Some properties of purified isocitric enzyme

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Abstract
The chemical and physical properties of the isocitric enzyme are described. The inactivation of the enzyme caused by dialysis against distilled water or 0.05 [image] aminotrishydroxymethylmethane buffer (pH 7.3) can be prevented by addition of 0.1 [image] ammonium sulfate or sodium sulfate to the dialyzing medium. The absolute activity of the dehydrogenase reaction of the isocitric enzyme at pH 7.3 at 24[degree] is 350 molecules of D-isocitrate oxidized/molecule of enzyme/minute (in presence of 0.00042 [image] triphosphopyridine nucleotide), and for the carboxylase reaction atp H 5.6 at 14[degree] is 560 molecules of oxalosuccinate decarboxylated/ molecule of enzyme/minute (in presence of 0.0013 [image] manganous chloride) when calculated from the velocity constants (Vm) of these reactions. The dehydrogenase reaction does not require Mn2- ions but, being closely linked to the carboxylase reaction, it will not proceed in a forward direction unless the carboxylase reaction also occurs. The backward carboxylase reaction depends on dehydrogenase activity. The Michaelis constant for D-isocitrate is 2.6 x 10-6 [image] at pH 7.3 at 24[degree]; that for oxalosuccinate is 5.6 x 10-4 [image] at pH 7.3 at 24[degree] for the reduction reaction, and 2.5 x 10-2 [image] at pH 5.6 at 14[degree] for the decarboxyla-tion reaction. A hypothesis to explain the mechanism of action of the isocitric enzyme is described. A similar mechanism is proposed for the malic enzyme.
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