Sulfhydryl and Disulfide Groups of Proteins Iv. Sulfhydryl Groups of the Proteins of ~uscle by A. E. Mirsky

By investigating the sulfhydryl groups of the muscle proteins I have attempted to throw some light on the state of the proteins in muscle and on the change in state of these proteins in muscular contraction. In previous papers i t has been shown that in a denatured protein the number of active SH groups is equivalent to the quantity of cysteine found in the hydrolyzed protein (Mirsky and Anson, 1934--35). In a native protein only a fraction of these groups is active but as the pH rises the number of active groups increases (Mirsky and Anson, 1935-36). The effect of denaturation is to activate SH groups at a constant pH. Since activation of SH groups and formation of insoluble protein both occur when denaturation is brought about by a considerable number of agents--by heat, acid, alkali, alcohol, urea, salicylate, surface action, and ultraviolet light--activation of SH groups without a change in pH is a reliable criterion of denaturation. This paper contains estimations of SH groups of the denatured muscle proteins and of the same proteins before they have been treated with denaturing agents. I t was first observed by Arnold (1911) that even when the proteins of muscle have not been treated with any reagent which would be expected to denature them, SH groups are detectable by the test with nitropmsside and ammonium hydroxide. In crystalline egg albumin Arnold could detect SH groups only after denaturation. Since egg albumin is a typical coagulable protein, and since none of the proteins of muscle have been isolated in a crystalline form, it was possible that the SH groups detectable in minced muscle in the isolated muscle proteins are due to the presence of denatured protein. And since the proteins of muscle are supposed to be unusually unstable, presence 559