title: Presence and regulation of the alpha-ketoglutarate dehydrogenase multienzyme complex in the filamentous fungus Aspergillus niger.
abstract: alpha-Ketoglutarate dehydrogenase has been demonstrated for the first time in cell extracts from the filamentous fungus Aspergillus niger. A minimum protein concentration of 5 mg/ml is necessary for detecting enzyme activity, but a maximum of ca. 0.060 mumol/min per mg of protein is observed only when the protein concentration is above 9 mg/ml. alpha-Ketoglutarate can partly stabilize the enzyme against dilution in the assay system. Neither bovine serum albumin nor a variety of substrates or effectors of the enzyme could stabilize the enzyme against inactivation by dilution. A kinetic analysis of the enzyme revealed Michaelis-Menten kinetics with respect to alpha-ketoglutarate, coenzyme A, and NAD. Thiamine PPi was required for maximal activity. NADH, oxaloacetate, succinate, and cis-aconitate were found to inhibit the enzyme; AMP was without effect. Monovalent cations including NH4+ were inhibitory at high concentrations (greater than 20 mM). The highest enzyme activity was found in rapidly growing mycelia (glucose-NH4+ or glucose-peptone medium). We discuss the possibility that citric acid accumulation is caused by oxaloacetate and NADH inhibition of the alpha-ketoglutarate dehydrogenase of A. niger.
mesh_terms: D000156:Aconitic Acid / Q000494:pharmacology; D001234:Aspergillus niger / Q000201:enzymology* / Q000254:growth & development; D002412:Cations; D003065:Coenzyme A / Q000378:metabolism; D007655:Ketoglutarate Dehydrogenase Complex / Q000037:antagonists & inhibitors / Q000302:isolation & purification / Q000378:metabolism*; D007656:Ketoglutaric Acids / Q000378:metabolism; D007658:Ketone Oxidoreductases / Q000378:metabolism*; D007700:Kinetics; D009243:NAD / Q000378:metabolism; D010071:Oxaloacetates / Q000494:pharmacology; D013386:Succinates / Q000494:pharmacology; D019802:Succinic Acid; D013835:Thiamine Pyrophosphate / Q000494:pharmacology
chemical_list: D002412:Cations; D007656:Ketoglutaric Acids; D010071:Oxaloacetates; D013386:Succinates; D009243:NAD; D000156:Aconitic Acid; D019802:Succinic Acid; D007658:Ketone Oxidoreductases; D007655:Ketoglutarate Dehydrogenase Complex; D013835:Thiamine Pyrophosphate; D003065:Coenzyme A