The kinetics of iron release from human transferrin by EDTA. Effect of salts and detergents.

The kinetics of iron release from diferric human transferrin by EDTA at pH 7.4 and 37 degrees C has been studied. Equations have been derived which give the expected absorbance-time curves when the two sites are kinetically homogeneous or heterogeneous and these have been compared to the experimental results. In the absence of added salts or detergents,the two sites are kinetically similar but not identical with respect to iron release. However, the two sites are kinetically homogeneous in 0.02 M laurylpyridinium chloride and 2 M LiCl. In NaClO4 (greater than 0.50 M) there are two parallel pathways for iron release; 80% of the total iron being released heterogeneously and 20% apparently homogeneously. For the simple electrolyte ions studied, the rate of iron release from diferric-transferrin increases in accordance with the lyotropic series: SO4 2- less than HCO-2 less than Cl- less than ClO4- and Na+ less than Li+. Sodium dodecyl sulphate (0.02 M) and LiCl (2.0 M) increase the rate of ion release by 0.1 M EDTA by 32- and 27-fold, respectively. The results show that conformational states are attainable from which iron can be released either homogeneously or heterogeneously and from which iron release is facilitated. It is proposed that the structure of the transferrin binding site on the cell membrane and/or the presence of another molecule may promote these conformers in vivo.