STUDIES ON KINETICS OF FORMATION AND DISSOCIATION OF ACTOMYOSIN COMPLEX

Changes in turbidity measured in a stopped flow spectrophotometer were used to study the rates of formation and dissociation of the actomyosin complex. The apparent rate constant for complex formation at pH 8 and 20° in 0.5 m KCl-5 X 10-3 m MgCl2 was 1.4 ± 0.5 X 105 m-1 sec-1. The value is based on the assumptions that myosin molecules are bound independently at sites on F-actin, and that each site consists of twoG-actin units. The dissociation of actomyosin by MgATP, MgGTP, MgUTP, MgITP, and magnesium pyrophosphate followed a simple exponential decay and the decay constant was proportional to the concentration of substrate. Dissociation by CaATP did not fit this simple description. The results for magnesium nucleotides and pyrophosphate are consistent with a kinetic scheme in which dissociation is produced by substrate binding by myosin at a site which is distinct from the actin binding site. © 1969, American Chemical Society. All rights reserved.