Degradation of Insulin and Insulin-Like Growth Factors by Enzyme Purified From Human Erythrocytes: Comparison of Degradation Products Observed With A14- and B26-[¹²⁵I]monoiodoinsulin

An insulin-degrading enzyme has been purified from human erythrocytes. This enzyme degraded ¹²⁵I-labeled insulin-like growth factor I (IGF-I) more slowly than ¹²⁵I—IGF-II and degraded IGF-II more slowly than ¹²⁵I-insulin. The time course of ¹²⁵I-insulin degradation suggested the presence of intermediates, each of which wasitself shown to be a substrate for the enzyme. One of these intermediates appeared to be made up entirely of B-chain residues and had His^B10 as its NH₂-terminal. The final major radiolabeled degradation product of A14-[¹²⁵I]monoiodoinsulin was a peptide with Tyr^A14 at the A-chain NH₂ terminal. This peptide could be reduced with dithiothreitol, suggesting that it contained amino acid residues from both A- and B-chains. It was partially precipitated by trichloroacetic acid and anti-insulin antibody but bound poorly to IM-9 lymphocytes. The final major degradation product of B26-[¹²⁵I]monoiodoinsulin was a peptide whose NH₂-terminal was Tyr^B26 and could not be reduced by dithiothreitol. It was partially precipitated by anti-insulin antibody but was precipitated poorly, if at all, by trichloroacetic acid and bound poorly to IM-9 lymphocytes. The results show that this enzyme degraded insulin by sequential cleavage of peptide bonds on both A- and B-chains. We identified Leu^A13-Tyr^A14, Ser^B9-His^B10, and Phe^B25-Tyr^B26 as three of the bonds that are cleaved.