Mechanism of formation of the complex between transferrin and bismuth, and interaction with transferrin receptor 1

The kinetics and thermodynamics of Bi(III) exchange between bismuth mononitrilotriacetate (BiL) and human serum transferrin as well as those of the interaction between bismuth-loaded transferrin and transferrin receptor 1 (TFR) were investigated at pH 7.4-8.9. Bismuth is rapidly exchanged between BiL and the C-site of human serum apotransferrin in interaction with bicarbonate to yield an intermediate complex with an effective equilibrium constant K(1) of 6 +/- 4, a direct second-order rate constant k(1) of (2.45 +/- 0.20) x 10(5) M(-1) s(-1), and a reverse second-order rate constant k(-1) of (1.5 +/- 0.5) x 10(6) M(-1) s(-1). The intermediate complex loses a single proton with a proton dissociation constant K(1a) of 2.4 +/- 1 nM to yield a first kinetic product. This product then undergoes a modification in its conformation followed by two proton losses with a first-order rate constant k(2) = 25 +/- 1.5 s(-1) to produce a second kinetic intermediate, which in turn undergoes a last modification in the conformation to yield the bismuth-saturated transferrin in its final state. This last process rate-controls Bi(III) uptake by the N-site of the protein and is independent of the experimental parameters with a constant reciprocal relaxation time tau(3)(-1) of (3 +/- 1) x 10(-2) s(-1). The mechanism of bismuth uptake differs from that of iron and probably does not involve the same transition in conformation from open to closed upon iron uptake. The interaction of bismuth-loaded transferrin with TFR occurs in a single very fast kinetic step with a dissociation constant K(d) of 4 +/- 0.4 microM, a second-order rate constant k(d) of (2.2 +/- 1.5) x 10(8) M(-1) s(-1), and a first-order rate constant k(-d) of 900 +/- 400 s(-1). This mechanism is different from that observed with the ferric holotransferrin and implies that the interaction between TFR and bismuth-loaded transferrin probably takes place on the helical domain of the receptor which is specific for the C-site of transferrin and HFE. The relevance of bismuth incorporation by the transferrin receptor-mediated iron acquisition pathway is discussed.