[The character of actin-myosin interaction at two stages of the superprecipitation reaction].

It has been shown that two-stage kinetics of superprecipitation (SPP) and ATPase of natural and synthetic actomyosin can be modulated by changing Mg-ATp2- concentration. The I stage is activated at low substrate concentrations, and the II stage--at high concentrations. Resynthesis of ATP completely inhibited the II stage of SPP (and ATPase) and produced no effect in the clearing phase, as well as in the I stage of these reactions. We conclude that active myosin bridges function during the I stage of SPP. However, the II stage ends with the formation of rigorous bridges. It is suggested that division of two different types of actomyosin complexes which participated in the alternative kinetic mechanisms of both, SPP and ATPase reactions, takes place at the moment when ATP is bound in active sites of myosin and dependent on substrate concentration.