Indirect detection of protein-metal binding: interaction of serum transferrin with In3+ and Bi3+

Transferrins comprise a class of monomeric glycoproteins found in all vertebrates, whose function is iron sequestration and transport. In addition to iron, serum transferrin also binds a variety of other metals and is believed to provide a route for the in vivo delivery of such metals to cells. In the present study, ESI MS is used to investigate interactions between human serum transferrin and two nonferrous metals, indium (a commonly used imaging agent) and bismuth (a component of many antiulcer drugs). While the UV-Vis absorption spectroscopy measurements clearly indicate that both metals bind strongly to transferrin in solution, the metal-protein complex can be detected by ESI MS only for indium, but not for bismuth. Despite the apparently low stability of the transferrin-bismuth complex in the gas phase, presence of such complex in solution can be established by ESI MS indirectly. This is done by monitoring the evolution of charge state distributions of transferrin ions upon acid-induced protein unfolding in the presence and in the absence of the metal in solution. The anomalous instability of the transferrin-bismuth complex in the gas phase is rationalized in terms of conformational differences between this form of transferrin and the holo-forms of this protein produced by binding of metals with smaller ionic radii (e.g., Fe3+ and In3+). The large size of Bi3+ ion is likely to prevent formation of a closed conformation (canonical structure of the holo-protein), resulting in a non-native metal coordination. It is suggested that transferrin retains the open conformation (characteristic of the apo-form) upon binding Bi3+, with only two ligands in the metal coordination sphere provided by the protein itself. This suggestion is corroborated by the results of circular dichroism measurements in the near-UV range. Since the cellular consumption of metals in the transferrin cycle critically depends upon recognition of the holo-protein complex by the transferrin receptor, the noncanonical conformation of the transferrin-bismuth complex may explain very inefficient delivery of bismuth to cells even when a high dosage of bismuth-containing drugs is administered for prolonged periods of time.