PRESENCE AND PROPERTIES OF A MG2+- DEPENDENT HCO3(-) STIMULATED ATPASE IN THE MALPIGHIAN TUBULES OF LOCUSTA-MIGRATORIA

A 50% stimulation of ATPase activity by added NaHCO3 was observed in homogenates of Malpighian tubules. The activities of the Mg2+-dependent and Mg2+-dependent HCO3--stimulated ATPase were highest in the 20,000 g fraction although substantial activity was present in the 100,000 g fraction. The properties of Mg2+-dependent HCO3--stimulated ATPase in the 100,000 g fraction have been determined. The pH optimum was ca. 7.5 although maximal stimulation due to the presence of HCO3- was nearer pH 8.0. Optimal concentrations of Mg2+ and HCO3- for Mg2+-dependent HCO3--stimulated ATPase were 2 mM and 20 mM respectively. In the presence of HCO3- the Km for ATP was increased from 0.16 mM to 0.26 mM. SCN- inhibited both the Mg2+-dependent and the Mg2+-dependent HCO3--stimulated ATPase whilst 1 mM ouabain and sodium acetazolamide had no effect on enzyme activity. © 1979.