Redox and metal ion binding properties of human insulin-like growth factor 1 determined by electrospray ionization mass spectrometry

Insulin-like growth factor 1 (IGF-1) is a 70-residue hormone containing three intramolecular disulfide bridges. IGF-1 and other growth factors are oxidatively folded in the endoplasmic reticulum and act primarily in the blood, under relatively oxidative conditions. It is known that IGF-1 exists in various intracellular and extracellular compartments in the oxidized form; however, the reduction potential of IGF-1 and the ability of fully reduced IGF-1, which contains six cysteine residues, to bind transition metal ions are not known. In this work, we determine that the redox potential of human IGF-1 is equal to -332 mV and the reduced form of hIGF-1 can bind cooperatively four Cu(+) ions, most probably into a tetracopper-hexathiolate cluster. The Cu(+) binding affinity of hIGF-1 is, however, approximately 3 times lower than that for the copper chaperones; thus, we can conclude that fully reduced hIGF-1 cannot compete with known Cu(+)-binding proteins.