Kinetics of the release of aluminum from human serum dialuminum transferrin to citrate

The kinetics of release of Al3+ from human serum dialuminum transferrin (Al2Tf) to citrate were investigated at 37 degrees C, pH 7.4, mu = 0.7 M, by difference UV spectrophotometry. The two metal-binding sites are not identical but behave in a kinetically similar manner to give apparent second-order rate constants of 0.60 and 0.38 M-1 s-1, respectively, for release of the first Al3+ from Al2Tf. The rate constants for release of the second metal ion from the monoaluminum transferrins are 0.27 and 0.12 M-1 s-1. The kinetic scheme for release of A13+ from Al2Tf is therefore similar to that for release of Fe3+ from Fe2Tf, but the rate of constants for metal ion release are between two and four orders of magnitude larger.