Dimethyl sulphoxide enhances the effects of Pi in myofibrils and inhibits the activity of rabbit skeletal muscle contractile proteins

In the catalytic cycle of skeletal muscle, myosin alternates between strongly and weakly bound cross-bridges, with the latter contributing little to sustained tension. Here we describe the action of DMSO, an organic solvent that appears to increase the population of weakly bound cross-bridges that accumulate after the binding of ATP, but before P(i) release. DMSO (5-30%, v/v) reversibly inhibits tension and ATP hydrolysis in vertebrate skeletal muscle myofibrils, and decreases the speed of unregulated F-actin in an in vitro motility assay with heavy meromyosin. In solution, controls for enzyme activity and intrinsic tryptophan fluorescence of myosin subfragment 1 (S1) in the presence of different cations indicate that structural changes attributable to DMSO are small and reversible, and do not involve unfolding. Since DMSO depresses S1 and acto-S1 MgATPase activities in the same proportions, without altering acto-S1 affinity, the principal DMSO target apparently lies within the catalytic cycle rather than with actin-myosin binding. Inhibition by DMSO in myofibrils is the same in the presence or the absence of Ca(2+) and regulatory proteins, in contrast with the effects of ethylene glycol, and the Ca(2+) sensitivity of isometric tension is slightly decreased by DMSO. The apparent affinity for P(i) is enhanced markedly by DMSO (and to a lesser extent by ethylene glycol) in skinned fibres, suggesting that DMSO stabilizes cross-bridges that have ADP.P(i) or ATP bound to them.