APOTRANSFERRIN PROTON DISSOCIATION AND INTERACTIONS WITH HYDROGENCARBONATE IN NEUTRAL MEDIA

A spectrofluorimetric titration analysis of the proton dissociation an d the interactions of human serum transferrin with hydrogencarbonate w as performed at pH 7.0-9.0, Apotransferrin loses a single proton proba bly per binding site with K-a = (6.80 +/- 0.35) x 10(-9) M. This proto n dissociation is independent of the hydrogencarbonate concentration, Apotransferrin does not interact with CO32-. However, it interacts wit h two HCO3-, exhibiting two different affinity constants; the dissocia tion constant presumably for the C-site is K-C = (4.40 +/- 0.15) x 10( -3) M and that for presumably the N-site is K-N = (3.60 +/- 0.30) x 10 (-2) M. These interactions are independent of pH and occur with the un protonated and protonated apotransferrin species with the same low aff inities, Such affinities are probably induced by ionic interactions in volving the side chain of the arginine residues in each of the two bin ding sites, As for the proton dissociation, it can occur with one of t he other sidechains of the amino acid residues of these binding sites.