Dissociation equilibrium of human recombinant interferon gamma

The biologically active form of interferon gamma is a dimer composed of two noncovalently bound identical polypeptide chains of 17 kDa each. In this study, it was found that dissociation of the dimer into monomers significantly reduced the fluorescence quantum yield and the efficiency of the intermolecular Tyr to Trp radiationless energy transfer. The same process caused significant changes in the fluorescence decay and in the fluorescence anisotropy decay. The kinetic and thermodynamic parameters of the dimer-monomer equilibrium were determined by fluorescence measurements at different temperatures and by a theoretical mathematical model. Dissociation of the dimers into monomers was an endothermic process and was favored by concentrations of the protein lower than 1 mu M and by increasing the temperature. It was accompanied by formation of aggregates, a slow and partially reversible process leading to inactivation of the interferon. It is suggested that certain monomeric conformers are competent for aggregation.