Factors affecting the dissociation and aggregation of human interferon gamma

The biologically active form of interferon gamma (IFN-gamma) is a dimer con sisting of two identical non-covalently bound polypeptide chains. We have s tudied spectroscopically the dimer-monomer dissociation equilibrium of huma n recombinant IFN-gamma and have found that the monomers possess approximat ely 50% lower Trp quantum yield than the dimers [Boteva et al. Biochemistry 1996;35:14825]. In the present study we characterise the conformational pr operties of the two states - monomeric and dimeric, and analyse the effects of the salt composition of human blood plasma, physiological cations K+, N a+, Ca2+ and Mg2+ and mechanical stress on the dimer-monomer equilibrium. A medium with electrolyte composition of human blood plasma increases both t he association and dissociation rate constants without shifting significant ly the dimer-monomer equilibrium. The physiological cations shift the equil ibrium towards dissociation of dimers into monomers by lowering the activat ion energy and the free energy of the process thus decreasing the stability of IFN-gamma. Mechanical stress caused by stirring of the protein solution reduces irreversibly the Trp fluorescence by 75-80% and decreases signific antly the alpha-helical content and favours the aggregation (C) 1999 Elsevi er Science B.V. All rights reserved.