The reactivity of the disulphide bonds of purified proteins in relationship to primary structure

With the aid of a coupled system involving glutathione reductase, the reaction of glutathione with the disulphide bonds of purified proteins has been studied. 2. Bovine serum albumin, conalbumin, lysozyme, trypsin inhibitors from egg white, lima bean and soya bean either did not react with glutathione or reacted only slightly. With these proteins reactivity was markedly increased by limited proteolysis. 3. Bovine and human gamma-globulins, fibrinogen and beta-lactoglobulin exhibited some reactivity (less than 15%) with glutathione and again this was increased by limited proteolysis. Pepsin, trypsin and chymotrypsin exhibited greater reactivity than the proteins previously mentioned. Di-isopropylphosphoryl-chymotrypsin exhibited less reactivity than chymotrypsin, suggesting that autolysis under the experimental conditions used contributed towards the reactivity of this protein. Proteolysis also increased the reactivity of these proteins. The three disulphide bonds of insulin were reduced by glutathione. 4. Above 35 degrees the disulphide bonds of serum albumin show a progressive increase in reactivity and at 55 degrees half of the bonds become accessible to glutathione. 5. From the results obtained with the proteins investigated, the conclusion reached is that the disulphide bonds of native proteins are structurally protected and do not react with glutathione under physiological conditions.