Ruthenium red was found to inhibit actin-activated myosin Mg2+-ATPase in smooth muscle and to bind to myosin heavy chain, but not to F-actin. The inhibition by Ruthenium red of actin-activated Mg2+-ATPase was of the competitive type with respect to actin (K i 4.4μM) and of the non-competitive type with respect to ATP (K i 6.6μM). However, Ruthenium red scarcely dissociated the acto-heavy meromyosin complex during the ATPase reaction. These results suggest that Ruthenium red interacts directly with the binding site for F-actin on the myosin heavy chain. This site is considered to be necessary not for maintaining the binding affinity of myosin for F-actin, but for activation of the Mg2+-ATPase.
