Oxygen exchange between Pi and water during ATP hydrolysis in activated rabbit skeletal muscle fibers provides direct evidence that Pi binds at the ATPase catalytic site in a force-generating state.
Oxygen exchange between (18O4)Pi in the medium and water accompanies ATP hydrolysis catalyzed by the calcium-regulated MgATPase of vertebrate skeletal muscle. Exchange was observed in chemically skinned fibers from rabbit psoas muscle held isometrically and activated by 30 microM free Ca2+. The rate of exchange was approximately proportional to Pi concentration (up to 10 mM) and was characterized by an apparent second order rate constant greater than or equal to 475 M-1 S-1 (pH 7.1, ionic strength 0.2 M, 22 degrees C). Much less exchange occurred in the absence of Ca2+ or when ATP was replaced by ADP. It has been inferred from mechanical experiments that Pi can bind to a force-generating ADP-bound state of actomyosin with resultant suppression of force (Hibberd, M. G., Dantzig, J. A., Trentham, D. R., and Goldman, Y. E. (1985) Science 228, 1317-1319). The oxygen exchange results support this inference by providing direct evidence that Pi in the medium binds at the ATPase catalytic site in activated isometric fibers. The inter-relationship of these two effects involving Pi on mechanochemical coupling in muscle is discussed.
Webb et al. (Sat,) reported a other. ATP hydrolysis with Pi in the medium vs. Absence of Ca2+ or ATP replaced by ADP was evaluated on Oxygen exchange rate between Pi and water. Oxygen exchange between Pi and water during ATP hydrolysis in activated rabbit skeletal muscle fibers provides direct evidence that Pi binds at the ATPase catalytic site in a force-generating state.
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