Search PubMed⌕ Search

Biomedical subjects

M A Becker

Publications and source records attributed to M A Becker.

At least 55 records · Page 3Linked to original sources
PubMed1985

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.

Allosteric Regulation↗
PubMed1984

The enzymatic synthesis of 5-amino-4-imidazolecarboxamide riboside triphosphate (ZTP).

5-Amino-4-imidazolecarboxamide riboside triphosphate (ZTP) is thought to play a regulatory role in cellular metabolism. Unlike other nucleoside triphosphates, ZTP is synthesized in a one-step reaction in which the pyrophosphate group of 5-phosphoribosyl-l-pyrophosphate is transferred to the riboside monophosphate (ZMP) in a reaction catalyzed by 5-phosphoribosyl-l-pyrophosphate synthetase; reversal of this reaction leads to dephosphorylation of ZTP to ZMP. This unusual route of synthesis (and catabolism) of ZTP may be important in defining its metabolic effects in the cell.

Adenosine Triphosphate↗
PubMed1984

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.

Adenine Phosphoribosyltransferase↗
PubMed1984

Human phosphoribosylpyrophosphate synthetase: radioimmunochemical quantitation in erythrocytes and fibroblasts.

PRPP synthetase catalyzes the synthesis of PRPP, a regulatory substrate in the pathway of purine nucleotide synthesis de novo. We have developed a specific assay for quantitative determination of PRPP synthetase immunologically cross-reactive material in human erythrocyte and fibroblast extracts. The sensitivity of the radioimmunoassay (0.3% and 0.08% of normal mean cross-reactive material in erythrocytes and fibroblasts, respectively) was equivalent to that of the enzymatic activity assay, but enzyme protein initially present in relatively inactive monomeric and smaller aggregated forms was radioimmunochemically measurable. The radioimmunoassay was utilized in conjunction with the enzymatic assay to study normal PRPP synthetase and PRPP synthetases from five affected male patients (in four families) in whom inherited enzyme superactivity was associated with increased rates of PRPP and purine nucleotide synthesis and gout with excessive uric acid excretion. Despite increased enzymatic activities in patients' cell extracts, values for cross-reactive material were within the ranges measured in the respective normal cell extracts. Thus, calculated absolute specific activities (nmol/hr/mg cross-reactive material) of patients' PRPP synthetases were substantially greater than those of normal PRPP synthetase. Moreover, absolute specific activities in hemolysates from both patients and normal individuals were in close agreement with the enzyme-specific activities measured in preparations of erythrocyte PRPP synthetase purified to homogeneity from the corresponding patient or normal source. These findings provided evidence for the accuracy and specificity of the radioimmunoassay and supported previous evidence for increased maximal reaction velocity as the basis of superactivity of the patients' enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)

Catalysis↗
PubMed1983

Mitogenic enhancement of purine base phosphoribosylation in Swiss mouse 3T3 cells.

Increased uptake of guanosine and the purine bases, adenine, hypoxanthine, and guanine, occurs within minutes after addition of fresh serum or purified growth-promoting agents to density-arrested cultures of Swiss 3T3, Swiss 3T6, and tertiary mouse embryo fibroblasts. Enhancement of uptake is maximal by about 50 min, is potentiated by combinations of growth promoters, and involves a process distinguishable from that of the enhanced uridine uptake on the basis of time course, pattern of growth-factor responsiveness and the failure of uridine to inhibit purine uptake. Enhanced purine uptake in 3T3 cells results from stimulation of the phosphoribosylation of purine bases and consequent trapping of nucleotide derivatives rather than from increased rates of membrane transport of the purine compounds. Increased availability of 5-phosphoribosyl 1-pyrophosphate (PRPP), identifiable within 30 min of serum addition, provides a likely explanation for increased purine base phosphoribosylation. Mitogenic stimulation of purine uptake is unaffected by cycloheximide but is markedly reduced in sodium-free medium. Enhanced purine uptake does not clearly depend upon changes in concentrations of effectors of intracellular PRPP synthesis. Nevertheless, when the possibility of direct mitogenic activation of PRPP synthase was studied, no differences were found in activities of this enzyme in extracts from stimulated and unstimulated 3T3 cells and mouse embryo fibroblasts.

