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Abstract
The activity of NADP-specific glutamate dehydrogenase (NADP-GDH; EC 1.4.1.4) increased at a linear rate of 2.1 x 103 units h?1 (g fresh wt)?1 following the transfer of Stemphylium botryosum mycelium grown in L-asparagine medium to nitrate medium. The maximum enzyme activity was reached after 5 h. De novo synthesis was demonstrated by density labelling of the enzyme with deuterium and inhibition of NADP-GDH synthesis by cycloheximide, p-fluorophenylalanine or 6-methylpurine. L-Asparagine or L-glutamine could serve as a corepressor of NADP-GDH synthesis whereas the D-isomers were ineffective. Of the various amide derivatives tested, only L-asparagine tert-butyl ester could mimic the effect of L-asparagine. Enzyme repression was not correlated with the internal pool of L-amides. After NADP-GDH had been induced to the maximum level, the addition of L-asparagine and cycloheximide resulted in a decrease of activity with half-lives of 4.5 h and 8 h respectively. The mean half-life, as measured by following the decay in specific radioactivity of the enzyme in nitrate medium after administration of 35SO4 2-, was 7 h. Mycelium starved of carbon and nitrogen sources showed a slow decrease (half-life of 17 h) in NADP-GDH activity. Depletion of energy by carbon starvation or the presence of sodium azide did not prevent the decrease in enzyme activity caused by L-asparagine. The decrease in NADP-GDH activity mediated by L-asparagine was inhibited by cycloheximide or ?-iodoacetamide. Sodium azide inhibited the decrease in enzyme activity caused by cycloheximide.
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