KINETICS OF IRON REMOVAL FROM MONOFERRIC AND COBALT-LABELED MONOFERRIC TRANSFERRINS BY DIETHYLENETRIAMINEPENTA(METHYLENEPHOSPHONIC ACID) AND DIETHYLENETRIAMINEPENTAACETIC ACID

Rate constants for thc removal of ferric ion from transferrin by the octadentate chelating agents diethylenetriaminepenta(methylenephosphonic acid) (DTPP) and diethylenetriaminepentaacetic acid (DTPA) have been measured for both N- and C-terminal monoferric transferrin in 0.1 M N-(2-hydroxyethyl)piperazine-N'-2-ehtanesulfonic acid buffer at pH 7.4 and 25-degrees-C. Iron removal by the amino carboxylate DTPA appears to follow simple first-order kinetics. However, plots of the observed pseudo-first-order rate constant for iron removal versus the concentration of DTPP follow neither simple first-order nor saturation kinetics. The results on DTPP are interpreted in terms of two parallel pathways for iron removal, one which shows saturation kinetics and another which is first order in DTPP. The site preferences for the two ligands are also different. While DTPA preferentially removes iron from the N-terminal site, DTPP preferentially removes ferric ion from the thermodynamically more stable C-terminal binding site. Labeling the vacant binding sites of the monoferric transferrins with kinetically inert cobalt(III) has little effect on the rates of iron removal by DTPP. Equilibrium constants for the binding of the anionic DTPP ligand to the apoprotein have also been determined. The successive macroscopic binding constants are log K1 = 6.25 +/- 0.16 and log K2 = 4.68 +/- 0.17.