The Changes of “Myosin B” during Storage of Rabbit Muscle

Abstract

The denaturation process of actomyosin during storage in 0.6 m KCl at pH 5.7 and 25°C was studied by measuring ATPase activity, turbidity and solubility. During the storage of actomyosin the inactivation rate of myosin A ATPase was similar to that of the ATPase activating ability of F-actin. While the inactivation rate of isolated F-actin was largely dependent on the initial protein concentration, that of F-actin in actomyosin was less dependent. When F-actin was stored alone, inactivation of the ATPase activating ability was accompanied with a remarkable increase in turbidity and a decrease in solubility. However, this was not true when F-actin was stored with myosin A. The denaturation process of actomyosin during storage in 0.6 m KCl at pH 5.7 and 25°C was studied with regard to changes in the binding ability of myosin A and actin. When F-actin was stored alone, it lost the ability to bind with myosin A in pararell to the decrease in the ATPase activating ability. When myosin A was stored alone for 3 hr, it maintained 60% of the initial ability to bind with F-actin, even if it lost 85% of the initial ATPase activity. Thus, the ATPase-less myosin A forms a myosin A-actin complex when it is mixed with intact F-actin. F-Actin was not stabilized in this modified myosin A-actin complex. This complex was dissociated with ATP into ATPase-less myosin A and intact F-actin. The modified myosin A-actin complex did not occur in the stored actomyosin. During the storage of actomyosin, the interaction between myosin A and F-actin did not break and became less dissociable by ATP. This feature is in contrast with the one observed previously on the actomyosin stored in 0.6 m KCl at pH 5.7 and 3°C.