A Synthesis of Human Proinsulin C-Peptide

A peptide corresponding to the thirty-one amino acid sequence of human proinsulin C-peptide (positions 33–63 of proinsulin) was synthesized by the solid-phase method. The product was purified consecutively by gel filtration, DEAE-cellulose chromatography, and high-performance liquid chromatography (HPLC). The purified material behaved as a single component in reversed-phase HPLC, gave correct amino acid ratios, and was not distinguished from natural human C-peptide in terms of immunoreactivity and chromatographic behaviors. The α→β transpeptidation at the Asp-Leu sequence, possible to occur associated with the HF cleavage, was studied using model peptides to demonstrate that the formation of β-peptide was 3–4% regardless of whether the β-carboxylic acid is free or protected as a benzyl ester.