Summary: The activity of NADP-specific glutamate dehydrogenase (NADP-GDH; EC increased at a linear rate of 2.1 x 10 units h (g fresh wt) following the transfer of mycelium grown in -asparagine medium to nitrate medium. The maximum enzyme activity was reached after 5 h. synthesis was demonstrated by density labelling of the enzyme with deuterium and inhibition of NADP-GDH synthesis by cycloheximide, -fluorophenylalanine or 6-methylpurine. -Asparagine or -glutamine could serve as a corepressor of NADP-GDH synthesis whereas the -isomers were ineffective. Of the various amide derivatives tested, only -asparagine -butyl ester could mimic the effect of -asparagine. Enzyme repression was not correlated with the internal pool of -amides. After NADP-GDH had been induced to the maximum level, the addition of -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 SO , 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 -asparagine. The decrease in NADP-GDH activity mediated by -asparagine was inhibited by cycloheximide or α-iodoacetamide. Sodium azide inhibited the decrease in enzyme activity caused by cycloheximide.


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