title: 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.
abstract: An extract of a neuroendocrine tumor of the human pancreas contained a high concentration of insulin and the C-peptide of proinsulin, as determined by radioimmunoassay, together with somatostatin, calcitonin, and thymosin beta 4. Analysis of the molecular forms of the proinsulin-derived peptides by high-performance liquid chromatography demonstrated that insulin was stored in the tumor as the intact peptide. In contrast, metabolites of C-peptide, representing the (1-21), (1-23), (1-25) and (1-29) N-terminal fragments, were isolated from the extract in addition to intact C-peptide. Generation of these metabolites involves cleavage of Xaa-Leu or Leu-Xaa bonds. Previous immunohistochemical studies have identified cathepsin B in secretory granules and lysosomes of human insulinoma cells. Synthetic human C-peptide was rapidly cleaved by purified human cathepsin B, primarily at the site of leucine residues, to give several metabolites, including the (1-25) and (1-23) fragments. The data indicate that the C-peptide of proinsulin is selectively metabolized in the neoplastic B cell by a mechanism that involves proteolytic cleavages in the C-terminal region of the peptide.
authors.affiliation: Department of Biomedical Sciences, Creighton University School of Medicine, Omaha, Nebraska 68178, USA.
authors.forename: J M
authors.initials: JM
authors.lastname: Conlon
authors.forename: A
authors.initials: A
authors.lastname: Höög
authors.forename: L
authors.initials: L
authors.lastname: Grimelius
mesh_terms: D000368:Aged; D000595:Amino Acid Sequence; D002096:C-Peptide / Q000235:genetics / Q000378:metabolism*; D002401:Cathepsin B / Q000378:metabolism*; D002845:Chromatography; D005260:Female; D006801:Humans; D007150:Immunohistochemistry; D007340:Insulinoma / Q000378:metabolism* / Q000473:pathology; D007424:Intracellular Fluid / Q000378:metabolism*; D008969:Molecular Sequence Data; D010190:Pancreatic Neoplasms / Q000378:metabolism* / Q000473:pathology; D011384:Proinsulin / Q000378:metabolism*; D011863:Radioimmunoassay
chemical_list: D002096:C-Peptide; D011384:Proinsulin; D002401:Cathepsin B