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D W Martin

Publications and source records attributed to D W Martin.

At least 91 records · Page 5Linked to original sources
PubMed1982

Purine oversecretion in cultured murine lymphoma cells deficient in adenylosuccinate synthetase: genetic model for inherited hyperuricemia and gout.

Alterations in several specific enzymes have been associated with increased rates of purine synthesis de novo in human and other mammalian cells. However, these recognized abnormalities in humans account for only a few percent of the clinical cases of hyperuricemia and gout. We have examined in detail the rates of purine production de novo and purine excretion by normal and by mutant (AU-100) murine lymphoma T cells (S49) 80% deficient in adenylosuccinate synthetase [IMP:L-aspartate ligase (GDP-forming), EC 6.3.4.4]. The intracellular ATP concentration of the mutant cells is slightly diminished, but their GTP is increased 50% and their IMP, four-fold. Compared to wild-type cells, the AU-100 cells excrete into the culture medium 30- to 50-fold greater amounts of purine metabolites consisting mainly of inosine. Moreover, the AU-100 cell line overproduces total purines. In an AU-100-derived cell line, AU-TG50B, deficient in adenylosuccinate synthetase and hypoxanthine/guanine phosphoribosyltransferase (IMP:pyrophosphate phosphoribosyltransferase, EC 2.4.2.8), purine nucleoside excretion is increased 50- to 100-fold, and de novo synthesis is even greater than that for AU-100 cells. The overexcretion of purine metabolites by the AU-100 cells seems to be due to the primary genetic deficiency of adenylosuccinate synthetase, a deficiency that requires the cell to increase intracellular IMP in an attempt to maintain ATP levels. As a consequence of elevated IMP pools, large amounts of inosine are secreted into the culture medium. We propose that a similar primary genetic defect may account for the excessive purine excretion in some patients with dominantly inherited hyperuricemia and gout.

Adenylosuccinate Synthase↗
PubMed1982

Mechanism of 2-aminopurine mutagenesis in mouse T-lymphosarcoma cells.

We investigated the mechanism of action of 2-aminopurine (Apur) in eucaryotic cells. By analogy with studies in procaryotic systems, the base analog is presumed to incorporate into DNA predominantly opposite T where, upon subsequent DNA replication, it can mispair with C, inducing an A:T leads to G:C transition. This model predicts that Apur-induced mutagenesis will be enhanced by factors that favor formation of Apur-C mispairs, e.g., high levels of dCTP or low levels of TTP. We describe the use of a mutant T-lymphosarcoma cell line, AraC-6-1, which has an abnormally high dCTP pool and a low TTP pool, to test this prediction. AraC-6-1 cells were three- to fivefold more mutable by Apur than their parental cell line, NSU-1. This enhanced mutability by Apur could not be explained by altered incorporation of 3H-labeled Apur, by generally impaired ability to repair DNA damage, or by a direct effect of Apur on the endogenous deoxynucleotide pools. The addition of 10 microM thymidine to the growth medium of AraC-6-1 cells lowered their high dCTP pool (two- to threefold), raised the TTP pool (two- to threefold), and abolished their enhanced mutability by Apur. Further manipulation to produce an abnormally high TTP/dCTP ratio suppressed Apur-induced mutagenesis (8- to 10-fold) in both AraC-6-1 and NSU-1 cells. These observations support the hypothesis that Apur induces A:T leads to G:C transitions in mammalian cells by a mispairing mechanism.

2-Aminopurine↗
PubMed1981

Demonstration of normal and mutant protein M1 subunits of deoxyGTP-resistant ribonucleotide reductase from mutant mouse lymphoma cells.

From a mutagenized population of mouse T-lymphoma cells (S49) in continuous culture a cell line has been isolated (Ullman, B., Gudas, L. J., Clift, S. M., Martin, D. W., Jr. (1979) Proc. Natl. Acad. Sci. U. S. A. 76, 1074-1978) with ribonucleotide reductase activity that is inhibited only 50% by concentrations of dGTP which abolish wild type enzyme activity. Ribonucleotide reductase activity from this dGuo-L cell line retains its normal sensitivity to dATP. The partial sensitivity/partial resistance of the ribonucleotide reductase suggests that the dGuo-L cell line is heterozygous for ribonucleotide reductase, possessing one normal allele and one allele which codes for a dGTP-resistant enzyme. Both homologous and heterologous mixing experiments between the separated nonidentical subunits of ribonucleotide reductase, protein M1 and protein M2, from wild type and dGuo-L cells showed that the dGTP- feedback sensitivity was governed by the source of the protein M1. A partial resolution of two dGuo-L protein M1 components was achieved by chromatography on dextran blue-Sepharose. In order to resolve the two dGuo-L protein M1 components more completely, we introduced into dGuo-L cells a second mutation which conferred resistance of the ribonucleotide reductase to dATP, while the original dGTP resistance was maintained. The chromatography of protein M1 from this latter clone, dGuo-L-Aphid-G5, on dATP-Sepharose resolved two kinetically distinct protein M1 components. The first component was sensitive to dGTP inhibition but stimulated by dATP; the second was absolutely refractory to dGTP but sensitive to dATP inhibition. This confirms the hypothesis that the dGuo-L parent is heterozygous for protein M1, containing one wild type and one mutant allele.

