Synthesis of Two Possible Disulfide Bonds Containing Peptide Fragments (Cys6–Cys47, Cys48–Cys52 (Type I), and Cys6–Cys48, Cys47–Cys52(Type II) of h-IGF-I) for the Identification of Disulfide Bond Linkage in Recombinantly Produced h-IGF-I

The primary structure of human IGF-I, except for the disulfide bond system, has been reported by Rinderknecht and Humbel. IGF-I afforded the corresponding characteristic peptide fragments on V8 protease digestion, which contained Cys6, Cys47, Cys48, and Cys52. Two possible fragments, Type I with Cys6–Cys47 and Cys48–Cys52 and Type II with Cys6–Cys48 and Cys47–Cys52 of h-IGF-I(4-9,47-53), were chemically synthesized. The disulfide bond system of IGF-I was unequivocally determined to be the Type-II form along with Cys18–Cys61. Interestingly, the Type-I system was included in the disulfide bond isomer produced as the main by-product in the refolding step on IGF-I synthesis by the recombinant DNA method.