Recent advances in the identification of enzyme abnormalities underlying excessive purine synthesis in man

Implications of disorders of purine metabolism for the kidney and the urinary tract

The spectrum of kidney and urinary tract disorders related to purines comprises acute hyperuricosuric nephropathy, chronic urate nephropathy and urolithiasis. Two factors in the development of acute hyperuricosuric nephropathy are increased uric acid concentration and low pH in the tubular fluid. Chronic urate nephropathy still possess several problems: incidence (although this seems to be decreasing, presumably owing to effective prevention), the source of interstitial urate, the cause of the interstitial deposition of urate, and the role of urate deposits in the pathogenesis of the interstitial nephropathy. The relation of the experimental nephropathy to the pathogenesis of chronic urate nephropathy in the human is not yet clear but a model is proposed according to which interstitial urate derives from two sources: hyperuricaemic plasma and hyperuricosuric tubular fluid. Urolithiasis related to purines leads to uric acid-urate stones, xanthine stones, 2,8-dihydroxyadenine stones, iatrogenic xanthine and oxipurinol stones, and possibly calcium stones. Pathogenetic factors in uric acid lithiasis are hyperuricosuria (whether due to an inborn enzyme abnormality or of unknown aetiology) and low urinary pH; oliguria is a contributory factor. There remain several open questions about uric acid lithiasis: incidence, the shift of its location from lower to upper urinary tract, the interplay of pathogenetic factors, and the role of compounds which inhibit crystallization.

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.

Isolation and characterization of a full-length expressible cDNA for human hypoxanthine phosphoribosyl transferase

We have cloned a full-length 1.6-kilobase cDNA of a human mRNA coding for hypoxanthine phosphoribosyltransferase (HPRT; IMP:pyrophosphate phosphoribosyltransferase, EC 2.4.2.8) into a simian virus 40-based expression vector and have determined its full nucleotide sequence. The inferred amino acid sequence agrees with a partial amino acid sequence determined for authentic human HPRT protein. Transfection of HPRT-deficient mouse LA9 cells with the purified plasmid leads to the expression of human HPRT enzyme activity in cells stably transfected and selected for enzyme activity in hypoxanthine/aminopterin/thymidine medium.

Isolation of a genomic clone partially encoding human hypoxanthine phosphoribosyltransferase

Mouse cells deficient in the enzyme hypoxanthine phosphoribosyltransferase (HPRT; EC 2.4.2.8) have been transfected with total human DNA, and cells producing human enzyme were isolated by growth in selective medium. DNA from several such cell lines has been used to generate secondary transfectants that make human HPRT. Blots of the DNA of these secondary cells have been hybridized with total human DNA probes or with cloned human Alu sequences, and one of several common bands has been cloned in pBR322. Colonies of transformed Escherichia coli containing human sequences were detected by their homology with human DNA, and subclones of resulting recombinant plasmids were prepared. Two subclones free of Alu sequences were found to contain human sequences that hybridized to human X chromosome DNA. One of these, pBR1.5, also hybridized to a single RNA band on gel blots of human and secondary transfectant cytoplasmic poly(A)+RNA but not to RNA from the parent mouse cell line. These results indicate that these clones represent human HPRT gene fragments. This has been confirmed by using pBR1.5 as a probe to isolate an authentic and expressible human HPRT cDNA clone from a library prepared by H. Okayama and P. Berg.

The mode of genetic transmission of gouty family with increased phosphoribosylpyrophosphate synthetase activity

The mode of genetic transmission of gout and increased activity of phosphoribosylphrophosphate synthetase (PRPPS) was studied in one family. Among 15 members of Family F, two male members had gout and had PRPPS activity of erythrocyte lysates three times higher than normal subjects. Five female members had activity 2.5 times higher than normal. The difference between the activities of male and female affected members was statistically significant (P less than 0.05). To examine the genetic trait of this abnormal PRPPS, the incorporation of 3H-adenine into erythrocytes or lymphocytes was studied using autoradiography. The number of grains which show the uptake of labeled adenine into cells revealed a normal distribution pattern in two normal persons and in two male patients, and a mixed pattern of the two cell populations in two female affected members. These results suggested mosaicism in female members and X-linked dominant transmission of this trait. Thermal inactivation of PRPPS of an affected female was intermediate between that from a normal subject and that from the affected males. This result showed the heterogeneity of the PRPPS from the hemolysate of an affected female. The genotype of PRPPS on the X-chromosome was assumed and the lod score between PRPPS and Xg was also estimated. From these findings and electrophoretical study, it was suggested that the abnormal enzyme was a mutant enzyme transmitted in an X-linked dominant trait, and that the mutation occurred on the structural gene of the PRPPS.

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.