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 Na(2)SO(4)), and sucrose on the thermal denaturation of diferric transferrin (Fe(2)Tf) and apotransferrin (apoTf) in the absence of chelating agents. At low [salt] (pH 7.3), Fe(2)Tf 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, T(m), for iron release (based on absorbance at 465 nm) and for unfolding of Fe(2)Tf and apoTf (based on ellipticity at 222 nm) decrease monoexponentially and plateau at approximately 0.35 M NaCl or approximately 0.15 M Na(2)SO(4), consistent with destabilization of Fe(2)Tf and apoTf by ionic screening of electrostatic interactions. Notably, the T(m) values for iron release and for unfolding of Fe(2)Tf vary with the rate of temperature increase, so these processes are kinetically controlled. Analysis of the time constant, tau(obs), for iron release from Fe(2)Tf (pH 5.7-7.3) as a function of [NaCl], [Na(2)SO(4)], or [sucrose] indicates that both ion-binding-induced conformation change and electrostatic screening of surface Coulombic interactions contribute to the acceleration of iron release from Fe(2)Tf.