Kinetics of the interaction between actin, ADP, and cardiac myosin-S1

The rate and equilibrium constants for the formation and dissociation of the bovine ventricular (BV) actomyosin-S1-ADP have been measured by stopped flow light scattering. A comparison of the rate constants obtained here with those for rabbit skeletal (RS) actomyosin-S1 indicates that there are large differences in several of the rate and equilibrium constants. 1) The rate constant of ADP dissociation from BV actomyosin-S1 is 65 +/- 10 s-1 at 15 degrees C compared to a lower limit of 500 s-1 previously observed for RS actomyosin-S1. 2) The association constant for ADP binding to actomyosin-S1 is increased from 6 X 10(3) M-1 for RS to 1.5 X 10(5) M-1 for BV at 15 degrees C. The following rate and equilibrium constants differ by less than a factor of 2 between RS and BV actomyosin-S1: 1) the second order rate constant for the dissociation of actomyosin-S1 by MgATP; 2) the second order rate constant of myosin-S1 and myosin-S1-ADP binding to actin; and 3) the association constant of myosin-S1 to actin. The rate constant for ADP dissociation from BV actomyosin-S1 is at least 10-fold greater than the Vmax for the steady state ATPase and therefore cannot be the rate-limiting step of ATP hydrolysis. However, at physiological temperature, 38 degrees C, and ATP concentration, greater than 3 mM, ADP dissociation is sufficiently slow to limit the rate of myosin-S1 dissociation from actin by ATP and is likely to be the rate-limiting step of cross-bridge dissociation in muscle. Moreover, the rate constant of ADP dissociation is sufficiently slow to be the molecular step which limits the unloaded shortening velocity in cardiac muscle.