AMMONIUM ACTIVATION OF THE HALOPHILIC ALANINE DEHYDROGENASE FROM HALOBACTERIUM-SALINARIUM

The effect of various concentrations of NH4Cl on the kinetic character istics and stability of alanine dehydrogenase (ADH) in the cell extrac t of the extremely halophilic bacterium Halobacterium salinarium was s tudied. The NH4+ ion, a product of the ADH-catalyzed oxidative deamina tion of alanine, was found to activate this reaction by increasing ten fold V-max while the Michaelis constant for NAD and alanine remained u nchanged. Competition between NH4+ and K+ ions was revealed. The disso ciation constants K-D of the enzyme-NH4+ and enzyme-K+ complexes were 0.025 and 0.1 M, respectively. It was suggested that the role of NH4in th deamination of amino acids in H. salinarium subjected to osmotic shock is to maximize the reaction rate under K+ deficiency.