Insulin degradation is an integral part of the cellular action of insulin. Recent evidence suggests that the enzyme insulin protease is involved m the degradation of insulin in mammalian tissues. Drosophila, which has insulin-like hormones and insulin receptor homologues, also expresses an insulin degrading enzyme with properties that are very similar to those of mammalian insulin protease. In the present study, the insulin cleavage products generated by the Drosophila insulin degrading enzyme were identified and compared with the products generated by the mammalian insulin protease. Both purified enzymes were incubated with porcine insulin specifically labeled with 125I on either the A19 or B26 position, and the degradation products were analyzed by HPLC before and after sulfitolysis. Isolation and sequencing of the cleavage products indicated that both enzymes cleave the A chain of intact insulin at identical sites between residues A13 and A14 and A14 and A15. Sequencing of the B chain fragments demonstrated that the Drosophila enzyme cleaves the B chain of insulin at four sites between residues BI0 and B11, B14 and B15, B16 and B17, and B25 and B26. These cleavage sites correspond to four of the seven cleavage sites generated by the mammalian insulin protease. These results demonstrate that all the insulin cleavage sites generated by the Drosopohila insulin degrading enzyme are shared in common with the mammalian insulin protease. These data support the hypothesis that there is evolutionary conservation of the insulin degrading enzyme and further suggest that this enzyme plays an important role in cellular function.
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