Reaction intermediates formed by myofibrils during the ATPase reaction under relaxed conditions

The species and amounts of intermediates formed by myosin in myofibrils during the ATPase reaction under relaxed conditions were examined. The amount of total nucleotides (ADP + ATP) bound to myofibrils, determined by a centrifugation method or a rapid filtration method, was 0.86 mol/mol myosin head. The amount of bound ADP, determined as the ADP remaining in the mixture after free ADP had been rapidly converted into ATP by an ATP-regenerating system, was found to be 0.67 mol/mol myosin head. We examined the time courses of free-Pi and total-Pi (TCA-Pi) formation after adding ATP to the myofibrils. The amount of Pi bound to myofibrils, calculated by subtracting the burst size of free Pi (0.23 mol/mol myosin head) from that of TCA-Pi (0.60 mol/mol myosin head), was found to be 0.37 mol/mol myosin head. The amount of tightly bound ATP determined by an ATP-quenching method was very low (0.03 mol/mol myosin head). If there is no myosin-phosphate complex, then the amounts of the myosin-phosphate-ADP complex, MADPP, and the tightly bound myosin-ATP complex, M*ATP, are 0.37 and 0.03 mol/mol myosin head, respectively, whereas the amounts of myosin-ADP and loosely bound myosin-ATP complexes are 0.30 and 0.16 mol/mol myosin head, respectively. Thus, half of the myosin heads forms MADPP or M*ATP, and the equilibrium between MADPP and M*ATP shifts to the MADPP side. These results agree with those obtained for myosin in solution (Inoue, A., Takenaka, H., Arata, T., & Tonomura, Y. (1979) Adv. Biophys. 13, 1-194). Therefore, in relaxed myofibrils the active site of myosin does not interact with actin.