Isolation and Characterization of Products Formed by the Action of Trypsin on Insulin

An examination of the Sanger structure of insulin (2) reveals that the one lysine and the one arginine in the molecule occur outside of the cystine bridges and in the last eight amino acid residues of the peptide chain having phenylalanine as the NHtterminal residue (B chain). Trypsin would be expected to cleave alanine and the heptapeptide, Gly . Phe . Phe . Tyr . Thr . Pro. Lys, from the molecule leaving a large portion of the molecule, desoctapeptide-insulin, intact. Sanger and Tuppy (3) and later Feitelson and Partridge (4) detected alanine and the heptapeptide on paper chromatograms of the products of tryptic digestion of the oxidized B chain of insulin, Harris and Li (5) obtained the same paper chromatographic pattern from tryptic digestion of crystalline zinc-insulin. Nicol and Smith (6, 7) have reported studies on the isolation of desoctapeptide-insulin (designated DHA-insulin by these investigators). They originally reported that desoctapeptide-insulin was biologically inactive in rabbits (6). However, Nicol recently (7) reported that desoctapeptide-insulin possessed about 15% of the biological activity of insulin in the lowering of the blood sugar of rabbits and also in the uptake of glucose by the rat diaphragm. Our studies were initiated several years ago with the idea of developing procedures for the preparation, isolation, and characterization of desoctapeptide-insulin. We used a somewhat different approach to the problem than that of Nicol and have obtained what appear to be conflicting results with regard to the biological activity of the desoctapeptide-insulin.