NITROGEN-SOURCE REGULATES EXPRESSION OF ALANINE DEHYDROGENASE ISOENZYMES IN STREPTOMYCES-AVERMITILIS IN A CHEMICALLY-DEFINED MEDIUM

Ammonium inns and alanine influence production of the macrolide averme ctin in Streptomyces avermitilis. L-Alanine dehydrogenase and alanine aminotransferase are the primary enzymes responsible for regulating th e intracellular concentration of alanine and also of ammonium ions. In cultures of S. avermitilis in a chemically defined medium with ammoni a or L-alanine as the only nitrogen source, specific activities of bot h enzymes increased during growth. The alanine dehydrogenase specific activity increased more than 86-fold after the culture was supplemente d with 0.2% L-alanine and 5-fold after addition of 0.5% ammonium sulfa te, whereas alanine aminotransferase specific activity increased 3- to 4-fold with either substrate. Five isoenzymes of alanine dehydrogenas e were detected histochemically in S. avermitilis after native gel ele ctrophoresis. Isoenzyme 1 was induced by alanine and temporarily repre ssed by high concentrations of ammonium sulfate. The presence of isoen zyme 1 was also related to changes in the kinetic properties of the al anine dehydrogenase reaction measured in elude desalted extracts. A no nlinear double-reciprocal plot was obtained in initial velocity studie s using L-alanine as a substrate in the sample induced with L-alanine. The nonlinearity was caused by both substrate inhibition and alloster ic regulation (positive cooperativity) by L-alanine. In contrast, the sample induced by ammonium sulfate showed a linear double-reciprocal p lot.