Transferrin recycling in reticulocytes: pH and iron are important determinants of ligand binding and processing

Iron uptake by rat reticulocytes is blocked by 20 mM NH₄Cl, while ¹²⁵I-diferric transferrin (Tf) uptake is relatively unaffected. At pH 5.0 both apo- and diferric Tf bind with high affinity; at pH 7.4 diferric Tf binds avidly, but apoTf binds very poorly. The dissociation rate (4°C) of diferric Tf is extraordinarily slow at pH 5.0 (extrapolated $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 32 hrs}$) and faster at pH 7.4 ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 101 min}$). At pH 5.0 apoTf also dissociates slowly ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 205 min}$), but at pH 7.4 apoTf exhibits a much faster dissociation rate ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 62 min}$). 20 mM NH₄Cl slows the release of Tf from cells at 37°C, but the rate of externalization of ligand is unaffected. Ligand dissociation at 37° involves both externalization of receptor-ligand complexes and receptor-ligand separation; the NH₄Cl effect may result from an increased fraction of externalized Tf in the differric form which may dissociate more slowly. Receptor-mediated movement of Tf through acid intracellular compartments provides a mechanism to remove iron from Tf and for apoTf to remain receptor-bound for externalization to the cell surface and subsequent dissociation.