Effects of D-Glutamate on Enzymes of Ammonia Assimilation in Bacillus megaterium NCIB 7581

Summary: Enzymes possibly concerned in the assimilation of ammonia were assayed in Bacillus megaterium NCIB 7581 after growth with ammonium sulphate as sole source of nitrogen. Alanine dehydrogenase and aspartase were not detected, but both NADPH-dependent glutamate dehydrogenase and glutamate synthase (NADH- or NADPH-dependent) were found. Glutamine synthetase was also present, though differing methods of assay gave different values for its activity. Glutamate dehydrogenase was competitively inhibited by d-glutamate, whereas glutamate synthase was insensitive to d-glutamate or d-glutamine (singly or together). d-Glutamate was a substrate for glutamine synthetase, and the use of l-glutamate by this enzyme was partly inhibited by a 10-fold molar excess of d-glutamate. l-Glutamine strongly inhibited glutamine synthetase, but d-glutamine had no effect A d-glutamate-resistant substrain (grown with ammonium sulphate as nitrogen source) contained 10 times more glutamate dehydrogenase activity than did the wild-type under the same conditions, and the enzyme showed altered affinity for 2-oxoglutarate and for Dglutamate.Activities of glutamate synthase and glutamine synthetase in the substrain were similar to those in the wild-type. Glutamate dehydrogenase was absent from organisms grown with nitrate as sole nitrogen source, yet growth under these conditions was still inhibited by d-glutamate.