Glucose 6-phosphate dehydrogenase from sweet potato: Effect of various ions and their ionic strength on enzyme activity

Activity of glucose 6-phosphate dehydrogenase (D-glucose 6-phosphate:NADP oxidoreductase, EC 1.1.1.49 EC] ) preparation from sweet potatoroot tissue was markedly altered in the presence of variousions. Cations or anions were effective in the following order:Na^$, K^$>Tris^$>NH₄^$>Mg^2$>Ca^2$, or Cl^–>NO₃^–,HPO₄^2–>SO₄^2–>HCO₃^–. Activity was inhibitedat high concentrations of Ca^2$, and HCO₃^–,. In an investigationon the dependence of the activity on pH, two activity peakswere clearly observed at low ionic strength. Ionic strength altered both the Km and Vmax for glucose 6-phosphate(G6P). A Lineweaver-Burk plot for the enzyme, with respect toG6P, showed a bimodal nature at low ionic strength; suggestingnegative cooperativity. Deviation from linearity of the plotwas less with an increase in the ionic strength. ¹ Present address: Institute of Applied Microbiology, Universityof Tokyo, Bunkyo-ku, Tokyo 113. (Received September 18, 1971; )