Animals↗
PubMed1981

Evidence for genetically independent allosteric regulatory domains of the protein M1 subunit of mouse ribonucleotide reductase.

Ribonucleotide reductase is responsible for the reduction of the 2'-hydroxy moiety of all four ribonucleoside diphosphates to the corresponding deoxyribonucleotides. The overall activity of the enzyme is regulated by the allosteric effectors ATP (activator) and dATP (inhibitor), and the enzyme's substrate specificity is also controlled by nucleotide effectors. For instance, wild type ribonucleotide reductase from mouse T-lymphoma (S49) cells requires dGTP as a positive effector for ADP reduction. This effect of dGTP causes a reciprocal inhibition of CDP reduction. The dGuo-L mutant cell line, resistant to growth inhibition by exogenous deoxyguanosine, contains a nucleotide-binding subunit, protein M1, that conveys to its CDP reductase an insensitivity to dGTP (and dTTP) inhibition. The dGuo-L protein M1 also shows a decreased capacity to use ADP as a substrate, and therefore, the regulation of the substrate specificity is altered in the mutant protein M1. Another mutant cell line, dGuo-200-1, is resistant to deoxyadenosine and its ribonucleotide reductase is abnormally resistant to inhibition by dATP. The isolated mutant protein M1 from dGuo-200-1 cells has a CDP reductase activity which is stimulated by dATP, unlike the wild type enzyme which is inhibited by dATP. It appears that this mutant enzyme has lost the capacity to distinguish between dATP and ATP, but is still sensitive to regulation by dGTP and dTTP. Thus, the site of protein M1 regulating overall activity is altered in the dGuo-200-1 mutant, while the site regulating substrate specificity is normal. These characteristics of the mutants provide genetic evidence for two independent allosteric domains of protein M1, each responsible for a different aspect of nucleotide sensitivity of ribonucleotide reductase.

Adenosine Triphosphate↗
PubMed1981

Ribonucleotide reductase in cultured mouse lymphoma cells. Cell cycle-dependent variation in the activity of subunit protein M2.

Ribonucleotide reductase is responsible for the production of the deoxyribonucleotides required for DNA synthesis. The enzyme is composed of two dissociable subunits, proteins M1 and M2, which are inactive alone, but are fully active when combined. From mouse S49 T lymphoma cells we have isolated and separated the two subunits and used each for determining the activity of the complementary subunit in extracts from cells of different phases in the cell cycle. Treatment of S49 cells with cAMP analogs (e.g. Bt2cAMP) results in the protein kinase-dependent arrest of the cells in the G1 phase of the cell cycle. Ribonucleotide reductase (holoenzyme) activity fell in S49 cells treated for more than 16 h with Bt2cAMP but was unchanged during short term treatments. The activity of protein M2 was decreased in parallel to the overall activity of ribonucleotide reductase, while protein M1 activity changed less. Removal of bt2cAMP after 24 h exposure resulted in increased holoenzyme and protein M2 activities. Centrifugal elutriation of exponentially growing S49 cells separated cells into a 90% pure G1 cell population a mixture of G1 and early S phase cells and a 95% pure S phase/G2 cell population. The specific catalytic activity of protein M1 was the same in all these fractions while that of protein M2 was decreased 60% in the G1 cell population. These results demonstrate that the ribonucleotide reduction necessary for DNA synthesis is regulated in a cell cycle-dependent fashion by the activity of the protein M2 subunit of ribonucleotide reductase.

Animals↗
PubMed1981

Phosphorylation of calcium adenosinetriphosphatase by inorganic phosphate: van't Hoff analysis of enthalpy changes.

The magnesium-dependent phosphorylation of sarcoplasmic reticulum (Ca2+)-AtPase by inorganic phosphate (Mg2+ + Pi + E = Mg.E-P) was studied in purified leaky AtPase vesicles as a function of temperature (20-30 degrees C). A bireactant scheme was used to determine equilibrium constants, and the corresponding enthalpy changes ( delta H degrees vh) were determined by van't Hoff analysis. At all temperatures, the binding of Pi and Mg(2+) to the enzyme was synergistic. The equilibrium constants showed only a modest temperature dependence, with delta H degrees vh varying from 3 to 13 kcal/mol. In particular, the delta H degrees vh Mg(2+) binding to the unoccupied enzyme was 3 +/ 2 kcal/mol. These data contrast with a recent calorimetric study under comparable conditions (Epstein, M., Kuriki, Y., Biltonen, R., & Racker, E. (1980) Biochemistry 17, 5564) which reported no significant binding synergism and a delta H degrees cal for Mg(2+) binding of -76 kcal/mol. A possible reason for the discrepancy between calorimetric and van't Hoff enthalpy determinations is given. In agreement with other previous work, the overall reaction was found to be accompanied by a positive entropy change.

