Actin Activation of Heavy Meromyosin Adenosine Triphosphatase

Abstract Actin activation of the ATPase activity of heavy meromyosin, a tryptic digestion product of myosin, was studied over a wide range of actin and ATP concentrations with creatine kinase and creatine phosphate added to hold the ATP concentration constant. Double reciprocal plots of ATPase against ATP concentration at varied actin concentration and of ATPase against actin concentration at varied ATP concentration were linear, suggesting that the system obeys simple Michaelis kinetics. From extrapolation of these plots we found that not only does ATP increase the dissociation of actin from heavy meromyosin but, analogously, the Michaelis constant of the acto-heavy meromyosin ATPase is greater than that of the heavy meromyosin ATPase. However, whereas the dissociation of actin from the heavy meromyosin-ATP complex is markedly dependent on ionic strength, the Michaelis constant of the acto-heavy meromyosin ATPase shows no salt dependence whatever. These data have important implications for the kinetic model which we previously proposed for the actin-heavy meromyosin-ATP system.