Polyphasic Character of ATP Hydrolysis in Actomyosin System

Abstract

The time course of ATP hydrolysis by reconstituted rabbit skeletal muscle actomyosin has been studied under various conditions of ionic strength and temperature with Mg2+ and Ca2+ as activating ions. Consistent with previous reports, a decrease in the rate of ATP splitting after the first several seconds of the reaction has been observed at low KCl concentrations (30–50 mM) in the presence of Mg2+ at 25 °C. The early fast phase of phosphate liberation was followed by an apparently linear phase lasting for about 2 min after which the ATPase rate further decreased. It has been confirmed that there is a correlation between the changes in the ATPase rate, particularly at early stages of the hydrolysis where the inhibition by products does not contribute to the decrease in rate, and superprecipitation of actomyosin gel. Extending previous studies on actin-myosin binding at low ionic strength [Dancker, P. and Hoffmann, M. (1973) Z. Naturforsch. 26c, 401–402; Strzelecka-Golaszewska, H., Jakubiak, M. and Drabikowski, W. (1975) Eur. J. Biochem. 55, 221–230], we were able to show that the decrease in rate of the Mg-ATPase is related to the release of a large portion of actin filaments from superprecipitating actomyosin gel. The contribution of other factors connected with the event of superprecipitation is discussed. It is concluded that the polyphasic character of ATP hydrolysis is not relevant to the kinetic mechanism of actomyosin ATPase.