A Biologically Active Insulin Analog with an Enlarged Intrachain Cyclic System

The synthesis and isolation in purified form of [Hcy6,11-A]insulin, a biologically active analog of sheep insulin, is described. This analog differs from the parent molecule in that the cysteine residues located at positions 6 and 11 in the A chain and participating in the formation of the intrachain cyclic system have been replaced by homocysteine residues. For the synthesis of this compound, the Hcy6,11-A chain of sheep insulin was chemically synthesized by the fragment condensation method and isolated as the S-sulfonated derivative. Conversion of the latter compound to the sulfhydryl form on exposure to 2-mercaptoethanol and combination of the reduced product with the S-sulfonated B chain of sheep insulin afforded [Hcy6,11-A]insulin. Purification of the insulin analog was accomplished by chromatography on a carboxymethylcellulose column with an exponential sodium chloride gradient. This analog, which possesses a 22-membered cyclic system in contrast to insulin which contains a 20-membered ring, had a potency of 7 to 8 i.u. per mg, as compared to 25 i.u. per mg possessed by the natural hormone when assayed by the mouse convulsion method or by the radioimmunoassay method. The implication therefore arises that the enlargement of the intrachain cyclic system by two methylene groups does not abolish the topochemical features responsible for the expression of the biological profile of insulin.