GMP synthetase from Ehrlich ascites cells

A spectrophotometric assay for quantitative determination of kcat of herpes simplex virus type 1 thymidine kinase substrates

A simple method to determine the in vitro catalytic turnover constant of several substrates of herpes simplex virus type 1 thymidine kinase is presented in this study. The method is based on a continuous spectroscopic enzyme-coupled assay and allows one to monitor the herpes simplex virus type 1 thymidine kinase activity in the presence of unlabeled substrates. A clear correlation between the catalytic turnover constant and the rate of decrease in absorbance over time during the assay has been demonstrated. Exploiting this correlation, this method has been used to determine rapidly and precisely the catalytic turnover constant of antiviral lead compounds not readily available in the radioactive labeled form.

Biochemical characterization of human GMP synthetase

GMP synthetase (EC 6.3.5.2) plays a key role in the de novo synthesis of guanine nucleotides. It is a potential target for immunosuppressive therapy. Recently, the human enzyme was purified to homogeneity (Hirst, M., Haliday, E., Nakamura, J., and Lou, L. (1994) J. Biol. Chem. 269, 23830-23837). We now report the characterization of this enzyme in terms of its biochemical and kinetic properties. We found that there are distinct features of the human enzyme that has not been reported for GMP synthetase from other sources. There are two variant forms of human GMP synthetase. Their catalytic properties are very similar, although their isoelectric points are different. They most likely represent post-translational modification variants. Magnesium ion is required for enzyme activity, and the requirement is beyond levels needed for ATP chelation. Magnesium appears to be an essential activator and there may be more than one binding site. Interaction of GMP synthetase with xanthosine 5'-monophosphate (XMP), a substrate, exhibits sigmoidal kinetics with a Hill coefficient of 1.48 +/- 0.07. This positive cooperativity is not due to ligand-induced oligomerization, since GMP synthetase remains a monomer in the presence of XMP and other substrates. Decoyinine, a selective inhibitor of GMP synthetase, inhibits the human enzyme reversibly with uncompetitive inhibition kinetics toward glutamine and XMP and non-competitive kinetics toward ATP.

Isolation and characterization of a mutant Chinese hamster cell line resistant to the glutamine analog 6-diazo-5-oxo-L-norleucine

A mutant cell line, called don801, was isolated from a wild-type population of V79.5 Chinese hamster cells by its ability to grow in the presence of the glutamine analog 6-diazo-5-oxo-L-norleucine (DON), which is toxic for V79.5 cells. The don801 cells were found not to be cross-resistant to another glutamine analog, O-diazoacetyl-L-serine (azaserine, AS). It was shown that guanine but neither hypoxanthine nor adenine protected V79.5 cells from the toxic effects of DON, while hypoxanthine and adenine, but not guanine protected them against AS toxicity. Exposure of wild-type cells to DON was shown to result in a specific reduction of intracellular GTP pools, while in the mutant cells there was no effect on GTP levels. These results strongly suggested that DON was specifically inhibiting guanylate synthetase (GMP synthetase;xanthosine-5'-phosphate: L-glutamine amidoligase, EC 6.3.5.2) in V79.5 cells and that the enzyme in don801 cells was resistant to inhibition. In vitro assays of GMP synthetase activities from V79.5 and don801 cells confirmed this hypothesis. The mutant phenotype was also found to be dominant in intraspecific cell hybrids.