Search PubMed⌕ Search

Biomedical subjects

M A Becker

Publications and source records attributed to M A Becker.

At least 91 records · Page 5Linked to original sources
PubMed1976

Patterns of phosphoribosylpyrophosphate and ribose-5-phosphate concentration and generation in fibroblasts from patients with gout and purine overproduction.

In the majority of patients with gout and excessive uric acid production, underlying enzyme abnormalities have not been identified. In the present study, measurement of both the rate of generation and concentration of phosphoribosylpyrophosphate (PP-ribose-P) and the concentration of ribose-5-phosphate in cultured cells were undertaken to establish a classification of purine overproducers to direct study of additional enzyme defects. Fibroblasts were cultured from 24 individuals assigned to 4 groups: group 1, 5 normal controls; group 2, 5 patients with gout and normal dialy urinary uric acid excretion (gouty controls); group 3, 7 patients with well-defined enzyme abnormalities and excessive urinary acid excretion (4 with hypoxanthine-guanine phosphoribosyltransferase deficiency and 3 with excessive PP-ribose-P synthetase activity); and group 4, 7 patients with gout and excessive uric acid excretion but without grossly abnormal activities of the above enzymes in erythrocyte lysates. In all 14 fibroblast strains from patients showing excessive production of uric acid (groups 3 and 4), rates of purine synthesis de novo and PP-ribose-P concentrations exceeded values for cells from control groups. Cells from group 3 patients with hypoxanthine-guanine phosphoribosyltransferase deficiency showed normal PP-ribose-P generation, while those with excessive PP-ribose-P synthetase activity demonstrated increased generation of this regulatory substrate. All strains from group 3 patients had normal ribose-5-phosphate concentrations. Five cell strains from group 4 patients showed one of the two patterns of abnormalities in these measurements seen in strains from group 3 patients: two resembled hypoxanthine-guanine phosphoribosyltransferase-deficient cells, and three resembled cells with excessive PP-ribose-P synthetase activity. Analyses of erythrocyte enzyme preparations from two of these patients in group 4 have led to identification of a kinetic variant of each enzyme as predicted from the foregoing patterns. Two additional group 4 cell lines that showed increased ribose-5-phosphate concentrations in addition to increased PP-ribose-P concentrations and generation were classified in a separate subgroup, since in the individuals excessive purine synthesis appeared to result from increases ribose-5-phosphate concentration, leading to increased availability of PP-ribose-P. No abnormality in either hypoxanthine-guanine phosphoribosyltransferase or PP-ribose-P synthetase has been found in erythrocyte preparations from one patient so classified.

Adenine Phosphoribosyltransferase↗
PubMed1975

Human phosphoribosylpyrophosphate synthetase. Comparison of purified normal and mutant enzymes.

Human phosphoribosylpyrophosphate synthetase has been purified 4500-fold to electrophoretic homogeneity from the erythrocytes of normal individuals and of two brothers in whom excessive activity of this enzyme results in excessive rated of purine nucleotide and uric acid synthesis de novo and gouty arthritis. Structural differences between the normal and mutant enzymes are indicated by a lower isoelectric point for the mutant enzyme (pI 4.85) than for the normal enzyme (pI 5.10); decreased electrophoretic mobility of the mutant preparation on cellulose acetate gel at low inorganic phosphate concentrations; increased (2.4-fold) inactivation of the mutant enzyme activity relative to the normal by identical amounts of a specific antiserum which precipitates identical quantities of normal and mutant enzyme; increased thermal lability of the mutant enzyme at 55 degrees; and an increased (2.2-fold) specific enzyme activity for the mutant enzyme despite the comparable purity of the preparations. Antibody inactivation, quantitative precipitin, and immunodiffusion studies as well as the disparity in specific enzyme activities during the course of purification suggest that a structural alteration in the mutant enzyme leads to increased catalytic activity per enzyme molecule, either from a primary alteration in the structural gene(s) for phosphoribosylpyrophosphate synthetase or from a post-transcriptional alteration in the enzyme. Purified preparations of normal and mutant enzymes showed nearly identical affinity constants for magnesium and the substrates, ATP and ribose 5-phosphate, as well as similar inhibition constants for the products, PP-ribose-P and AMP, and the inhibitors ADP, GDP, and 2,3-diphosphoglycerate. An increased maximal velocity of the reaction was, thus, the sole kinetic difference identified. The increased velocity of the mutant enzyme reaction was constant over a range of inorganic phosphate concentrations from 0.1 to 100 mM. Subunit molecular weights of the enzyme preparations, estimated by sodium dodecyl sulfate-polyacrylamide electrophoresis, were identical (32,000), although the undenatured mutant enzyme showed a greater proportion of stainable protein in the smaller of two molecular weight forms (both greater than 500,000) of the enzyme demonstrated on polyacrylamide gel electrophoresis in the presence of 1 mM sodium phosphate.

