Evidence that the NADP-linked glutamate dehydrogenase of Coprinus cinereus is regulated by acetyl-CoA and ammonium levels

Abstract

During development of the mushroom carpophore of the basidiomycete Coprinus cinereus, and through the operation of endogenous control mechanisms, the enzyme NADP-linked glutamate dehydrogenase (l-glutamate: NADP⁺ oxidoreductase (deaminating), EC 1.4.1.4) increases greatly in activity in the developing cap, while remaining at a barely detectable level in the stipe and parental mycelium. This behaviour can be reproduced in vegetative mycelium which, after growth in a rich medium, is transferred to a medium lacking in nitrogen source and containing 100 mM pyruvate as sole carbon source. Such treatment immediately causes induction of activity of NADP-linked glutamate dehydrogenase. Only glucose, fructose, dihydroacetone, acetate and propan-1-ol share with pyruvate the ability to induce this enzyme activity. A mutant mycelium which is known to lack the enzyme acetyl-CoA synthetase failed to show induction of glutamate dehydrogenase activity on acetate medium, although normal induction occurred on medium containing either glucose or pyruvate. It is concluded that induction requires synthesis of acetyl-CoA and that this latter compound is the probable intracellular regulator. Inclusion of as little as 2 mM NH₄Cl in the transfer medium is sufficient to prevent enzyme induction. Some other nitrogen sources are also able to prevent induction but all seem to operate through the formation of ammonium which is excreted into the medium. Other compounds, like alanine or glutamate are unable either to promote or prevent induction. External concentrations of ammonium which are able to prevent induction do not correlate with elevated internal ammonium levels, so it is concluded that, perhaps through some membrane-reaction, the external level of ammonium determines whether induction will occur. The regulation mechanism is therefore interpreted as one in which the enzyme is induced by elevated intracellular levels of acetyl-CoA providing external levels of ammonium are low.