Investigation of soluble mitochondrial ATPase by the reacting enzyme sedimentation method

Soluble mitochondrial ATPase from bovine heart (factor F1) loses its activity during ATP hydrolyses. The inactivation is accelerated by moderate pressure, which is generated in an ultracentrifuge cell. The rate of inactivation slows down if the concentration of the substrate (MgATP) is diminished. ATP hydrolysis proceeds at an almost constant rate if the substrate concentration is as low as 0.05 mM. One intersubunit cross-link formed by dimethylsuberimidate per molecule of factor F1, prevents its inactivation during the ATPase reaction both without pressure and in an ultracentrifuge. Sedimentation coefficients measured by the reacting enzyme centrifugation method of both unmodified factor F1 at a low (about 0.05 mM MgATP) substrate concentration and of its dimethylsuberimidate cross-linked form in the presence of 10 mM MgATP, were determined to be s20, w = 12.4 +/- 0.4 S. The value is the same as that obtained by the conventional boundary sedimentation method in the absence of the substrate. This result testifies to the fact that the conformation of reacting factor F1 in solution is similar to that of the enzyme in the absence of the substrate.