Enzyme Activation↗
PubMed1973

Purine overproduction in man associated with increased phosphoribosylpyrophosphate synthetase activity.

In hemolyzates from red cells of two brothers with purine overproduction and gout, activity of phosphoribosylpyrophosphate synthetase is more than twofold greater than that measured in normal or other gouty individuals. The increased enzyme activity, which is also demonstrable in fibroblasts of the one patient tested, is associated with increased production of 5-phosphoribosyl-1-pyrophosphate by intact cells, an indication that the enzyme abnormality is the basis for the purine overproduction. This genetic abnormality is an example of an increased enzyme activity producing a disease state.

Adenosine Triphosphate↗
PubMed1973

Human phosphoribosylpyrophosphate synthetase: increased enzyme specific activity in a family with gout and excessive purine synthesis.

Among affected members of a family (B. family) with excessive purine production and gout, activity of phosphoribosylpyrophosphate synthetase (EC 2.7.6.1) is 2.5- to 3.0-fold higher than among normal people. The molecular basis for this increased enzyme activity was studied. Antibody to purified human phosphoribosylpyrophosphate synthetase of erythrocytes was obtained from immunized rabbits. Studies with the IgG fraction of this antiserum show the presence of normal quantities of immunoreactive enzyme, but 2.5- to 3.0-fold higher activity per molecule in affected members of the B. family. In addition, by use of a stain for phosphoribosylpyrophosphate synthetase activity, a difference in electrophoretic mobility was demonstrated on cellulose acetate gel between the partially purified enzyme from normal people and an affected member of the B. family. These studies suggest that the enzyme aberration responsible for purine overproduction and gout in the B. family results from a structurally altered enzyme with increased activity per molecule.

Animals↗
PubMed1973

Purine metabolism in normal and thioguanine-resistant neuroblastoma.

Purine metabolism has been examined in a clonal line of mouse neuroblastoma cells resistant to the cytotoxic effects of 6-thioguanine. Comparative studies in the resistant and parental lines indicate that the former cells have less than 1% of normal hypoxanthine phosphoribosyltransferase (EC 2.4.2.8) activity. The activities of other enzymes important in the de novo and salvage pathways of purine biosynthesis were not significantly different in the two lines. Hypoxanthine phosphoribosyltransferase deficiency in this neuroblastoma line was associated with increased intracellular concentrations of 5-phosphoribosyl-1-pyrophosphate, an increased rate of purine biosynthesis de novo, and failure to incorporate hypoxanthine, but not adenine, into nucleotides. There are essentially the same alterations in purine metabolism that occur in hypoxanthine phosphoribosyltransferase-deficient fibroblasts or lymphoblasts derived from individuals with the Lesch-Nyhan syndrome. Clonal lines of hypoxanthine phosphoribosyltransferase-deficient neuroblastoma cells may therefore be of use in elucidating the mechanisms by which the enzyme defect leads to the neurologic dysfunction seen in children with this disease.

Adenine↗