title: The formation of Fe3+-transferrin-CO3(2-) via the binding and oxidation of Fe2+.
abstract: This paper examines the reaction pathway in which Fe2+ is bound by apotransferrin and subsequently oxidized by O2 to yield Fe3+-transferrin-CO3(2-). The time course of the reaction follows a curved first order function suggesting somewhat different reactivities of the two transferrin binding sites. The initial velocity of the oxidation reaction follows saturation kinetics with regard to apotransferrin, Fe2+, and NaHCO3. We suggest an equilibrium between these components and Fe2+-transferrin-CO3(2-). The initial velocity is a linear function of O2 concentration. This is consistent with the rate-limiting step of the overall reaction being the oxidation of the Fe2+-transferrin-CO3(2-). A second order rate constant of approximately 4 X 10(3) M-1 s-1 was estimated for the oxidation of Fe2+-transferrin-CO3(2-) by O2. Oxidation by H2O2 is about 30 times faster. The reaction velocity increases with increasing pH between pH 6.0 and 7.5 Fe3+-transferrin-anion complexes are formed by the binding and oxidation of Fe2+ iun the presence of O2 and synergistic anions. The anion is found to have a strong effect on the reaction rate and provides additional evidence for the proposed reaction route. The presence of chelating agents also strongly affects the rate of Fe3+-transferrin-CO3(2-) formation. EDTA and N-(2-hydroxyethyl)ethylenediaminetriacetic acid severely depress the rate, while other chelating reagents have a moderately inhibiting effect. Thioglycolate is found to enhance the reaction by a factor of 9. The formation of a quaternary complex consisting of thioglycolate-Fe2+-transferrin-CO3(2-) is suggested. The results are correlated with an earlier study on the reductive release of iron from transferrin (Kojima, N., and Bates, G. W. (1979) J. Biol. Chem. 254, 8847-8854).
mesh_terms: D002254:Carbonates / Q000378:metabolism*; D002614:Chelating Agents / Q000494:pharmacology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000378:metabolism*; D007700:Kinetics; D010084:Oxidation-Reduction; D010100:Oxygen; D011485:Protein Binding; D014168:Transferrin / Q000378:metabolism*
chemical_list: D002254:Carbonates; D002614:Chelating Agents; D014168:Transferrin; D007501:Iron; D010100:Oxygen