Rates of production of alanine and heptapeptide and of loss of biological activity during digestion of insulin with trypsin.

The tryptic hydrolysis of bovine insulin has been the subject of several recent studies (l-4). During the hydrolysis, alanine and the heptapeptide, Gly-Phe-Phe-Tyr-Thr-Pro-Lys, are produced, leaving a large portion of the molecule, desoctapeptideinsulin, intact (l-4). The structure of the heptapeptide has recently been confirmed by synthesis (5). Conflicting results have been reported with regard to the biological activity of the desoctapeptide-insulin. In a preliminary note, Nicol and Smith (1) reported that the desoctapeptide-insulin was inactive in lowering the blood sugar of rabbits. However, in a later publication, Nicol (3) reported that the desoctapeptide-insulin possessed approximately 15% of the activity of insulin in lowering the blood sugar of rabbits and also in the uptake of glucose by the rat diaphragm. On the other hand, Young and Carpenter (4) reported that desoctapeptide-insulin, which had been purified by partition column chromatography and characterized by amino acid composition and NHrterminal groups, was essentially inactive, having less than 4% of the activity of intact insulin, in the mouse convulsion assay. The present studies were undertaken in an attempt to resolve the conflicting results on biological activity of desoctapeptideinsulin. In order to eliminate different responses in different assays, the desoctapeptide-insulin prepared by Young and Carpenter (4) has been subjected to assay in rabbits. In addition, an experiment has been performed in which the rate of production of alanine and heptapeptide has been compared with the rate of loss of biological activity both in mice and rabbits during the digestion of insulin with trypsin.