Backbone cyclic insulin.

Backbone cyclic insulin was designed and prepared by reverse proteolysis in partial organic solvent of a single-chain precursor expressed in yeast. The precursor contains two loops to bridge the two chains of native insulin. The cyclisation method uses Achromobacter lyticus protease and should be generally applicable to proteins with C-terminal lysine and proximal N-terminal. The presence of the ring-closing bond and the native insulin disulfide patterns were documented by LC-MS peptide maps. The cyclic insulin was shown to be inert towards degradation by CPY, but was somewhat labile towards chymotrypsin. Intravenous administration of the cyclic insulin to Wistar rats showed the compounds to be equipotent to HI despite much lower insulin receptor affinity.