Dimethyl sulphoxide enhances the effects of P-1 in myofibrils and inhibitsthe activity of rabbit skeletal muscle contractile proteins

In the catalytic cycle of skeletal muscle, myosin alternates between strong ly and weakly bound cross-bridges, with the latter contributing little to s ustained tension. Here we describe the action of DMSO, an organic solvent t hat appears to increase the population of weakly bound cross-bridges that a ccumulate after the binding of ATP, but before Pi release. DMSO (5-30 %, v/ v) reversibly inhibits tension and ATP hydrolysis in vertebrate skeletal mu scle myofibrils, and decreases the speed of unregulated F-actin in an in vi tro 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 attr ibutable to DMSO are small and reversible, and do not involve unfolding. Si nce DMSO depresses S1 and acto-S1 MgATPase activities in the same proportio ns, without altering acto-S1 affinity, the principal DMSO target apparently lies within the catalytic cycle rather than with actin-myosin binding. Inh ibition by DMSO in myofibrils is the same in the presence or the absence of Ca2+ and regulatory proteins, in contrast with the effects of ethylene gly col, and the Ca2+ 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.