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Isotope-exchange reactions (H2(18)O in equilibrium with KH2PO4) during ATP hydrolysis catalyzed by myosin and its subfragment 1 from rabbit, dog, and human cardiac muscle were studied. All preparations of myosin and subfragment 1 in the presence of Mg2+ catalyzed two types of 18O-exchange reactions similar to those of skeletal muscle: intermediate and direct 18O exchange. The dependences of both reactions on divalent metals and nucleotides were studied. Data on 18O exchange of subfragment 1 from rabbit, dog, and human cardiac muscle were obtained for the first time. They indicate the similarity of molecular mechanisms of ATP energy use in cardiac and skeletal muscle contraction.

18O-exchange reactions of skeletal, cardiac and smooth muscle myosin were studied. All investigated preparations of myosin catalyse two types of 18O-exchange--intermediate and direct exchanges in the presence of Mg2+. The dependence of 18O-exchange extent on divalent cations have appeared to be similar for different muscle types. This supports the hypothesis of similarity of ATP hydrolysis molecular mechanisms by myosin of different origin.

Myosin, HMM and HMM S1 catalyze 18O-exchange between P1 and H218O of the medium at an intermediate stage of ATP hydrolysis ("intermediate 18O-exchange") in the presence of Mg2+. Natural complexes of actomyosin and acto-HMM S1 do not catalyze intermediate 18O-exchange but facilitate "direct" or "medium" 18O-exchange (KH2P18O4 in equilibrium H2O) even without ATP. Reconstituted complexes of actomyosin, acto-HMM, acto-HMM S1, PABC-HMM S1, congo-myosin and TNP-myosin do not catalyze direct 18O-exchange in the presence of Mg2+ and absence of ATP. From the data obtained a hypothetical sequence of phosphorylation and 18O-exchange reactions in myofibril action has been suggested.