Diagnostic evaluation of phosphoribosylpyrophosphate synthetase activities in hemolysates

Superactivity of phosphoribosylpyrophosphate (PRPP) synthetase is one of several hereditary enzyme abnormalities associated with gout and excessive uric acid excretion. Although measurement of PRPP synthetase activities in erythrocyte lysates should provide a practical means to detect abnormalities of the enzyme, reported values for normal individuals have varied considerably. We describe a radioisotopic procedure for measurement of PRPP synthetase activities in dialyzed hemolysates under a variety of conditions permitting evaluation of enzyme catalytic function and responsiveness to inhibitors and activators. Utilizing this procedure, enzyme activities for normal individuals were higher than generally reported, a difference attributable in part to the following measures undertaken to assure accuracy in activity determinations: precise control of pH of reaction mixtures, provision of verified excesses of the auxiliary enzyme adenine phosphoribosyltransferase, and measurement of all of the radiolabeled products of the assay. Under each condition of measurement, enzyme activities in 44 normal individuals, 13 patients with gout and normal uric acid excretion, and 10 patients with gout and uric acid overproduction were indistinguishable. In four additional individuals with uric acid overproduction, however, excessive enzyme activities were identifiable at all inorganic phosphate concentrations, but responses to purine nucleotide inhibitors were normal. In hemolysates from a patient with an inhibitor-resistant PRPP synthetase, an altered pattern of inorganic phosphate activation and diminished nucleotide inhibitor response was demonstrated. Our studies confirm the ability of the assay procedure to detect kinetically distinct variant forms of PRPP synthetase. Application of this procedure should aid in evaluation of the prevalence of derangements of PRPP synthetase among patients with gout and uric acid overproduction.