Characterization of two glutamate dehydrogenases from the symbiotic microalga Symbiodinium microadriaticum isolated from the coral Acropora formosa.

In view of their possible involvement in ammonium assimilation in the coral/algal symbiosis, we have purified two distinct glutamate dehydrogenase isoenzymes from the symbiotic dinoflagellate Symbiodinium microadriaticum (Freudenthal) extracted from the staghorn coral Acropora formosa collected from Magnetic Island, North Queensland, Australia, in 1986-1987. An NADPH-specific glutamate dehydrogenase (GDH) displayed biphasic kinetics with respect to ammonium as the variable substrate; at low substrate concentrations the apparent K was below 1 m M, whereas at high substrate concentrations the corresponding value was approximately 200 m M. The NADPH-GDH displayed extremely low activity in the direction of glutamate oxidation; together with the kinetic data this suggests a probable role in ammonium assimilation. A second (NADH-specific) GDH was found to have both amination and deamination activities, and presumably functions in vivo in glutamate oxidation. Kinetic constants are reported for both GDH isoenzymes. [ABSTRACT FROM AUTHOR]