Initial site of insulin cleavage by insulin protease.

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Abstract
Exposure of insulin to [rat] insulin protease (insulinase, EC 3.4.22.11), a degradative enzyme with considerable specificity toward insulin, results in alterations in the properties of the insulin molecule. Limited degradation by the enzyme results in a decrease in the ability of insulin to bind to membrane receptors with less change in the immunoprecipitability or trichloroacetic acid precipitability of the hormone. Limited degradation by insulin protease also alters insulin so that the molecule becomes susceptible to attack by nonspecific endopeptidases which have no effect on unaltered insulin. An intermediate is produced in the proteolytic degradation of insulin. By labeling with [14C]dansyl chloride, an insulin intermediate with 3 amino-terminal residues, Glyc, Phe and Leu, was identified. Analysis of this intermediate demonstrated that it was composed of an intact A chain and a B chain cleaved between residues B16 and B17, with the 3 peptide chains held together by disulfide bonds. A stepwise degradation of insulin evidently occurs in vivo and an early step in the process is the cleavage between B16 and B17 that renders the molecule susceptible further degradation by nonspecific proteases.