Sulphydryl groups in proteins

Abstract

THE work recorded here is an attempt to obtain quantitative information concerning the sulphydryl groups of proteins. It has long been realized that the combination in which sulphur exists in cystine and methionine is incapable of explaining all the reactions of protein sulphur. As instances of such reactions may be mentioned the suspected existence of free sulphydryl groups in protein (cysteine never having been isolated from protein), the recognition of labile and non-labile sulphur, recently the subject of yet further researches [Blumenthal & Clarke, 1935] and the exposure of sulphydryl groups on denaturation of protein. Certain alternative suggestions have been put forward; e.g. the -CS-NHlinkage of On the contrary, modern work [e.g. Speakman & Whewell, 1936] seems to favour simple hydrolysis of the disulphide linkage as the explanation of the sulphydryl group on denaturation. R. S. S. R + HOH -R. SH + R. SOH. We ourselves were more particularly interested in the distribution of sulphur in muscle proteins. That muscular tissue gives a nitroprusside reaction is an old-established fact and that muscle powder (i.e. denatured protein) contains a thermostable sulphydryl group was recorded by Dixon & Hopkins [1922]; whilst more recently Astbury [1934] has commented upon the similarity of the X-ray diffraction pictures of keratin (which is rich in cystine) and the muscle protein myosin, without, however, attributing functional significance to the sulphur groupings. The reactivity of the sulphydryl group and the simultaneous presence of glutathione in close association with the muscle proteins offer free play to the imagination. There seemed, however, little hope of eliciting useful information from muscle unless some quantitative method of estimating sulphydryl could be applied. The present communication is concerned with such a method and the preliminary results of its application. Whilst this work was in progress Mirsky & Anson [1935] published an account of their work directed to the same end but using quite different methods. A preliminary account of our own work has already appeared [Todrick & Walker, 1936]. Method Briefly the method consists in measuring the amount of oxidation-reduction indicator, phenolindo-2:6-dichlorophenol, reduced by a known weight of protein.