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Effect of Chaotropic Salts and Protein Denaturants on the Thermal Stability of Mouse Fibroblast Interferon

Department of Microbiology, The Medical College of Wisconsin, Milwaukee, Wisconsin 53233, U.S.A.

Altering the aqueous environment, especially with agents that affect hydrogen bonds, markedly affects the stability of mouse L cell interferon. Low pH stabilizes interferon whereas high pH labilizes it; heavy water further enhances interferon thermostability at pH 2 but not at pH 9. Exposure to the protein denaturants, 4 M-guanidine hydrochloride and 6 M-urea, significantly decreases the activity of interferon at pH 2 and pH 9; however, the residual interferon activity is relatively thermostable. Certain chaotropic salts protect interferon against thermal destruction, and in terms of effectiveness, their sequence is in the order SCN^- > I^- Cl^- = ClO₄^- = Br^- > NO₃^-. Interferon becomes more stable to heat as the NaSCN concentration is increased from 0.25 M to 2.0 M. Molecular sieve chromatography of interferon in the presence of 1.5 M-NaSCN at pH 7 shows a shift in its apparent mol. wt. from 25000 to 42000. Unlike most proteins, the unfolded conformation of interferon appears to be more stable to heat than the molecule with a smaller Stokes' radius.

Received 30 June 1976; accepted 3 November 1976.