TRANSFERRIN - INTERACTIONS OF LACTOFERRIN WITH HYDROGEN CARBONATE

The interaction of apolactoferrin with hydrogen carbonate (bicarbonate ) has been investigated in the pH range 6.5-9.2. In the absence of bic arbonate apolactoferrin loses a single proton with pK(1a) of 8.10. Thi s proton loss is independent of the interaction with the synergistic a nion. The C-site of apolactoferrin interacts with bicarbonate with a v ery low affinity (K-C(-1) = 3.2 M-1). This process is accompanied by a proton loss, which is probably provided by the bicarbonate in interac tion with the protein. This proton loss can possibly be the result of a shift in the proton dissociation constant, pK(a), of the bicarbonate /carbonate acid/base equilibrium, which would decrease from pK(a) 10.3 5 to pK(2a) 6.90 in the bicarbonate-lactoferrin adduct. The N-site of the protein interacts with bicarbonate with an extremely low affinity, which excludes the presence of the N-site-synergistic anion adduct in neutral physiological media. Contrary to serum transferrin, the conce ntration of the apolactoferrin in interaction with bicarbonate is pH d ependent. Between pH 7.4 and pH 9 with [HCO3-] about 20 mM, the concen tration of the serum transferrin-bicarbonate adduct is always about 30 %, whereas that of the apolactoferrin-synergistic anion adduct varies from 25% at pH 7.5 to 90% at pH 9. This implies that, despite an affin ity for bicarbonate two orders of magnitude lower than that of serum t ransferrin, lactoferrin interacts better with the synergistic anion. T his can be explained by the possible interaction of lactoferrin with c arbonate in neutral media, whereas transferrin only interacts with bic arbonate.