Reversible effects of okadaic acid and microcystin-LR on the ATP-dependent interaction between actin and myosin

Okadaic acid, a toxin from black sponge, and microcystin-LR, a toxin from blue-green algae, were found to stimulate and inhibit, respectively, the actin-activated ATPase activity of skeletal muscle myosin. These effects were confirmed by monitoring the sliding movement of actin-filaments on myosin. This technique also enabled us to demonstrate the reversibility of these effects, a property that is essential for their use as a pharmacological tool for the analysis of the mechanochemical characteristics of muscular contraction. The sites of action of both toxins were within the myosin molecule, as demonstrated by monitoring (i) their effects on the intrinsic tryptophan fluorescence of heavy meromyosin and (ii) their ATP-dependent effects on the ATPase activity. The former effects further suggest that myosin heads are their actual sites of action, and the latter effects suggest that they interact with the ATPase active sites located within the heads.