Disulfide arrangement of human insulin-like growth factor I derived from yeast and plasma

The disulfide arrangement of yeast derived human insulin-like growth factor I (yIGF-I) was determined using a combination of Staphylococcus aureus V8 protease mapping, fast-atom-bombardment mass spectrometry as well as amino acid sequence and composition analysis. Three disulfide bridges were found between the following cysteine residues: Cys6-Cys48, Cys47-Cys52 and Cys18-Cys61. IGF-I isolated from human plasma (pIGF-I) was found to have an identical disulfide configuration. A yeast-derived isomeric form of IGF-I (yisoIGF-I) exhibited an altered disulfide arrangement: Cys6-Cys47, Cys48-Cys52 and Cys18-Cys61. Radioreceptor analysis of pIGF-I and yIGF-I showed high specific activity, 20,000 U/mg. However, yisoIGF-I demonstrated a severely reduced ability to bind to the IGF-I receptor (19%) and was less potent in provoking a mitogenic response in Balb/C 3T3 fibroblasts (50% at doses 10-100 ng/ml). The data demonstrate the importance of correct disulfide arrangement in IGF-I for full biological activity.