic background degradation of the substrate, carbamoyl phosphate, are used. Since the assay procedure is relatively rapid, it is especially attractive in situations where results are desired immediately. The method can be used for the assay of any enzyme which releases inorganic phosphate, even in the presence of labile organophosphate compounds.