Structural changes involved in the folding of proinsulin.

The circular dichroism spectra of reduced proinsulin (precursor of proinsulin), reduced insulin (A plus B chain), and C-peptide, all of bovine origin, were recorded. The difference spectrum between reduced proinsulin and the sum of reduced insulin and C-peptide was constructed. The difference spectrum indicates the presence of structures which may be those necessary to keep the proinsulin precursor in position for correct formation of the three cystine bridges of proinsulin and insulin. The difference spectrum can be interpreted as being due to approximately 6 additional amino acids in theα-helix and 9 additional amino acids in theβ-structure in reduced proinsulin, all 15 being in random coil state in the separated chains.