THERMAL-INDUCED DENATURATION OF 2 MODEL PROTEINS - EFFECT OF POLOXAMER-407 ON SOLUTION STABILITY

In an attempt to enhance the physical stability of two model proteins in solution toward thermal-induced denaturation, we investigated the effect of the nonionic surfactant poloxamer 407 (Pluronic(R) F-127) with a nonrecombinant (urease) and recombinant-derived (interleukin 2) protein. Our aim was to determine the time-dependent loss in biological activity when urease and recombinant interleukin-2 (rIL-2) were dissolved in pH 7 phosphate buffer (PB) with or without poloxamer 407 (P-407) and stored at elevated temperatures for 96 h. In addition, urease was selected to determine whether denaturation due to the thermal stress was reversible. Resulting percent activity remaining vs time data for each protein were fitted to a mono- (75-degrees-C) or biexponential (37 and 50-degrees-C) equation and the area-under-the-curve (AUC \ 0 --> 96) calculated. A significantly (p < 0.05) greater fraction of the enzymatic activity of urease was irreversibly lost when urease was dissolved in PB only and incubated at 37 and 50-degrees-C compared to urease incubated in a PB solution which contained P-407. However, for urease/PB solutions incubated at 75-degrees-C, addition of P-407 resulted in an increased rate of enzyme inactivation. Results with rIL-2 suggest that P-407 bad no protective effect against thermal-induced denaturation at 50-degrees-C. Activity-time profiles described by biexponential equations may suggest an initial rapid unfolding phase followed by a slower unfolding/refolding phase for both proteins evaluated in either solvent system.