Rates of iron release from both sites of free transferrin at pH 7.4 are critically dependent upon ionic strength, because release appears to require binding of a simple nonchelating anion such as chloride to a kinetically active site of the protein. This site is distinct from the synergistic anion-binding site, occupancy of which is required for binding of iron to occur at all. Complexing of transferrin to its receptor also modulates release of iron, but in a more complex fashion. At extracellular pH, 7.4, receptor retards release, but at the pH of the endosome in which release occurs within the cell, 5.6, receptor accelerates release. The present study was undertaken to determine whether the kinetically active anion requirement is maintained at pH 5.6 and whether the effects of anion binding and receptor binding are independent of each other. A spectrofluorometric method was developed to monitor release of iron from C-terminal monoferric human transferrin and its complex with the transferrin receptor. At pH 5.6, as at pH 7.4, profiles of iron release to pyrophosphate from free and from receptor-complexed monoferric transferrin show curvilinear dependence on pyrophosphate concentration, consistent with a previously described kinetic scheme and suggestive of a similar release mechanism in all cases. Furthermore, at pH 5.6 release rates depend upon anion (chloride) concentration in free and in receptor-complexed transferrin as in free transferrin at pH 7.4, extrapolating nearly to zero as chloride concentration approaches zero. The enhancing effect of receptor on release is displayed at all concentrations of chloride tested, indicating that the release-promoting effects of receptor and chloride are independent of each other. Since release is thought to occur from a lobe of transferrin when the two domains enclosing the binding site of the lobe rotate about their hinging strands to an ''open'' conformation, one possibility is that in the C-terminal lobe the anion- or receptor-binding sites, or both, are located in the hinging strands.
