Aorta caldesmon inhibits actin activation of thiophosphorylated heavy meromyosin Mg2+-ATPase activity by slowing the rate of product release.

Activation of aorta thiophosphorylated heavy meromyosin (HMM[SP]) Mg2+-ATPase activity by aorta actin and the fraction of HMM[SP]-substrate intermediate complexes bound to actin were measured simultaneously. At 25 degrees C the Km for ATPase activation and the dissociation constant for the binding reaction were similar, irrespective of the presence or absence of tropomyosin. Aorta caldesmon (0.1 mol/mol actin) inhibited ATPase activation by 80-90% but did not alter the binding of HMM[SP]-product intermediates to actin. It is concluded that caldesmon inhibits by slowing the rate-limiting release of products from the actin-HMM[SP].ADP.Pi complex.