EFFECT OF 2,3-BUTANEDIONE MONOXIME ON MYOSIN AND MYOFIBRILLAR ATPASES - AN EXAMPLE OF AN UNCOMPETITIVE INHIBITOR

2,3-Butanedione monoxime (BDM) reversibly inhibits force production in muscle. At least part of its action appears to be directly on the contractile apparatus. To understand better its mechanism of action, we studied the effect of BDM on the steps of myosin subfragment 1 Mg2+-ATPase in 0.1 M potassium acetate, pH 7.4. Because of the rapidity of certain processes, we experimented at 4-degrees-C and our main technique was the rapid flow quench method. By varying the experimental conditions (relative concentrations of reagents, time scale, quenching agent), it was possible to study selectively the different steps of the S1 Mg2+-ATPase: [GRAPHICS] At saturation (20 mM), BDM had two major effects on the ATPase. First, it increased the equilibrium constant of the cleavage step (K3) from 2 to > 10. Second, it slowed the kinetics of the release of P(i) by an order of magnitude (k4; from 0.054 to 0.004 s-1). By contrast, the kinetics of the binding of ATP (k) and the release of ADP (k6) were little affected by BDM. Thus, the oxime appears to interact specifically with M**.ADP.P(i), and it is a rare example of an uncompetitive inhibitor. Its effect is to reduce the steady-state concentration of the "strong" actin binding state M*.ADP and to increase that of the "weak" binding state, M**.ADP-P(i). The effect of BDM on the initial ATPase of Ca2+ activated myofibrils was very similar to that on S1 ATPase. Thus, with myofibrils too BDM seems to exert its main effect subsequent to the initial binding and cleavage steps. We discuss our results with reference to the effect of BDM on the contractile properties of muscle fibers.