Enzyme kinetic measurements of the effects of actin on the heavy meromyosin adenosine triphosphatase (ATPase) are used for a study of the binding of heavy meromyosin to actin. In the absence of free magnesium ions, the potassium-activated ATPase of heavy meromyosin is inhibited by actin, and this inhibition is used as a quantitative measuee of binding. The activation of the heavy meromyosin ATPase by actin in the presence of magnesium ions is also used as a measure of binding. Both kinds of data, when analyzed according to the theory of multiple equilibria, give simple binding curves with a binding stoichiometry of 1 mole of heavy meromyosin/mole of monomsrs in F-actin.
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