Circular dichroism studies of relaxin and insulin peptide chains

The circular dichroism (CD) spectra of pig relaxin and pig insulin are similar, reflecting the known structural similarities between the two hormones. However, the conformational characteristics of the separate chains of insulin and relaxin show significant differences. The S-sulfo forms of insulin A and B chains and S-sulfo relaxin A chain have CD spectra consistent with largely unordered structures whereas the S-sulfo form of relaxin B-chain has at least 90%β-structure. This β-structure may explain the unusual solubility and adsorptive properties of the relaxin B-chain and the poor combination yields with A-chain. The relaxin B-chain changes to a largely unordered conformation if the peptide is shortened at the carboxyl terminus by six amino acid residues. This conformational change has important implications in planning relaxin synthesis strategy. Significant interactions and conformational changes are observed between the oxidized forms of the A and B chains of both relaxin and insulin. In using CD to monitor chain recombination of native relaxin peptides it was found that the spectra obtained differ depending on whether the reduced chains are separated or not separated from the reaction mixture prior to reoxidation. Although the spectra differ the biological activity was 20% in both cases. The remainder of the reoxidized but inactive material contains β-structures which make a greater contribution to the CD spectrum when the chains have been separated and processed than when they are reoxidized in situ.