Differentiation between Several Types of Phosphohydrolases in Light Microsomes of Corn Roots

The phosphohydrolase activity of a light microsomal fraction isolated from corn roots (Zea mays L. cv LG 55) was investigated. The fraction, which appears to be enriched in endoplasmic reticulum and Golgi membranes, has ATPase and pyrophosphatase activities that hydrolyze ATP and pyrophosphate at an optimum pH of 7.0, with K_m values of about 160 and 240 micromolar and with V_max values of about 200 and 50 nanomoles substrate hydrolyzed per milligram protein per minute, respectively. These enzymes differ in their sensitivity to anions and inhibitors. The ATPase is stimulated by sulfate anions, whereas pyrophosphatase is inhibited by molybdate. Furthermore, the simultaneous addition of ATP and pyrophosphate to the reaction medium increases phosphohydrolysis, suggesting that separate enzymes are operating in the membranes. We also observed that pyrophosphate competitively inhibits the ATPase, whereas ATP has no significant effect on the pyrophosphatase. On the other hand, we observed a detergent-stimulated, molybdate-insensitive inosine diphosphatase activity which, in the native state, hydrolyzes inosine diphosphate with a K_m of about 700 micromolar and a V_max of about 450 nanomoles inosine diphosphate hydrolyzed per milligram protein per minute. In the solubilized form, the enzyme appears to be fully active, exhibiting lower K_m values to hydrolyze inosine diphosphate. Furthermore, we found that native inosine diphosphatase is inhibited either by ATP or pyrophosphate, whereas inosine diphosphate inhibits the ATPase, but has no significant effect on the pyrophosphatase. It appears that inosine diphosphate is a positive modulator of the inosine diphosphatase, whereas ATP and pyrophosphate act as negative modulators of this enzyme.