Binding of trivalent chromium to serum transferrin is sufficiently rapid to be physiologically relevant

•In 25 mM (bi)carbonate, chromic ions bind rapidly and tightly to transferrin.•Binding constants for chromic ions to conalbumin have been confirmed.•EPR spectra of Cr3 +-conalbumin are similar to those of Cr3 +-transferrin.•Conformational changes accompany binding of Cr3 + to transferrin.

Transferrin, the major iron transport protein in the blood, also transports trivalent chromium in vivo. Recent in vitro studies have, however, suggested that the binding of chromic ions to apotransferrin is too slow to be biologically relevant. Nevertheless, the in vitro studies have generally failed to adequately take physiological bicarbonate concentrations into account. In aqueous buffer (with ambient (bi)carbonate concentrations), the binding of chromium to transferrin is too slow to be physiologically relevant, taking days to reach equilibrium with the protein's associated conformational changes. However, in the presence of 25 mM (bi)carbonate, the concentration in human blood, chromic ions bind rapidly and tightly to transferrin. Details of the kinetics of chromium binding to human serum transferrin and conalbumin (egg white transferrin) in the presence of bicarbonate and other major potential chromium ligands are described and are consistent with transferrin being the major chromic ion transporter from the blood to tissues.

Graphical abstractKinetics of Cr binding to conalbumin as function of [HCO3−].Details of the kinetics of chromium binding to human serum transferrin and conalbumin (egg white transferrin) in the presence of bicarbonate and other major potential chromium ligands are described and are consistent with transferrin being the major chromic ion transporter from the blood to tissues.Figure optionsDownload full-size imageDownload as PowerPoint slide