Identification of an insulin fragment produced by an insulin degrading enzyme, neutral thiopeptidase

Previous studies have shown that neutral thiopeptidase (E.C.3.4.22.11, insulinase) degrades (processes) insulin with a high affinity ( K m = 30 × 10 −9 M). In the current studies, insulin was subjected to digestion with a highly purified rat liver neutral thiopeptidase and the peptides generated were separated by HPLC using a C 8 column. With the use of structural analysis (which included the determination of amino terminal residues and amino acid composition), the major product was identified as a peptide containing portions of both chains of insulin, A 1 to A 13 and B 1 to B 9 having two disulfide bonds, an interchain disulfide bond and presumably the intra-A chain disulfide bond as well. Examination of insulin-like biological activity using a primary cultured hepatocyte test system showed that the fragment promoted neither short-term (α-aminoisobutyric acid uptake) nor long-term (glycogen synthesis) bioactivities of insulin.