Studies on Human Proinsulin

Human proinsulin C-peptide has been extracted from human pancreas with acid-ethanol and purified by means of gel filtration, carboxymethyl-cellulose chromatography, paper electrophoresis, and partition chromatography. The purified material was judged to be about 98% pure by compositional analysis. Amino acid sequence analysis was carried out on the intact C-peptide and fragments derived by hydrolysis with chymotrypsin and thermolysin. On the basis of these results, and in accord with the compositional stoichiometry, the following 31-residue amino acid sequence is proposed: Glu-Ala-Glu-Asp-Leu-Gln-Val-Gly-Gln-Val-Glu-Leu-Gly-Gly-Gly-Pro-Gly-Ala-Gly-Ser-Leu-Gln-Pro-Leu-Ala-Leu-Glu-Gly-Ser-Leu-Gln. Although sufficient human proinsulin has not been available thus far for detailed structural studies, the probable identity of this pancreatic C-peptide with the corresponding region of human proinsulin is supported by both compositional and immunological studies on human proinsulin. Comparison of the amino acid sequences of the bovine, porcine, and human C-peptides reveals homology but also much greater variability in both length and sequence than occurs among the insulins in these species. The conformation and role in peptide chain folding of the connecting polypeptide region of proinsulin is evaluated in the light of these findings.