Evaluation of the role of 5-phosphoribosyl-alpha-1-pyrophosphate synthetase in congenital hyperuricemia and gout: a simple isotopic assay and an activity stain for the enzyme

Phosphoribosylpyrophosphate synthetase superactivity. A study of five patients with catalytic defects in the enzyme

Superactive phosphoribosylpyrophosphate (PRPP) synthetases were characterized in fibroblasts and erythrocytes from 5 unrelated men with gout and/or hyperuricemia and uric acid overproduction. The kinetic basis of enzyme superactivity in all patients was increased maximal reaction velocity. Affinities of the enzymes for substrates and activators and responsiveness to inhibitors were normal, and levels of immunoreactive enzyme in patient and control fibroblast and erythrocyte extracts were comparable. Enzymes purified to homogeneity from 2 patients confirmed the presence of isolated catalytic defects. Altered physical properties of certain of the superactive enzymes suggested the presence of several distinctive structural defects among the aberrant forms. Fibroblasts from each affected patient showed increased PRPP concentration and generation, as well as accelerated rates of all PRPP-requiring purine nucleotide synthetic pathways. These findings support the concept that enzyme superactivity results in uric acid overproduction as a consequence of increased rates of PRPP and purine nucleotide synthesis. Cultured cells from female relatives of 2 patients showed evidence for the heterozygous carrier state, as measured both by enzyme activities and by rates of PRPP and purine synthesis. The clinical phenotype in 4 patients was limited to early adult-onset gout and its consequences, whereas the fifth patient expressed a familial constellation of hyperuricemia, sensorineural deafness, ataxia, and renal insufficiency. The severity of the derangements in PRPP synthetase and in PRPP and purine synthesis in cells from the 5 patients, however, was comparable. The neurologic accompaniments of enzyme superactivity found in 1 family described here, and in 2 others described previously, thus may not necessarily be consequences of primary defects in PRPP synthetase.

Selective expression of phosphoribosylpyrophosphate synthetase superactivity in human lymphoblast lines

Phenotypic expression of 5-phosphoribosyl 1-pyrophosphate (PRPP) synthetase superactivity was examined in lymphoblast lines derived from six unrelated male patients. Fibroblasts from these individuals have increased rates of PRPP and purine nucleotide synthesis and express four classes of kinetic derangement underlying enzyme superactivity: increased maximal reaction velocity (catalytic defect); inhibitor resistance (regulatory defect); increased substrate affinity (substrate binding defect); and combined catalytic and regulatory defects. Lymphoblast lines from three patients with catalytic defects and from three normal individuals were indistinguishable with respect to enzyme activities, PRPP concentrations and generation, and rates of purine synthesis. Enzyme in lymphoblasts from a patient with combined defects also showed normal maximal reaction velocity but expressed purine nucleotide inhibitor resistance. A second regulatory defect and a substrate binding defect were also demonstrable in lymphoblasts and were identical to the enzyme defects in fibroblasts from the respective patients. Regulatory and substrate binding defects in lymphoblasts were accompanied by increased rates of PRPP and purine nucleotide synthesis. Among explanations for selective expression of enzyme superactivity, reduced concentrations of catalytically superactive enzymes seemed unlikely: immunoreactive PRPP synthetase was comparable in normal-derived and patient-derived cells. Activation of normal enzyme in transformed lymphocytes was also unlikely because absolute specific activities of lymphoblast PRPP synthetases corresponded to those of normal fibroblast and erythrocyte enzymes. Abnormal electrophoretic mobilities and thermal stabilities, identified in certain catalytically superactive fibroblast PRPP synthetases, were not found in the corresponding lymphoblast enzymes. Thus, lymphoblast PRPP synthetases from patients with catalytic superactivity appeared to differ structurally and functionally from their fibroblast counterparts.

A spectrophotometric assay of phosphoribosyl pyrophosphate synthetase

A simple spectrophotometric assay of phosphoribosyl pyrophosphate synthetase from human erythrocytes is described. The kinetic properties of the enzyme have been determined and are in good agreement with published results. Conditions for storing haemolysates at -20 degrees C are described. The method is suitable for semi-automation.

Variant human phosphoribosylpyrophosphate synthetase altered in regulatory and catalytic functions

