The coagulation of myosin in muscle

The coagulation of myosin is one of the few changes in the proteins of muscle known to take place during contraction and rigor. Although under certain conditions as much as one-third of the total protein of muscle may become insoluble, the significance of this change for the shortening of muscle is not understood. And yet the recognition of a definite transformation in the substance of muscle should be of value in investigating the mechanism of contraction, especially when one recalls that nearly all of those abortive theories of contraction that have been formulated since the time of Descartes have been based on knowledge of systems supposed to be analogous to muscle, rather than on a knowledge of the properties of living muscle itself. As a step towards an understanding of the chemical properties of the "living machinery" of muscle (as distinguished from the metabolic transformations in muscle) I have, accordingly, investigated the coagulation of myosin. I have already shown how the coagulation of myosin in muscle is related to the denaturation and coagulation of isolated myosin (Mirsky, 1935-36 and 1936-37). By measurements of protein sulfhydryl groups it was found that the coagulation of myosin in muscle differs from the coagulation of myosin and other proteins brought about by the usual denaturing agents (such as heat and acid) but resembles the coagulation of myosin caused by dehydration. At this point it is important to recall that when myosin is said to coagulate in muscle it is not supposed that myosin actually precipitates from solution. In muscle probably only a very small part of the myosin present is dissolved (Smith, 1934). That there is a change in myosin in muscle is inferred from the fact that at one time the protein can be dissolved in certain media in which at another time it cannot be dissolved; the myosin is said to have coagulated. Apparently myosin in muscle can pass from one gel state to another. How these states differ will be considered in this paper