The electroreduction of the disulphide bonds of insulin and other proteins

Abstract

The polarographic behavior of five proteins of five proteins containing SS groups and one containing SH groups has been studied by using the dropping-mercury electrode. The disulphide proteins all gave a single cathodic wave at pH 1 which was shown to involve the reduction of some of the SS groups. The number of SS groups involved with each protein was calculated from the Ilkovic equation. The height of these waves increased linearly with protein concentration until a limit was reached. The limiting current, and the concentration at which it occurred, varied between proteins and was shown to be related to the number of reducible SS groups/molecule and the molecular weight. At pH 7[image]1 and 9[image]2 a second cathodic wave was observed in some cases. This appeared after the first wave had reached its limiting height and was also shown to involve the reduction of SS groups. The behavior of both types of polarographic wave has been explained in terms of the adsorption of protein on to the surface of the mercury drop. Hemoglobin, which contains SH groups but no SS groups, does not give a polarographic wave under these conditions. Insulin and ribonuclease were reduced at a stirred-mercury cathode. At-1[image]35v only the interchain SS bonds of insulin were reduced but at [long dash]1[image]8 v all three bonds were reduced. At-2[image]0 v all four SS bonds of ribonuclease were reduced.