Studies on NADP-isocitrate dehydrogenase from maturing castor bean seeds
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概要
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NADP-specific isocitrate dehydrogenase from the soluble fraction of maturing castor bean endosperm was partially purified (approximately 180-fold) and some of its enzymatic properties were studied. Mg^<++>, Mn^<++>, Cd^<++>, Ba^<++>. Co^<++>, Zn^<++>, and Sr^<++> were activators of the enzyme reaction at a concentration, of 6.7 × 10^<-5> M. The optimum pH of this enzyme was about 8.5. The enzyme was stable in the narrow range from pH 7.0 to pH 8.0. Km values for isocitrate and NADP at pH 8.5 were 3.5 × 10^<-6> M: and 3.6 × 10^<-6> M, respectively. Enzyme stability was not affected by NaCl concentration and enzyme reaction was inhibited at 5 × 10^<-6> M PCMB (80% inhibition). It is suggested that the condensation product of glyoxylate and oxalacetate also inhibits the reaction. NADP-IDH in the crude extract from maturing castor bean endosperm was heat-stable but the dialyzed enzyme preparation and the partially purified enzyme were labile against heat treatment at 57℃. When Mg^<++> was added to the partially purified enzyme in the presence of isocitrate or NADP, the enzyme was stabilized against heat treatment. Mn^<++>, Ca^<++>, Co^<++>, Sr^<++> or Ba^<++> could be substituted for Mg^<++>. Addition of only one of the factors, Mg^<++> isocitrate or NADP, had no effect on the heat stability. Moreover, a combination of isocitrate and NADP did not establish stabilization. A divalent cation plays a central role, while adenine nucleotide, especially ATP, may have an important part in stabilization.
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