Contraction of Muscle and Denaturation of Myosin

It is recognized that the protein myosin takes part directly in the contraction of muscle, and there is even reason to believe that myosin itself is the contractile substance of muscle., If so significant a role in contraction is ascribed to myosin, it is important to know how myosin changes during contraction. Experiments concerning the effect of heat on both muscle and myosin show that, under some conditions at least, shortening of muscle may be made possible by the denaturation of myosin. In this note I shall briefly describe these experiments. A thermal stimulus can cause either a reversible or an irreversible shortening of muscle. If the sartorius muscle of a frog is gradually warmed, its length remains unchanged until a temperature of about 37° is reached, when the muscle suddenly shortens. On cooling, the muscle recovers its original length. If the muscle is heated to higher temperatures a further shortening is observed, beginning at 39° and reaching an end-point at about 45 °C. This time the muscle does not relax on cooling. The 2 responses of muscle to heat were first sharply distinguished from each other by Jensen, who referred to the reversible shortening as thermal contraction and to the irreversible shortening as thermal rigor. In experiments on the effect of heat on myosin isolated from frog muscle I find that heat denatures myosin in 2 distinct steps, one of which can be identified with thermal contraction and the other with thermal rigor. In these experiments myosin was in a medium bearing some resemblance to its environment in the muscle fiber—in isotonic KG with a small amount of phosphate adjusting the pH to 6.9. Under these conditions very little protein is in solution and myosin has the appearance of a transparent semi-fluid gel.