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R J Maloney
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The role of divalent cations in the activation of the NADP+-specific isocitrate dehydrogenase from Pisum sativum L.
A divalent cation electrode was used to measure the stability constants (association constants) for the magnesium and manganese complexes of the substrates for the NADP+-specific isocitrate dehydrogenase (EC 1.1.1.42) from pea stems. At an ionic strength of 26.5 mM and at pH 7.4 the stability constants for the Mg2+-isocitrate and Mg2+-NADP+ complexes were 0.85 +/- 0.2 and 0.43 +/- 0.04 mM-1 respectively and for the Mn2+-isocitrate and Mn2+-NADP+ complexes they were 1.25 +/- 0.07 and 0.75 +/- 0.09 mM-1 respectively. At the same ionic strength but at pH 6.0 the Mg2+-NADPH and Mn2+-NADPH complexes had stability constants of 0.95 +/- 0.23 and 1.79 +/- 0.34 mM-1 respectively. Oxalosuccinate and alpha-ketoglutarate do not form measureable complexes under these conditions. Saturation kinetics of the enzyme with respect to isocitrate and metal ions are consistent with the metal-isocitrate complex being the substrate for the enzyme. NADP+ binds to the enzyme in the free form. Saturation kinetics of NADPH and Mn2+ indicate that the metal-NADPH complex is the substrate in the reverse reaction. In contrast the pig heart enzyme appears to bind free NADPH and Mn2+. A scheme for the reaction mechanism is presented and the difference between the reversibility of the NAD+ and NADP+ enzyme is discussed in relation to the stability of the NADH and NADPH metal complexes.
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