Silk sericin-insulin bioconjugates: synthesis, characterization and biological activity.

When silk fiber derived from Bombyx mori was subjected to degumming treatments twice in water and subsequent degraded processing in slightly alkaline aqueous solution under high-temperature and high-pressure, the water-soluble silk sericin peptides (SS) with different molecular mass from 10 to 70 kDa were obtained. The sericin peptides could be conjugated covalently with insulin alone with cross-linking reagent glutaraldehyde. The physicochemical properties of the silk sericin-insulin (SS-Ins) conjugates were determined by Enzyme-Linked Immunosorbent Assay (ELISA). The biological activities of SS-Ins bioconjugates were investigated in vitro and in vivo. The results in human serum in vitro indicated that the half-life of the synthesized SS-Ins derivatives was 2.3 and 2.7 times more than that of bovine serum albumin-insulin (BSA-Ins) conjugates and intact insulin, respectively. The pharmacological activity of SS-Ins bioconjugates lengthened to 21 h in mice in vivo, which was over 4 times longer than that of the native insulin. The immunogenicity of silk sericin and the antigenicity of SS-Ins derivatives were not observed in both rabbits and mice. The bioconjugation of insulin with silk sericin protein evidently improved both physicochemical and biological stability of the polypeptide.