Single turnovers of adenosine 5'-triphosphate by myofibrils and actomyosin subfragment 1.

The ATPase of myofibrils has been investigated by using single-turnover methods which avoid contraction to unphysiologically short sarcomere lengths. A substoichiometric amount of ATP is mixed with myofibrils in rigor, and the rate of decay of bound ATP to ADP and Pi is followed. At 0 degree C, the rate is 0.45 s-1 and is not dependent on ionic strength in the range I = 0.035-0.175. The steady-state ATPase of myofibrils under these conditions is considerably slower (0.072 s-1) than the rate of ATP decay and must be controlled by a subsequent step. Unlabeled ATP chase experiments were used to determine the rate of release to the medium of bound ATP. This rate is comparable to that of hydrolysis and dissociation as products but is dependent on ionic strength, changing from 0.53 s-1 at I = 0.035 to 0.12 s-1 at I = 0.085. Knowledge of the rate of release of bound ATP, together with the rate of ATP binding, allows the basic equilibrium constant between the states AM + ATP and A + M*.ATP to be estimated. The value is 3 X 10(4), which corresponds to about 40% of the total basic free-energy change between physiological concentrations of medium ATP and medium ADP and Pi. Single turnovers of acto-S1 ATPase were also investigated. At 0 degree C and I = 0.02, the rate of decay of bound ATP was equal to the rate of steady-state ATPase over a range of actin concentrations up to 10 times the Km. This observation suggests that it is the same enzymatic step which controls the rates of both processes; acto-S1 models of this type are discussed.