Abstract
A 50% stimulation of ATPase activity by added NaHCO 3 was observed in homogenates of Malpighian tubules. The activities of the Mg 2+-dependent and Mg 2+-dependent HCO 3 −-stimulated ATPase were highest in the 20,000 g fraction although substantial activity was present in the 100,000 g fraction. The properties of Mg 2+-dependent HCO 3 −-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 HCO 3 − was nearer pH 8.0. Optimal concentrations of Mg 2+ and HCO 3 − for Mg 2+-dependent HCO 3 −-stimulated ATPase were 2 mM and 20 mM respectively. In the presence of HCO 3 − the K m for ATP was increased from 0.16 mM to 0.26 mM. SCN − inhibited both the Mg 2+-dependent and the Mg 2+-dependent HCO 3 −-stimulated ATPase whilst 1 mM ouabain and sodium acetazolamide had no effect on enzyme activity.
Published Version
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