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Biomedical subjects

T Spector

Publications and source records attributed to T Spector.

At least 109 records · Page 6Linked to original sources
PubMed1977

Mammalian adenylosuccinate lyase. Participation in the conversion of 2'-dIMP and beta-D-arabinosyl-IMP to adenine nucleotides.

Adenylosuccinate synthetase (EC 6.3.4.4) from rabbit muscle efficiently catalyzes the formation of 2'-deoxyadenylosuccinate and beta-D-arabinosylade-nylosuccinate from 2'-dIMP and beta-D-arabinosylIMP (Spector, T. and Miller, RL. (1976) Biochim. Biophys. Acta 445, 509-517). These novel analogs of adenylosuccinate were synthesized with this enzyme and their kinetic constants were determined with adenylosuccinate lyase purified from Ehrlich ascites cells. 2'-Deoxyadenylosuccinate and beta-D-arabinosyladenylosuccinate were readily cleaved to 2'-dAMP and beta-D-arabinosylAMP, respectively. Their Km values were similar to that of adenylosuccinate (3-6 micronM) and their substrate efficiencies (V/Km) were 120 for 2-deoxyadenylosuccinate and 32 for beta-D-arabinosyl-adenylosuccinate, compared to a value of 100 for adenylosuccinate. The products of the reactions, 2'-dAMP and beta-D-arabinosylAMP, were competitive inhibitors with Ki values of 5 and 87 micronM, respectively. ATP and ADP were considerably weaker competitive inhibitors with Ki values of 200-300 micronM. IMP, GMP, xanthosine 5'-monophosphate, 6-thioIMP and 6-thioGMP had Ki values greater than 200 micronM.

Animals↗
PubMed1976

Guanosine monophosphate synthetase from Ehrlich ascites cells. Multiple inhibition by pyrophosphate and nucleosides.

GMP synthetase (xanthosine-5'-phosphate: ammonia ligase (AMP-forming), EC 6.3.4.1) from Ehrlich ascites cells was found to be subject to multiple inhibition by its reaction product, PPi, and some analogs of adenosine. PPi and the nucleoside (N) inhibitors were also capable of individually inhibiting this enzyme. Under no conditions did the inhibition appear to be irreversible or "pseudoinactivating" in nature. The individual inhibition by PPi was competitive with respect to ATP (KI = 0.42 mM). Conversely, in the absence of PPi, the binding of N was noncompetitive with ATP, but shifted to a competitive pattern when PPi was present. Furthermore, with the inhibitors in concert, there was an apparent lowering of the KI values for both inhibitors. This data was consistent with either PPi functioning to tighten the binding of N at a noncatalytic site (positive cooperativity) or with PPi actually opening a second binding site for N in addition to the non-catalytic site. Although this study did not distinguish which of these events was occurring, it did reveal that the intensity of the effect of PPi appeared to be constant. That is, for various N inhibitors with a range of independently determined KI values from 26 to 1650 muM, the ratio of their KI values determined in the absence of PPi to the values determined in the presence of PPi was always 38 +/- 1.

Ammonia↗
PubMed1975

Studies with GMP synthetase from Ehrlich ascites cells. Purification, properties, and interactions with nucleotide analogs.

GMP synthetase has been purified 57-fold from Ehrlich ascites cells. The enzyme was found to be stable and to have an approximate molecular weight of 85,000 (determined by gel filtration). Its activity was stimulated by dithiothreitol and inhibited by 2-mercaptoethanol, p-chloromercuribenzoate, and hydroxylamine. Both ammonia and glutamine could serve as amino group donors. While none of the 10 triphosphate purine and pyrimidine nucleotides studied were able to substitute for ATP as the energy donor for the reaction, all of these compounds were able to bind to the ATP site. The Ki values for CTP, beta-D-arabinofuranosyl-ATP, and 1-N6-ethenoATP were slightly lower than the Km of ATP (0.28 mM). Six monophosphate nucleotides were aminated by this enzyme. Listed in order of their substrate efficiency (Vmax/Km), they are: xanthosine 5'-phosphate(XMP) (28,000); 2'-dXMP (1,200); 8-azaXMP (320); 6-thioXMP (200); beta-D-arabinofuranosyl-XMP (72); 1-ribosyloxipurinol 5'-phosphate (0.5). 6-ThioXMP was a strong alternative substrate inhibitor with a Ki of 5 muM. The aminated products of the reaction (four studied) were competitive inhibitors with respect to XMP.

Animals↗
PubMed1975

Guanosine monophosphate synthetase from Escherichia coli B-96. Inhibition by nucleosides.

The mechanism of inhibition of GMP synthetase by purine and purine-analog nucleosides was investigated. It was found that in addition to allowing the nucleoside to bind to the enzyme (Udaka, S., and Moyed, H. S. (1963) J. Biol. Chem. 238, 2797)PPi was also a competitive inhibitor with respect to ATP. A rate equation was derived to describe this inhibitory model for two competitive inhibitors where the binding of one inhibitor is contingent upon the binding of the other. The inhibition constants for a large number of nucleosides were then determined. It was found that the initial enzyme-inhibitor complex (of all nucleoside inhibitors) was slowly (0.2 min-1) transformed into a secondary (nondissociating) complex. The two inhibitory complexes appeared to exist in equilibrium. While decoyenine, N6-allyladenosine, and adenosine had similar inhibition constants for the initial complex (0.7 to 1.0 muM), their apparent inhibition constants for the secondary complex were 0.004, 0.06, and 0.5 muM respectively. These differences in the apparent dissociation constants from the secondary complexes are due to different equilibria between the initial and the secondary complexes. The ratios of the secondary complex to the initial complex at equilibrium were 3,250, 290, and 11 for decovenine, N6-allyladenosine, and adenosine, respectively.

Adenosine↗