Immunological and chemical characterization of bovine preproinsulin

Fetal bovine pancreatic poly(A)-containing RNA directs the synthesis of an insulin immunoreactive polypeptide that is larger than proinsulin, preproinsulin, in the wheat germ cell-free translation system. We have characterized this peptide in detail both immunologically and chemically and have shown that it is 2500 daltons larger than bovine proinsulin (8700 daltons), possesses both insulin and bovine C-peptide-specific antigenic determinants, and contains all the tryptic peptides found in bovine proinsulin. Preproinsulin synthesized in the wheat germ cell-free system was precipitated with approximately 4-fold greater efficiency by bovine proinsulin antiserum than by insulin antiserum. Additional evidence was obtained which indicated that the preprotein folds and undergoes correct sulfhydryl oxidation less efficiently than proinsulin, perhaps due to the presence of the hydrophobic NH2-terminal extension. Automated sequential Edman degradation of bovine preproinsulin revealed the presence of an additional NH2-terminal sequence of 23 residues, preceding the B chain segment of proinsulin. The positions of 6 of the 7 leucine residues found in the bovine preproinsulin extension were identical to those reported previously for the rat preproinsulins. This close sequence similarity between the extensions of the bovine and rat preproinsulins supports the hypothesis that these molecules fulfill similar biosynthetic functions in vivo.