Factors affecting the reactivation of the mitochondrial adenosine 5'-triphosphatase and the release of ATPase inhibitor protein during and following the reenergization of mitochondria from ischemic cardiac muscle

In the present study we examined three factors affecting the reversal of the ischemia-induced inhibition of the mitochondrial ATPase described by us earlier (W. Rouslin (1983) J. Biol. Chem. 258, 9657-9661). These factors were the pH, the MgATP concentration, and the pCa of the medium in which mitochondria were sonicated following their reenergization in vitro. It was found that the extent of ATPase reactivation, on the one hand, and the extent of inhibitor protein release, on the other, following the reenergization in vitro and subsequent sonication of intact mitochondria isolated from 20-min-ischemic canine cardiac muscle were affected differently by each of the three factors studied. While raising the pH of the medium in which the mitochondria were sonicated subsequent to reenergization from approximately 7.0 to approximately 8.2 resulted in marked parallel increases in both ATPase reactivation and inhibitor protein release, lowering the pH of the medium to approximately 6.4 resulted in a marked decrease in ATPase reactivation but also in the apparent irreversible binding and/or denaturation of a portion of the ATPase inhibitor. Increasing the MgATP concentration of the sonication medium from zero to 2.0 mM resulted in approximately a one-third decrease in ATPase reactivation. The effect upon inhibitor release was more dramatic. MgATP at 2 mM decreased inhibitor release by approximately two-thirds. The pCa of the sonication medium was varied between 9.0 and 3.5 using Ca-ethylenebis(oxyethylenenitrilo)-tetraacetic acid (EGTA) buffers. Decreasing the pCa of the medium from 9.0 to 3.5 had a paradoxical effect. It resulted in increases both in ATPase reactivation and in the amount of inhibitor bound to the particles. Such a paradoxical effect may be explained if one assumes the existence of two kinds of inhibitor-enzyme interaction sites, namely, regulatory and nonregulatory binding sites. Thus, decreasing the pCa may decrease interaction at regulatory sites while enhancing interaction at nonregulatory inhibitor binding sites.