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Biomedical subjects

M A Becker

Publications and source records attributed to M A Becker.

At least 73 records · Page 4Linked to original sources
PubMed1980

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.

Adolescent↗
PubMed1979

Regional localization of the gene for human phosphoribosylpyrophosphate synthetase on the X chromosome.

Sixty-eight independent hybrid clones were isolated after irradiated normal human lymphocytes were fused with Chinese hamster fibroblasts lacking hypoxanthine-guanine phosphoribosyltransferase activity. The cells were grown under selective conditions requiring retention of the X chromosome-linked locus for human hypoxanthine-guanine phosphoribosyltransferase. The frequency and patterns of cotransference of human phosphoribosylpyrophosphate synthetase with the selected marker and with additional X-linked enzymatic markers confirm X linkage of the structural gene for human phosphoribosylpyrophosphate synthetase and support assignment of this gene to a position on the long arm of the X, between the loci for alpha-galactosidase and hypoxanthine-guanine phosphoribosyltransferase.

Chromosome Mapping↗
PubMed1978

Evidence for X-linkage of human phosphoribosylpyrophosphate synthetase.

The mode of genetic transmission of human phosphoribosylpyrophosphate synthetase (ribosephosphate pyrophosphokinase; ATP:D-ribose-5-phosphate pyrophosphotransferase; EC 2.7.6.1) was studied in fibroblasts cultured from members of a family with a structurally and electrophoretically altered phosphoribosylpyrophosphate synthetase that has increased activity per enzyme molecule. Enzyme activity in fibroblast lysates from the daughter of an affected male patient was intermediate to the activities in lysates from her father (and her affected paternal uncle) and from her mother and other normal individuals. Two bands of enzyme activity corresponding to normal and mutant phosphoribosylpyrophosphate synthetases were found in fibroblast lysates from the daughter after cellulose acetate strip electrophoresis. In contrast, only mutant enzyme was detectable in lysates derived from the male patients. Fibroblasts cloned from the daughter contained two phenotypically distinct (normal and mutant) populations of cells with respect to phosphoribosylpyrophosphate synthetase activity and electrophoretic mobility. These studies support assignment of the structural gene for human phosphoribosylpyrophosphate synthetase to the X-chromosome. No evidence for the presence of the normal enzyme was found in erythrocyte or lymphocyte lysates or in partially purified erythrocyte enzyme preparations from the heterozygous daughter, suggesting either nonrandom X-chromosome inactivation in precursors of these cells or selection against hematopoietic cells bearing the normal enzyme after random X-chromosome inactivation.

Cells, Cultured↗
PubMed1977

Uricosuric effects of niridazole.

Niridazole, an antischistosomal drug, caused a 44% decrease in the serum uric acid (SUA) concentration in 11 patients with schistosomiasis. The mean SUA (+/- SE) was 6.3 +/- 1.4 mg/100 ml at baseline and 3.5 +/- 1.5 mg/100 ml (p less than 0.01) on day 7 of treatment. There was a significant increase in the urinary uric acid/creatinine ratio, from 0.446 +/- 0.165 at baseline to 0.550 +/- 0.145 on day 3. There was no significant difference on day 7. The fractional clearance of uric acid rose from 7.2 +/- 6.8% to 13.9 +/- 17.3% (p less than 0.01), indicating a uricosuric effect. Oxypurine excretion was unchanged. In a separate study on 7 other patients, the SUA remained low for 4 to 7 days after the last dose. Niridazole, although not an organic acid, has uricosuric effects.

Creatinine↗
PubMed1977

Effects of inosine on purine synthesis in normal and HGPRT-deficient human fibroblasts.

1. Incubation of normal and HGPRT-deficient fibroblasts with inosine results in increased PP-ribose-P concentrations. 2. The increased PP-ribose-P concentrations are accompanied by decreased rates of purine synthesis de novo, more marked in normal cells 3. Increased purine nucleotide concentrations during incubation with inosine provide a likely explanation for the inhibition of purine synthesis in normal cells 4. The lack of accelerated purine synthesis in mutant cells under these conditions is not fully explained by consideration of PP-ribose-P and purine nucleotide concentations.

Cells, Cultured↗
PubMed1976

Regulation of purine nucleotide synthesis. Effects of inosine on normal and hypoxantine-guanine phosphoribosyltransferase-deficient fibroblasts.

Incubation of normal and hypoxanthine-guanine phosphoribosyltransferase-deficient (mutant) human fibroblasts with inosine results in increased intracellular concentration of 5-phosphoribosyl 1-pyrophosphate (PP-ribose-P). The magnitude of this increase is dependent on the concentration of the nucleoside and results from donation of the ribose moiety of inosine to the ribosyl phosphate moiety of PP-ribose-P through ribose phosphate intermediates. During incubation, rates of purine nucleotide synthesis de novo, estimated by incorporation of (14C) formate into formylglycinamide ribotide, are diminished in both normal and mutant cells: 5 mM inosine inhibits purine synthesis by 60-80% in normal cells and 2-20% in hypoxanthine-guanine phosphoribosyltransferase-deficient cells. The rates of purine synthesis in both normal and mutant cells are increased, however, during incubation with methylene blue at concentrations (50-100 muM) which result in more modest increases in ribose 5-phosphate and PP-ribose-P concentrations than are observed with inosine. Saturation of the PP-ribose-P amidotransferase reaction by PP-ribose-P does not appear, therefore, to explain the failure of increased PP-ribose-P concentration to stimulate the rate of purine synthesis in either type of fibroblast during incubation with inosine. Although the dissociation between PP-ribose-P concentration and the rate of purine nucleotide synthesis in normal fibroblasts incubated with inosine may be explained at least in part by an accompanying increase in intracellular concentrations of purine nucleotide feedback inhibitors, purine nucleotide concentrations are unchanged in mutant cells during incubation with inosine; these cells, in addition, show minimal (less than 3% of normal) incorporation of labeled hypoxanthine or the hypoxanthine moiety of inosine into purine nucleotides. The effect of inosine on purine synthesis de novo in hypoxanthine-guanine phosphoribosyltransferase-deficient fibroblasts is not explained in full by consideration of the concentrations of purine nucleotides and of PP-ribose-P, the factors frequently invoked as antagonistic regulators controlling the rate of this process.

Adult↗