Immunological specificity of human insulin: application to immunoassay of insulin.

Previous investigations have revealed evidence for species-specificity among various mammalian insulins in immunologic reactions (1-5). For this reason, immunoassay of plasma insulin (610) in any species generally requires that the insulin employed in standard solutions be derived from the same species. Immunoassay of plasma insulin in man has been carried out by comparing the effects of unknown plasma samples with those of standard solutions of human insulin in their ability to inhibit competitively the binding of I131labeled beef insulin by guinea pig antibodies to beef insulin (7-10). Since supplies of human insulin are relatively scarce, the immunoassay of endogenous plasma insulin in man would be expedited if an animal insulin in more plentiful supply could be substituted as a standard for human insulin. Insulins from four different ungulate species (beef, hog, sheep, horse) have been found to react quantitatively with human antiserums to beef-pork insulin in a manner that could be correlated (5) with the similarity or dissimilarity of the amino acid sequences as elucidated by Harris, Sanger and Naughton (11). Of these four insulins, pork insulin (11) resembles human insulin (12) most closely in amino acid sequence, differing only in the C-terminal residue of the B chain (alanine in pork insulin, threonine in human insulin). The other animal insulins differ from human insulin also in one or more of the 8 to 10 positions of the A chain. It seemed worthwhile, therefore, to examine various antiserums for similarity of reaction of pork and human insulin. The present communication describes insulin antiserums obtained from guinea pigs immunized with crystalline pork insulin that react identically with human insulin and pork insulin in respect to the competitive inhibition of binding of pork insulin131.