Studies on the phosphomannose isomerase of Amorphophallus konjac C. Koch II. Effect of divalent metal ions on the EDTA-treated enzyme1

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Abstract
Konjak phosphomannose isomerase was inactivated in a time-dependent process by metal binding agents, and the inactivated enzyme was instantaneously reactivated by adding such metal ions as Zn++, Co++, Fe++, Mn++ and Cu++. However, neither Ca++ nor Mg++ were effective for reactivation. Zn++, at a low concentration, brought about complete reactivation of the enzyme at pH 6–7. The EDTA-treated enzyme was more susceptible to heat denaturation when compared with the native enzyme, but the addition of various metal ions caused the recovery of the thermal stability of the EDTA-treated enzyme. The magnitude of the recovery depended on the metal ion species and the concentrations. The most effective metal ion was Co++, which caused the recovery of thermal stability to a level higher than that of the native enzyme. Phosphomannose isomerase was inhibited by pchloromercuribenzoate and HgCl2; the inhibition by p-chloromercuribenzoate being more pronounced as incubation progressed. In contrast, the EDTA-treated enzyme was more readily inhibited by the mercurial ion than was the native enzyme. Zn++, when added to the EDTA-treated enzyme, markedly restored its resistance to the mercurial-induced inhibition. The metal-substituted enzyme was also inhibited by EDTA in a time-dependent process.