INTRACELLULAR DEGRADATION OF THE C-PEPTIDE OF PROINSULIN, IN A HUMAN INSULINOMA - IDENTIFICATION OF SITES OF CLEAVAGE AND EVIDENCE FOR A ROLE FOR CATHEPSIN-B

An extract of a neuroendocrine tumor of the human pancreas contained a high concentration of insulin and the C-peptide of proinsulin, as det ermined by radioimmunoassay, together with somatostatin, calcitonin, a nd thymosin beta(4). Analysis of the molecular forms of the proinsulin -derived peptides by high-performance liquid chromatography demonstrat ed that insulin was stored in the tumor as the intact peptide. In cont rast, metabolites of C-peptide, representing the (1-21), (1-23), (1-25 ) and (1-29) N-terminal fragments, were isolated from the extract in a ddition to intact C-peptide. Generation of these metabolites involves cleavage of Xaa-Leu or Leu-Xaa bonds. Previous immunohistochemical stu dies have identified cathepsin B in secretory granules and lysosomes o f human insulinoma cells. Synthetic human C-peptide was rapidly cleave d by purified human cathepsin B, primarily at the site of leucine resi dues, to give several metabolites, including the (1-25) and (1-23) fra gments. The data indicate that the C-peptide of proinsulin is selectiv ely metabolized in the neoplastic B cell by a mechanism that involves proteolytic cleavages in the C-terminal region of the peptide.