Stimulation of rat liver mitochondrial adenosine triphosphatase by anions

The hydrolysis of MgATP by isolated rat liver mitochondrial ATPase (EC 3.6.1.3) at pH 8.0 was stimulated by various anions. The rate of hydrolysis was increased from 18 to 170 mumol per min per mg, a 9.4-fold stimulation, by HSeO3 at 1 mM MgATP. In the absence of a stimulatory anion, reciprocal plots of initial velocity studies with MgATP as the variable substrate were curved (Hill coefficient approximately 0.5). With the addition of anion, the reciprocal plots became linear. When the substrate was MgITP or MgGTP with the isolated enzyme or MgATP with submitochondrial particles, no curvature of the reciprocal plots was observed. With purified ATPase, anions stimulated the hydrolysis of MgITP, MgGTP, MgUTP or MgCTP only slightly. With submitochondrial particles the stimulation by anions of MgATP hydrolysis was limited to approximately 2-fold. These data are interpreted to indicate the existence of two substrate sites for MgATP and an anion-binding site on the isolated enzyme.