Accumulation of a nondegradable mannose ligand within rabbit alveolar macrophages. Receptor reutilization is independent of ligand degradation.

Synthetic neoglycoproteins were made by reacting 5-( azidocarbonyl )pentyl 1-thio-alpha-D-mannopyranoside with poly(L-lysine) and poly(D-lysine). The 125I- Man90 -poly-D-Lys and 125I- Man104 -poly-L-Lys were tightly bound at 2 degrees C by the mannose receptor of the rabbit lung macrophage (Kd = 0.66 +/- 0.18 and 0.59 +/- 0.26 nM, respectively). Under saturating conditions in the cold, the macrophage bound 98 200 +/- 7000 and 84 200 +/- 10 500 ligand molecules per cell for the D- and L-polylysine derivatives, respectively. The cell-surface-bound ligands were dissociable by ethylenediaminetetraacetic acid and mannose at 2 degrees C. At 37 degrees C, the macrophages internalized both 125I- Man90 -poly-D-Lys and 125I- Man104 -poly-L-Lys efficiently. Although the internalized 125I- Man104 -poly-L-Lys was degraded quickly by the macrophage to small radiolabeled peptide, the internalized 125-I- Man90 -H-poly-D-Lys apparently could not be degraded or exocytosed . The amount of 125I- Man90 -poly-D-Lys which accumulated within the cell was 7-fold higher than the combined amount of surface and intracellular mannose receptors, strongly indicating reutilization of the receptors independent of degradation of the ligand.