[Biochemical characteristics of an initial degradation product of insulin by insulin-degrading enzyme].

We previously reported on the detection and HPLC separation of the initial degradation product (peak VI) of native insulin from the reaction of monocomponent porcine insulin with affinity-purified pig skeletal muscle insulin-degrading enzyme (IDE). In the present study, we investigated the biological characteristics of the initial degradation product. Structural analysis of peak VI by amino acid composition and glucose oxidation revealed that peak VI was composed of intact B-chain and a fragment of A-chain. In vivo, peak VI exhibited a hypoglycemic effect on rats. In vitro, this peptide had the binding capacity to insulin receptor of rat adipocytes and the ability to stimulate the glucose oxidation on rat adipocytes and the activity of insulin receptor kinase. However, the biological potencies of peak VI were 1/40 to 1/160 of those of insulin proper. Its reduced biological potencies were probably due to a decrease of affinity for insulin receptor, because both biological potencies of peak VI to bind to insulin receptor and to stimulate the glucose oxidation were 2.5% of insulin. Moreover peak VI showed the same full agonistic effect as insulin on the glucose oxidation at higher concentration. On the other hand, a cross-linking study suggested that peak VI preserves almost the same affinity to IDE as insulin. These findings may indicate the possibility that pig skeletal muscle IDE cleaves peptide bonds within A-chain at an early step of degradation of native porcine insulin and generates peak VI, which is the next substrate to insulin for IDE and keeps reduced insulin-like biological potencies, and then peak VI is converted into several relatively low molecular weight products.