The combination of insulin with thiocyanate ions.

Measurements of the electrophoretic mobilities of insulin reported by Hall (1, 2) are limited to regions higher than pH 7 and lower than pH 4.5, since the protein is practically insoluble in common buffer solutions of the intermediate pH range. While it has been possible to determine the elect.rophoretic mobility of adsorbed insulin and of suspensions of insulin crystals in the isoelectric range by the microscopic method (3), strict compar’son of mobility data obtained under these two different experimental conditions is not always permissible (4). Both sets of data, however, indicate that the isoelectric point of insulin lies within the range of pH 4.9 to 5.9. Attempts have been made in the present study to increase the solubility of insulin in its isoelectric region sufciently to afford determinations of the electrophoretic mobility in this range by the moving boundary method. Of various ions of the lyotropic series that have been tested, thiocyanate was found to exert a marked peptizing effect. The lower pH limit of solubility just sufficient for mobility measurements was approximately pH 5, the solubility increasing with increasing pH. In the presence of thiocyanate, insulin remained insoluble, however, at all pH regions below 5, including those in which it is soluble in the absence of this ion, i.e. pH 2. The electrophoretic data reported in this paper, together with the observed effect of thiocyanate on the solubility of insulin, suggest an interaction of this anion with certain basic groups of the protein. Considerable evidence has already been given for the binding by proteins of fatty acid anions (5--S), anionic detergents (g-13), and even anions of common buffer salts (14-16). Although some of these combinations may occur with the non-polar residue of the anion (17-18)) all of them will be reflected by measurable changes in electrophoretic mobilities.