Comparison of the myosin and actomyosin ATPase mechanisms of the four types of vertebrate muscles

The mechanism of ATP hydrolysis by myosin and actomyosin was investigated for the four major classes of vertebrate muscles: fast white (posterior latissimus dorsi), slow red (anterior latissimus dorsi), cardiac and smooth (gizzard). The kinetic behavior of all classes of muscle was consistent with the scheme developed previously for rabbit fast white muscle, but quantitative differences were observed for the rate constants of some of the steps in the hydrolysis cycle. The rate of the hydrolysis step of myosin subfragment-1 was similar for the striated muscles and two to three times smaller for smooth muscle. Two isomerizations of the enzyme occurred in the pathway leading to the formation of the myosin-products intermediate. The rate of dissociation of acto S–1 by ATP was slower for slow muscles and a maximum rate was observed at low temperature. The rate of association of the S-1-products intermediate with actin was equal to the turnover rate of acto S–1 ATPase at low concentrations of actin. The rate of dissociation of ADP from an acto S–1-ADP complex was also much slower for slow muscle. It was shown by Barany (1967) that the maximum turnover rate of actomyosin ATPase (V_M) is proportional to the velocity of contraction of the muscle. The only step in the mechanism that is correlated with V_M is the apparent second-order rate constant for the formation of a complex of the S-1-product state with actin. The evidence is discussed in terms of a mechanism in which the release of reaction products from actomyosin is the step that is of primary importance in determining the value of V_M and the velocity of contraction.