Effects of Calcium on NAD(H)-Glutamate Dehydrogenase from Turnip (Brassica rapa L.) Mitochondria

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The effect of Ca^<2+> and ammonia on mitochondrial NADH-glutamate dehydrogenase (GDH: EC 1.4.1.2) isolated from turnip root (Brassica rapa L.) activity was examined. Increasing the ammonia [(NH_4)_2SO_4] concentration led to significant substrate inhibition which could be reversed by micromolar levels of Ca^<2+>. The sensitivity of the enzyme to ammonia inhibition and its reversal by Ca^<2+> was affected by proteolysis. After treatment with various proteases, lower concentrations of Ca^<2+> were capable of fully activating the enzyme or overcoming the inhibitory effects of high ammonium, compared to non-treated enzyme. However, the protease-treated enzyme was still sensitive to ethylene glycol-bis(β-aminoethyl ether) N,N,N',N'-tetraacetate (EGTA). In contrast, NADH-GDH activity was inhibited approx. 30% by organic mercurials (200 μM), but the residual activity was not affected by the subsequent additions of EGTA. NADH-GDH activity could also be stimulated by additions of high concentrations of NaCl (300 mM) in the absence of added Ca^<2+>. These results suggest that hydrophobic and -SH groups may be involved in the regulation of mitochondrial NADH-GDH activity by Ca^<2+>.
日本植物生理学会の論文