Effects of zinc ion on the conformation of antigenic determinants on insulin

Abstract

Comparison of c.d. [circular dichroism] spectra of Zn-insulin with Zn2+-free insulin demonstrated significant differences. It was proposed that these differences are due to changes in the structure of insulin monomers within aggregated insulins or the results of insulin aggregation. The effect of Zn2+ on the immunological activity of insulin indicated that the antigenic determinants of insulin were also altered. The apparent loss of immunological activity of monoiodotyrosylinsulin was demonstrated to be due to the loss of Zn2+ rather than the substitution of iodine. The immunological activity of Zn-insulin and Zn2+-free insulin was compared in the radioimmune and immune hemolysis-inhibition assays by using an identical population of guinea pig antibodies and concentrations of inhibitor. Relative to Zn-insulin, Zn2+-free insulin had a markedly attenuated immunological activity in the immune hemolysis-inhibition assay, whereas in the radioimmune assay slightly greater immunological activity was observed with the Zn2+-free insulin. The removal of Zn2+ probably perturbs the conformation of determinants that react with antibodies operative in the immune hemolysis-inhibition assay (immune hemolysis determinants) and has a minimal effect on the conformation of determinants that react with antibodies operative in the radioimmune assay (radioimmune determinants).