Abstract
We have identified a small molecule inhibitor, MY461, of the cardiac myosin ATPase. This agent was characterized in steady state and transient kinetic assays to understand its mechanism of action. MY461 decreases the steady-state rate of the ATPase activity of purified bovine β-cardiac myosin subfragment-1 (S1) as well as that of bovine cardiac myofibrils, wild type and the mutant R453C of recombinant human β-cardiac myosin S1. We also find that MY461 inhibits cardiac myosin selectively as compared to systems containing rabbit skeletal or chicken smooth muscle myosins Analysis of the individual steps of the chemo-mechanical cycle of cardiac myosin suggests that MY461 exerts its effect by inhibiting the actin-stimulated release of phosphate, presumably by stabilizing the detached state of cardiac myosin prior to the release of phosphate. We find no evidence to suggest that MYK000461 inhibits cardiac myosin in a strongly bound state and no other steps in the chemo-mechanical cycle are affected by MY461. Thus, the enzymatic step governing the weak to strong transition of S1 binding to actin is inhibited without affecting the release from the strongly bound states. This decrease in the rate of transition from the weak to strongly bound state should decrease force production and may underlie its ability to decrease cardiac contractility in cellular and in vivo models of cardiac function. An agent such as MY461 could potentially be used to treat cardiac disorders that stem from hyper contractility such as the genetic hypertrophic cardiomyopathies (HCM). By decreasing the net force of contraction and restoring it back to normal level could potentially be useful in treating patients that suffer from this disease.
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