Animals↗
PubMed1981

Mutator phenotypes in mammalian cell mutants with distinct biochemical defects and abnormal deoxyribonucleoside triphosphate pools.

Recent studies of in vitro DNA synthesis have shown that fidelity of replication is influenced by the relative concentrations of deoxyribonucleoside triphosphates (dNTPs). Several investigators have used reconstituted prokaryotic replication systems to copy defined natural templates and have shown that specific incorporation errors can be induced by an appropriate bias of the precursor pools. The recent demonstration of mutator phenotypes among mutant Chinese hamster ovary cell lines with altered intracellular dNTP pools has allowed extension of the in vitro observations to eukaryotic replication and repair mechanisms. We describe here three mutant murine T-lymphosarcoma cell lines with altered dNTP pools and increased rates of spontaneous mutation to dexamethasone resistance and 6-thioguanine resistance. Unlike previously described mammalian cells with mutator phenotypes, these three lines have demonstrable defects in known structural gene products. Two of these cell lines are heterozygous for mutations affecting the M1 subunit of ribonucleoside diphosphate reductase; the other mutant is deficient in deoxycytidylate deaminase. In each cell line these mutations result in deranged endogenous dNTP pools and increased rates of spontaneous mutation, which are shown to be characteristic of the cell line and independent of the two genetic markers examined. Furthermore, normalization of the dNTP pools of the deaminase-deficient cells suppresses its mutator phenotype. Thus, abnormal dNTP pools seem to cause enhanced mutagenesis in mammalian cells.

Animals↗
PubMed1980

Submolecular characterization of a mutant human purine-nucleoside phosphorylase.

Purine-nucleoside phosphorylase deficiency in humans is associated with a severe defect in thymus-derived lymphocyte function. We have used the consanguineous parents of one purine-nucleoside phosphorylase-deficient patient as a source of mutant protein since the enzyme is totally lacking in the homozygous deficient child. When analyzed by denaturing two-dimensional gel electrophoresis, purified erythrocytic purine-nucleoside phosphorylase from normal individuals has four major and two minor subunits of identical molecular weight (29,700) but different isoelectric points. Similar analysis of phosphorylase purified from the parents of the purine-nucleoside phosphorylase-deficient child reveals the presence of two additional, more alkaline subunits with molecular weights significantly greater (approximately 500) than the normal subunits. Comparison of the two-dimensional thin layer peptide maps of the complete tryptic hydrolysates of the 125I-labeled subunits demonstrates that all eight subunits have a majority of their peptides in common, and thus, the structures of all are closely related. Peptide maps of the major subunits of normal purine-nucleoside phosphorylase are all identical with each other but different in several peptides from those of the normal minor subunits. Peptide maps of the mutant subunits differ from those of the normal major subunits only in the loss of one normal peptide and the presence of two new peptides. Tryptic peptide analysis of overlapping partial proteolytic cleavage fragments of the mutant and normal major subunits of purine-nucleoside phosphorylase has been used to determine the order of the iodinated tryptic peptides in the molecule. These results indicate that the mutant subunits are altered at an internal position rather than at the termini of the protein. A likely explanation of this internal alteration is an insertion of several tandem amino acid residues, perhaps resulting from an unequal genetic crossover or an intron processing abnormality.

Amino Acid Sequence↗
PubMed1980

Abnormal regulation of purine metabolism in a cultured mouse T-cell lymphoma mutant partially deficient in adenylosuccinate synthetase.

The isolation and characterization of a mutant mouse T-cell lymphoma (S49) with altered purine metabolism is described. This mutant, AU-100, was isolated from a mutagenized population of S49 cells by virtue of its resistance to 0.1 mM 6-azauridine in semisolid agarose. The AU-100 cells are resistant to adenosine mediated cytotoxicity but are extraordinarily sensitive to killing by guanosine. High performance liquid chromatography of AU-100 cell extracts has demonstrated that intracellular levels of GTP, IMP, and GMP are all elevated about 3-fold over those levels found in wild type cells. The AU-100 cells also contain an elevated intracellular level of pyrophosphoribosylphosphate (PPriboseP), which accounts for its resistance to adenosine. However AU-100 cells synthesize purines de novo at a rate less than 35% of that found in wild type cells. Furthermore, the intact cells of this mutant S49 cell line cannot efficiently incorporate labeled hypoxanthine into nucleotides since the salvage enzyme HGPRTase is inhibited in situ. The AU-100 cell line was found to be 80% deficient in adenylosuccinate synthetase, but these cells are not auxotrophic for adenosine or other purines. The significant alterations in the control of purine de novo and salvage metabolism caused by the defect in adenylosuccinate synthetase are mediated by the resulting increased levels of guanosine nucleotides.

Adenylosuccinate Synthase↗