The Inactivation of Crystalline Insulin by Cysteine and Glutathione* by Vincent

In some earlier work on insulin (1, 2) we have shown that the sulfur of insulin existed mainly if not entirely in the disulfide form, and further that the major portion of the disulfide content could be accounted for by cystine. In continuation of this study of the sulfur of insulin we felt it would be worth while to study the behavior of the reduced or sulfhydryl form of insulin. If the disulfide grouping alone could be reduced, it would give much information as to whether the disulfide linkage as such were necessary to the action of insulin. It has been shown by many different workers that the reduction of various insulin preparations destroyed their activity. Practically no such work has been carried out on crystalline insulin. The reducing agents used in the above work, however, were quite vigorous ones and there could be no justification for believing that no other part of the molecule besides the disulfide grouping had been reduced. There is some reason to believe that the action of cysteine might prove to be fairly specific for reducing the disulfide grouping. Mirsky and Anson (3) applied somewhat the reverse reaction to proteins in devising a method to measure the sulfhydryl groups in denatured proteins. They oxidized the sulfhydryl groups by using an excess of some disulfide. They fully realized that the