Interaction of transferrin with rat alveolar macrophages

Abstract

Binding of rat transferrin to isolated alveolar macrophages was investigated in the 0.125 nM to 2 μM range. Computer analysis of the data revealed two classes of binding sites, a small number (<1000 exposed/cell) having high affinity (dissociation constant (Kd), 3.4 nM) and a large number (approximately 4 × 106/cell) having low affinity (Kd48 μM). Measurements with a monoclonal antibody to the rat transferrin (rTf) receptor yielded values in the same range as the high-affinity sites derived from studies of ligand binding. Binding to the low-affinity sites at pH 5.8 was nearly one order of magnitude stronger than that at pH 7.3. Bovine lactoferrin (12 μM), cationized bovine serum albumin (14 μM), L-arginine (50 mM), and L-lysine (50 mM) did not compete against rTf binding to the low-affinity sites. Removal of an average of 2.6 × 108sialyl residues from each cell did not affect binding. Heparan sulphate proteoglycan purified from alveolar macrophages bound strongly to immobilized rTf, thus raising the possibility that the low-affinity interaction of transferrin with these cells may be mediated, at least in part, by this glycosaminoglycan.Key words: heparan sulphate proteoglycan, macrophage, transferrin, transferrin receptor.