Ca2+-binding properties of a unique ATPase inhibitor protein isolated from mitochondria of bovine heart and rat skeletal muscle.

Previous studies showed that Ca2+ induced monomer to active dimer interconversion of a mitochondrial ATPase inhibitor protein from bovine heart or rat skeletal muscle (Yamada, E.W., Huzel, N.J. and Dickison, J.C. (1981) J. Biol. Chem. 256, 10203-10207). Initial equilibrium dialysis measurements of Ca2+ binding showed that this unique protein possesses three binding sites of high affinity with a maximum of one mol of Ca2+ bound/mol of protein monomer. Magnesium (1 mM) did not affect the first association constant but increased the second and third by about 1.2 and 1.5 fold, respectively. That the apparent association constants varied with concentration of protein monomer was in agreement with the self-associating nature of the protein. Scatchard plots at three concentrations of protein intersected at a molar ratio of about 0.5 (Ca2+/monomer). Ka1 and Ka2 values of 4.2 microM and 12.1 microM, respectively, were estimated by extra-polation of apparent constants to infinite dilution of protein. Ka3 (51.3 microM) was estimated by extrapolation of double reciprocal plots of apparent constants versus protein concentration to infinite levels of protein. A model for Ca2+ binding by this self-associating protein is described. Trifluoperazine had no effect on the activity of the inhibitor protein from either tissue.