Adenine↗
PubMed1982

Test performance as a function of the hard-driving and speed components of the type A coronary-prone behavior pattern.

Male and female students in an undergraduate psychology class completed a student version of the Jenkins Activity Survey (JAS) and several weeks later performed on the first course test of the semester. Test-completion times and test scores were unobtrusively recorded and a series of Pearson product-moment correlations were subsequently performed. These revealed the following: (1) The time urgency component of the Type A coronary-prone behavior pattern was negatively related to the amount of time that students spent working on the test (p less than 0.05) but unrelated to the obtained tests scores. (2) The hard-driving and competitive component of the Type A coronary-prone behavior pattern bore a marginal positive relationship with time-on-test (p less than 0.10) and a strong positive relationship with obtained test scores (p less than 0.002). (3) Global JAS scores and time were not correlated. However, global JAS scores were found to be positively related to the scores that students obtained on the test (p less than 0.05). It is suggested that the failure of previous research to reach any definite conclusions concerning the relationship between Type A/B behavior and task performance may be due to their utilization of global measures of Type A behavior while ignoring the effects of the component behavioral tendencies. When time urgency and hard-drivingness do not interfere with one another and when the nature of the task makes at least one of these components salient, performance differences should emerge between Type A's and Type B's.

Achievement↗
PubMed1982

Gout with superactive phosphoribosylpyrophosphate synthetase due to increased enzyme catalytic rate.

We have studied families C and A in which superactivity of PRPP synthetase (E.C. 2.7.6.1) is associated with gout and uric acid overproduction in affected hemizygous males. PRPP synthetase catalyzes synthesis of PRPP, a regulatory substrate in purine synthesis de novo. Activities of the enzyme in erythrocyte and fibroblast extracts from the male index cases, T.C. and R.A., were nearly threefold greater than normal at each Pi concentration tested. PRPP synthetase superactivity was accompanied by increased intracellular PRPP concentration and generation in erythrocytes and fibroblasts from these patients, and enhanced rates of PRPP-dependent purine synthesis reactions, including purine synthesis de novo, were demonstrable in their fibroblasts. These findings suggested that increased intracellular synthesis dut to enzyme superactivity underlay purine nucleotide and uric acid overproduction in these patients. Similar studies in cells from the sister of T.C. and the mother of R.A. showed increased values that were, however, intermediate between normal values and those of the affected males, indicating that these women are heterozygous carriers of the traits for enzyme superactivity. The enzymatic basis for increased PRPP synthetase activity in both families was investigated. Immunochemical studies in dialyzed erythrocyte lysates and highly purified erythrocyte enzyme preparations provided evidence for increased enzyme activity per molecule of immunoreactive enzyme. In addition, purified T.C. and R.A. PRPP synthetases showed 3.1- and 2.8-fold greater enzyme specific activities, respectively, than comparably purified normal enzymes. Kinetic constants of purified T.C. and R.A. PRPP synthetases for substrates, activators, and inhibitors were indistinguishable from normal, and increased maximal reaction velocity alone appeared to account for enzyme superactivity. Despite an apparently similar kinetic mechanism for superactivity, the diminished electrophoretic mobility of T.C. PRPP synthetase and increased thermal lability of R.A. PRPP synthetase suggested distinct structural alterations leading to enzyme superactivity in families C and A.

Adult↗
PubMed1981

The phosphogluconate pathway and synthesis of 5-phosphoribosyl-1-pyrophosphate in human fibroblasts.

