A kinetic study has been made of the effects of various nucleotide species as modifiers of the reaction catalyzed by nucleoside diphosphatase when magnesium-inosine diphosphate is used as the substrate. The results showed that the magnesium complexes of both nucleoside and deoxy-nucleoside triphosphates, as well as free inosine diphosphate, are capable of activating the enzyme at lower concentrations of substrate. The experimental data have been analyzed in terms of the mechanism previously proposed for the reaction (Biochemistry 8, 3821 (1969)) and values obtained for the various kinetic constants. These indicate that the structure of the modifier determines the strength of its binding to free enzyme and that the resulting enzyme-modifier complexes have different abilities to combine with the substrate. Certain nucleotide species were also able to significantly lower the maximum velocity of the reaction.
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