Glutamate dehydrogenases from tissues of the ribbed mussel Modiolus demissus: ADP activation and possible physiological significance.

Glutamate dehydrogenase (E.C. 14.1.3) was localized in the mitochondria from heart, gill, mantle and hepatopancreas of this euryhaline bivalve mollusc. Activity levels were low (0.1-0.4 mumoles/min/g wet weight) in all tissues when assayed in the glutamate forming direction. Partially purified gill mitochondrial GDH was most active at pH 8.5. The rate in the glutamate deaminating direction was 10-20% of the rate in the glutamate forming direction. ADP at apparent Ka concentrations of micrometer (glutamate formation) and 170 micrometer (glutamate deamination) enhanced GDH activity, 8- and 4-fold respectively. GDH, in vivo, is probably in the activated form and appears to function in glutamate synthesis rather than ammonia formation. However, based on the low activities obtained, the role of GDH in salinity induced amino acid synthesis seems minimal.