The phosphogluconate pathway (pentose phosphate cycle) of normal fibroblasts was stimulated 20-fold by methylene blue and inhibited to 14% of the baseline rate by 6-aminonicotinamide. In fibroblasts deficient in glucose-6-phosphate dehydrogenase activity (an average of 1.5% of normal mean), the pentose phosphate cycle was unaffected by either methylene blue or 6-aminonicotinamide. In normal cells, neither the intracellular concentration nor the rate of generation of 5-phosphoribosyl-1-pyrophosphate was altered by the marked and opposite changes in the rate of the phosphogluconate pathway caused by methylene blue and 6-aminonicotinamide. Intracellular ribose 5-phosphate concentration was increase by methylene blue (an average increase of 83%) but not significantly altered by 6-aminonicotinamide. In fibroblasts deficient in glucose-6-phosphate dehydrogenase activity, the 5-phosphoribosyl-1-pyrophosphate concentration and rate of generation were higher rather than lower in comparison to normal cells under all conditions studied. The data suggest a predominant role for the nonoxidative branch of the phosphogluconate pathway in supplying ribose 5-phosphate for nucleotide biosynthesis. Pentose phosphate supply cannot be considered an essential function of the oxidative branch in fibroblasts.

Cell Line↗
PubMed1981

Regulation of 5-phosphoribosyl-1-pyrophosphate synthesis in human fibroblasts by the concentration of inorganic phosphate.

During the growth cycle of normal fibroblasts and of fibroblasts deficient in glucose-6-phosphate dehydrogenase activity, the concentration of 5-phosphoribosyl-1-pyrophosphate and of Pi, as well as the activity of 5-phosphoribosyl-1-pyrophosphate synthetase, decreased to stable values in confluent cultures. A high degree of correlation (0.89 and 0.91 for two normal and 0.69 for one glucose-6-phosphate dehydrogenase-deficient cell strain, respectively) was shown between intracellular Pi, and 5-phosphoribosyl-1-pyrophosphate concentrations under varying culture and incubation conditions. 5-Phosphoribosyl-1-pyrophosphate concentrations were elevated in normal fibroblasts incubated with methylene blue only if intracellular Pi levels were high. Neither methylene blue nor 6-aminonicotinamide, singly, affected intracellular Pi concentrations. However, when normal cells were pretreated with 6-aminonicotinamide and then with methylene blue, intracellular Pi decreased, 5-phosphoribosyl-1-pyrophosphate was depleted, and its rate of generation decreased. Under similar conditions, glucose-6-phosphate dehydrogenase-deficient fibroblasts maintained unaltered Pi levels, and 5-phosphoribosyl-1-pyrophosphate concentration and generation were slightly increased. The decrease in intracellular Pi in normal cells after the combined treatment was commensurate with an accumulation of 6-phosphogluconate, which did not take place in mutant cells. The changes in 5-phosphoribosyl-1-pyrophosphate synthesis, whether due to the stage of growth or various experimental manipulations, were always concordant with changes in intracellular Pi level. The regulatory role of Pi is consistent with the known enzymic properties of 5-phosphoribosyl-1-pyrophosphate synthetase.

Cells, Cultured↗
PubMed1981

Coronary-prone behavior, social insecurity and stress among college-aged adults.

One hundred and seventy-four male college students completed a student version of the Jenkins Activity Scale, the Macmillan stress inventory, measures of social insecurity, and items concerning personal habits and preferences. Social insecurity and Type A behavior emerged as independent but additive factors associated with negative stress experience. Type A's as compared to Type B's, reported feeling more stress, having and desiring less social support, being more interested in various performance standards and being upset for a longer period of time following poor school performance. Interestingly, no difference emerged between A's and B's regarding the temporal impact of successful school performance. High Social Insecurity subjects reported feeling more stress, having less social support, being more upset following poor school performance and being more interested in social comparison information as compared to their Low Social Insecurity counterparts. The results are interpreted as supportive of the notion that social insecurity and Type A behavior may contribute to coronary heart disease by affecting one's exposure and physiological reactions to everyday life stressors.

Adult↗