Measurement of the reversibility of ATP binding to myosin in calcium-activated skinned fibers from rabbit skeletal muscle. Oxygen exchange between water and ATP released to the solution

We have measured the rate constant for ATP release from myosin heads of Ca²⁺-activated, demembranated muscle fibers using the technique of phosphate-water oxygen exchange. Single rabbit psoas fibers were held in an activating solution in [¹⁸O]water ([MgATP] = 8 mM, ionic strength = 0.2 M, pH = 7.0, 24°C). After about 20% hydrolysis of ATP, product P(i) and remaining ATP were isolated, and the distribution of ¹⁸O in both molecules was analyzed using a mass spectrometer. The exchange in P(i) was similar to that previously reported (Hibberd, M.G., Webb, M.R., Goldman, Y.E., and Trentham, D.R. (1985) J. Biol. Chem. 260, 3496-3501). The amount of ¹⁸O in ATP gave a rate constant of about 4 s^-1 for ATP release, if it is assumed that each rate constant in the pathway of ATP hydrolysis has the same value for all myosin ATPase sites. However, the distribution of ¹⁸O in both released P(i) and ATP is not well explained by a single pathway for ATP hydrolysis. We present a model that indicates how such distributions could arise from a range of values for the rate constants for P(i) and ATP release from actomyosin, and this range is determined by differences in the amounts of strain in attached cross-bridges. The kinetic information obtained from these isotope exchange experiments is compared to show that they give a compatible set of rate constants for actomyosin in fibers.