Structure and activity of insulin, XIII. Specificity of the arginine-guanidino group in biologically active tetrapeptides of the insulin sequence B 22-25 (Arg-Gly-Phe-Phe).

The biologically active partial sequence Arg-Gly-Phe-Phe (position B 22-25 of the insulin B chain) in the form of the synthetic tetrapeptidamide, was compared in several bioassays with the following analogous synthetic peptides: homoarginyl-, ornithyl-, lysyl-, citrullyl-, alanyl- and NG-nitroarginyl-Gly-Phe-Phe-NH2. The syntheses of the lysyl- and alanyl-tetrapeptidamides are described. After intraperitoneal injection of the peptides in doses of 3-100 mumol per 100 g rat, together with [U-14C]glucose, the natural sequence Arg-Gly-Phe-Phe showed the highest insulin like activity (incorporation of labeled carbon into the diaphragm). The activity of the homoarginyl peptide was a little weaker. The ornithyl- and the lysyl-peptide, however, showed a remarkably diminished activity. The activity of the citrullyl-peptide was even lower and the alanyl-peptide was inactive. In vitro assays with rat diaphragm showed the same range of effects for the elevation of glucose uptake and glycogen content of the diaphragm. The activity decreased in the following order: Arg- greater than Har- greater than Orn- greater than Cit-Gly-Phe-Phe-NH2. Alanyl- and Nitroarginyl-Gly-Phe-Phe-NH2 were without effect. In isolated fat cells the glucose oxidation was enhanced significantly only by the arginyl-peptide. The results show that among the structures examined the guanidino group carried by the C5 chain of arginine is the most effective. The results are in accordance with our preceding work [1] using semisynthetic insulins obtained from natural A-chain and synthetic B-chain variants. In these products the replacement of Arg B 22 by ornithine or lysine also led to drastically diminished activity and after replacement of Arg B 22 by alanine the activity also disappeared.