Atypical effects of salts on the stability and iron release kinetics of human transferrin.

The contributions of electrostatic interactions to the thermal stability of human transferrin have been evaluated by analysis of the effects of pH, salts (NaCl and Na2SO4), and sucrose on the thermal denaturation of diferric transferrin (Fe2Tf) and apotransferrin (apoTf) in the absence of chelating agents. At low [salt] (pH 7.3), Fe2Tf and apoTf are highly thermostable, but decreasing pH or increasing [salt] substantially decreases the thermal stability of both. As [salt] is increased, the midpoint transition temperatures, Tm, for iron release (based on absorbance at 465 nm) and for unfolding of Fe2Tf and apoTf (based on ellipticity at 222 nm) decrease monoexponentially and plateau at ∼0.35 M NaCl or ∼0.15 M Na2SO4, consistent with destabilization of Fe2Tf and apoTf by ionic screening of electrostatic interactions. Notably, the Tm values for iron release and for unfolding of Fe2Tf vary with the rate of temperature increase, so these processes are kinetically controlled. Analysis of the time constant, τobs...