The relationship between sulfhydryl groups and the activation of myosin adenosinetriphosphatase.

Greenstein and Edsall (1) in early observations on the quantitative estimation of sulfhydryl groups in the muscle protein myosin classified the SH groups of the native protein into “free” and unavailable groups on the basis of porphyrindine titration with nitroprusside as an external indicator. In subsequent experiments, Singer and Barron (2) observed that combination of the “free” groups of myosin wit.h p-chloromercuribenzoate (PCMB) resulted in little alteration of the adenosinetriphosphatase (ATPase) activity of myosin, while addition of PCMB equal to the total SH led to complete inhibition. It has been observed by one of us (3) that low concentrations of PCMB resulted in some increase in ATPase activity and this was also observed occasionally by Polis and Meyerhof (4). On reexamination of the influence of PCMB titration of myosin SH, we observed that a marked increase in ATPase activity occurred when approximately one-half the sulfhydryl groups had been titrated, when Ca++ was employed as ATPase activator. On the other hand, only inhibition occurred when ethylenediaminetetraacetic acid (EDTA) was employed as activator (cf. (5, 6) for EDTA activation). The reaction of myosin with N-ethylmaleimide (NEM) gave similar results. After the work presented here was completed, two reports (7,8) appeared on the influence of phenylmercuric acetate and dinitrophenol on myosin ATPase. In these the authors appear to be dealing with similar phenomena.