The exchange of Fe3+, Tb3+, In3+, Ga3+, and Al3+ between the C-terminal metal-binding site of the serum iron transport protein transferrin and the low-molecular-mass serum chelating agent citrate has been studied at pH 7.4 and 25 degreesC. The removal of Ga3+, In3+, and Al3+ follows simple saturation kinetics with respect to the citrate concentration. In contrast, removal of both Fe3+ and Tb3+ shows a combination of saturation and first-order kinetic behavior with respect to the citrate concentration. The saturation component is consistent with a mechanism for metal release in which access to the bound metal is controlled by a rate-limiting conformational change in the protein. The first-order kinetic pathway is very rapid for Tb3+, and this is attributed to a direct attack of the citrate on the Tb3+ ion within the closed protein conformation. It is suggested that this pathway is more readily available for Tb3+ because of the larger coordination number for this cation and the presence of an aquated coordination site in the Tb3+-CO3-Tf ternary complex. There is relatively little variation in the k(max) values for the saturation pathway for Tb3+, Ga3+, Al3+, and In3+, but the k(max) value for Fe3+ is significantly smaller. It is suggested that protein interactions across the interdomain cleft of transferrin largely control the release of the first group of metal ions, while the breaking of stronger metal-protein bonds slows the rate of iron release. The rates of metal binding to apotransferrin are clearly controlled in large part by the hydrolytic tendencies of the free metal ions. For the more amphoteric metal ions Al3+ and Ga3+, there is rapid protein binding, and the addition of citrate actually retards this reaction. In contrast, the nonamphoteric In3+ ion binds very slowly in the absence of citrate, presumably due to the rapid formation of polymeric In-hydroxo complexes upon addition of the unchelated metal ion to the pH 7.4 protein solution. The addition of citrate to the reaction accelerates the binding of In3+ to apoTf, presumably by forming soluble, mononuclear In-citrate complexes.