Iron removal from monoferric human serum transferrins by 1,2-dimethyl-3-hydroxypyridin-4-one, 1-hydroxypyridin-2-one and acetohydroxamic acid

Abstract

The kinetics of iron removal from both forms of human serum monoferric transferrin by three ligands, 1,2-dimethyl-3-hydroxypyridin-4-one (L1), 1-hydroxypyridin-2-one and acetohydroxamic acid, have been evaluated at pH 7.4 and 25.0°C. In almost all cases the rate of iron removal follows simple saturation kinetics with respect to the ligand concentration. No spectroscopically distinct intermediates are observed during the iron removal reaction, which is consistent with a mechanism in which the rate-limiting step in iron removal is a protein conformational change. In the presence of chloride or perchlorate, most systems continue to follow simple saturation kinetics, but with significantly different k max values. Chloride accelerates iron release from both transferrin binding sites, while perchlorate accelerates iron release from the C-terminal site but retards iron release from the N-terminal site. When the hydrochloride salt of L1 is used to prepare the L1 stock solution, the allosteric effect of the chloride produces a continuing increase in the rate of iron removal with increasing ligand concentration, so that one no longer observes simple saturation kinetics. A least squares fit of k obs vs. the ligand concentration for L1·HCl shows that the allosteric effect of the chloride not only enhances the first-order term for iron removal but also doubles the apparent k max for the saturation term. This supports the view that allosteric binding of anionic ligands contributes to the observed variation in k max among different ligands. A detailed description of this allosteric effect is not yet possible because the effect varies significantly from system to system, depending upon the specific anion that is binding at the allosteric site, the ligand that is used to remove the iron, and the transferrin lobe from which iron is removed.