On the mechanism of actomyosin ATPase from fast muscle

The labeled inorganic phosphate formed by enzymatic hydrolysis of [gamma-18O]ATP in normal water was derivatized to trimethyl phosphate and analyzed for the proportions of [18O3]Pi, [18O2]Pi, [18O1]Pi, and [18O0]Pi. The proportions observed were correlated with the kinetics of intermediate exchange by using a kinetic relationship in which it is assumed that binding of ATP and subsequent release of products are irreversible. Actomyosin and acto-heavy meromyosin catalyze intermediate exchange at a mean rate that is more than 1 order of magnitude slower than that predicted by rapid kinetic studies or implied by the essentially complete intermediate exchange observed with myosin alone. The reason for the slow apparent exchange is that there are two ATPase activities with different exchange properties. The effect of varying heavy meromyosin concentrations at a constant actin concentration shows that the two activities are interrelated and suggests further that one is due to ATP hydrolysis by the undissociated actomyosin crossbridge.