Immunological and biological properties of iodoinsulin labeled with one or less atoms of iodine per molecule

Experiments were designed to compare the distribution of free and antibody-bound unlabeled insulin to the distribution of free and antibody-bound insulin-(125)I. The insulin antibody was incorporated in a specific immune precipitate similar to the one used by Hales and Randle for the radioimmune assay of insulin. Insulin which was not bound by the specific immune precipitate was measured by the immune hemolysis inhibition assay. This report contains evidence that the addition of the unlabeled insulin in the radioimmune assay results in relatively more insulin-(125)I which remains free and less bound by antibodies than is the case with the unlabeled insulin. Methods are described for the separation of an electrophoretically homogeneous iodoinsulin from samples of crude iodoinsulin with average incorporations of less than 0.2 atoms iodine per molecule. These purified iodoinsulin fractions have a markedly attenuated biological activity. Evidence is presented which supports the postulate that only a portion of the antibodies in guinea pig insulin antiserum are capable of effectively binding with purified iodoinsulin.