Control of ammoniagenesis in the kidney of water- and air-breathing osteoglossids: characterization of glutamate dehydrogenase
Glutamate dehydrogenases (EC 1.4.1.2) from the kidney of Osteoglossum bicirrhosum (called aruana) and Arapaima gigas were kinetically characterized. The two enzymes exhibited several common characteristics including Vmax activity ratio, pH optimum, affinity for cofactors, a marked preference for NAD(H) over NADP(H), and a very low affinity for NH4+. A variety of regulatory metabolites affected both enzymes. GTP and GDP were inhibitory while ADP, ATP, AMP, and leucine activated the enzymes. Both enzymes displayed potent product inhibition which was partially reversed by low levels of ADP. Arapaima kidney glutamate dehydrogenase was tightly regulated by the adenylate and guanylate nucleotides, inhibition by GTP and GDP and deinhibition by ADP and AMP being much stronger for this enzyme than for the aruana enzyme. Aruana glutamate dehydrogenase, however, was more responsive to NAD–NADH control. The enzyme was more sensitive to NAD(H) product inhibition and this inhibition was poorly reversed by ADP. From these data, it was concluded that both fish kidney glutamate dehydrogenases could function in glutamate oxidation in vivo. However, the Arapaima enzyme appeared most clearly adapted to a catabolic role, activity being more tightly linked to the energy status of the mitochondrion. Conversely, the aruana enzyme displayed regulatory properties allowing it the potential to function in NADH oxidation during periods of hypoxic stress.