Mechanism of the actomyosin ATPase: effect of actin on the ATP hydrolysis step.

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Abstract
The Lymn-Taylor model for the actomyosin ATPase suggests that during each cycle of ATP hydrolysis the complex of myosin subfragment 1 (S-1) with actin must dissociate into S-1-ATP plus actin before ATP hydrolysis can occur. Whether such a mandatory detachment step occurs was studied by measuring the effect of actin on the rate and magnitude of the ATP hydrolysis step (initial Pi burst) and on the steady-state ATPase rate. The rate of the initial Pi burst markedly increases at high actin concentration, although the Lymn-Taylor model predicts the rate should remain nearly constant or decrease. At high actin concentration the magnitude of the initial Pi burst is much larger than predicted by the Lymn-Taylor model. At 360 .mu.M actin, at which more than 90% of the S-1-ATP is bound to actin, there is no inhibition of the steady-state ATPase activity, although the Lymn-Taylor model predicts that 70% inhibition should occur. The acto-S-1 complex apparently is not dissociated by ATP during each cycle of ATP hydrolysis. The rate of the initial Pi burst appears to be faster when S-1-ATP is bound to actin than when it is dissociated.