An NAD+-dependent alanine dehydrogenase from a methylotrophic bacterium

A study was made of the NAD+-dependent alanine dehydrogenase (EC 1.4.1.1) elaborated by the methylotrophic bacterium Pseudomonas sp. strain MA when growing on succinate and NH4Cl. This enzyme was purified 400-fold and was found to be highly specific for NH3 and NAD+; however, hydroxypyruvate and bromopyruvate, but not alpha-oxoglutarate or glyoxylate, could replace pyruvate to a limited extent. The Mr of the native enzyme was shown to be 217,000, and electrophoresis in SDS/polyacrylamide gels revealed a minimum Mr of 53,000, suggesting a four-subunit structure. The enzyme, which has a pH optimum of 9.0, operated almost exclusively in the aminating direction in vitro. It was induced by NH3 or by alanine, and was repressed by growth on methylamine or glutamate. It is suggested that this enzyme has two roles in this organism, namely in NH3 assimilation and in alanine catabolism.