An inherited, structurally abnormal and superactive form of the enzyme 5-phosphoribosyl 1-pyrophosphate (PP-ribose-P) synthetase (EC 2.7.6.1) has been characterized in fibroblasts cultured from a 14-yr-old male (S.M.) with clinical manifestations of uric acid overproduction present since infancy. PP-ribose-P synthetase from the cells of this child showed four- to fivefold greater than normal resistance to purine nucleotide (ADP and GDP) feedback inhibition of enzyme activity and hyperbolic rather than sigmoidal inorganic phosphate (Pi) activation in incompletely dialyzed extracts. Excessive maximal velocity of the enzyme reaction catalyzed by the mutant enzyme was indicated by: enzyme activities twice those of normal at all concentrations of Pi in chromatographed fibroblast extracts; normal affinity constants for substrates and for the activator, Mg2+; and twofold greater than normal activity per immunoreactive enzyme molecule. The mutant enzyme thus possessed deficient regulatory and superactive catalytic properties, two mechanisms previously demonstrated individually to underlie the excessive PPRribose-P and uric acid synthesis of affected members of families with superactive PP-ribose-P synthetases. Increased PP-ribose-P concentration (4-fold) and generation (2.7-fold) and enhanced rates of PP-ribose-P dependent purine synthetic reactions, including purine synthesis de novo, in S.M. fibroblasts confirmed the functional significance of this patient's mutant enzyme. Diminished stability of the variant PP-ribose-P synthetase was manifested in vitro by increased thermal lability and in vivo by deficiency of enzyme activity at Pi concentrations greater than 0.3 mM in hemolysates and by an accelerated, age-related decrement in enzyme activity in lysates of erythrocytes separated by specific density. Despite the diminished amount of PP-ribose-P synthetase in the S.M. erythrocyte population, S.M. erythrocytes had increased PP-ribose-P concentration and increased rates of incorporation of [14C]adenine and hypoxanthine into acid-soluble nucleotides during incubation at 1 mM Pi. These findings provided further confirmation of the extent to which PP-ribose-P synthesis is modulated in the normal cell at physiological Pi concentration by purine nucleotide inhibition of PP-ribose-P synthetase. The activity and kinetic characteristics of PP-ribose-P synthetase from fibroblasts of the mother of patient S.M. indicated that this woman was a heterozygous carrier of the enzyme defect expressed in hemizygous manner by her son.

A spectrophotometric method for the determination of 5-phosphoribosyl-1-pyrophosphate

A new, sensitive, specific and simple spectrophotometric method for the determination of 5-phosphoribosyl-l-pyrophosphate (PRPP) is presented. PRPP is reacted with excess hypoxanthine in the presence of hypoxanthine-guanine phosphoribosyltransferase. At the end of the reaction, PRPP concentration is measured from the extent of conversion of hypoxanthine to inositate. The concentration of the purine base is determined spectrophotometrically in the presence of xanthine oxidase.

Electrophoretic heterogeneity of 5-phosphoribosyl-1-pyrophosphate synthetase within and among humans

The product of the enzyme 5-phosphoribosyl-1-pyrophosphate (PPriboseP) synthetase is a substrate for purine, pyrimidine, and pyridine biosynthesis and may be rate limiting for purine biosynthesis. A system developed to electrophoretically separate and histochemically detect PPriboseP synthetase in crude cell extracts has facilitated the identification of electrophoretically variant enzyme forms in the erythrocytes of five patients from a patient population of 200. Additional studies of human organs obtained at autopsy revealed a unique electrophoretic mobility for nearly each organ within the same individual.

In vitro and in vivo age-related modification of human erythrocyte phosphoribosyl pyrophosphate synthetase

Upon storage, human erythrocyte phosphoribosyl pyrophosphate synthetase (PRibPP synthetase, EC 2.7.6.1) from normal individuals was found to undergo a spontaneous dissociation into active enzyme components of much smaller molecular mass (60 000--90 000). These modified forms of enzyme exhibit kinetic properties different from the original large molecular weight enzyme (over 200 000). The small active components can be reversibly associated to form larger molecules in the presence of purine ribonucleotides as well as phosphoribosyl pyrophosphate (PRibPP). ATP was found to be most effective in associating PRibPP synthetase, while guanylate nucleotides seem to have no effect. The large molecular weight components, once separated from the milieu, were not able to undergo further dissociation. Fresh or stored human white cell tissue homogenates were found to lack the low-molecular-weight enzyme under all our experimental conditions. A characteristic enzyme modification similar to that observed in stored erythrocyte was also noted in erythrocytes of increasing ages. The physiological significance of these findings to the regulatory function of PRibPP synthetase in purine metabolism in vivo is discussed.

A simple and sensitive method for estimating the concentration and synthesis of 5-phosphoribosyl 1-pyrophosphate in red blood cells

A method is presented for the determination of 5-phosphoribosyl 1-pyrophosphate (PRPP), which is based on the release of 14CO2 from [carboxyl-14C]-orotic acid by the consecutive action of orotate phosphoribosyltransferase and orotidine-5'-monophosphate decarboxylase. The assay is simpler and less time-consuming than most methods currently employed and is equally sensitive. The method proved to be suitable for measuring low concentrations of PRPP such as found in human erythrocytes and fibroblasts. An increased PRPP concentration was observed in erythrocytes from patients with partial or complete deficiency of hypoxanthine-guanine phospho-ribosyltransferase. frp, sp,e (but not all) gouty patients and from a patient with deficiency of purine nucleoside phosphorylase. PRPP synthetase activity was measured with a method similar to the assay for PRPP. In erythrocytes with an increased PRPP concentration, PRPP synthetase activity was found to be normal at both optimal and suboptimal substrate concentrations.