The ligand binding specificity and tissue localization of a rat alveolar macrophage lectin

The parameters of the reaction between a rat alveolar macrophage lectin (Mr = 180,000) and its ligands have been examined. The reaction is dependent on Ca2+ over the optimal pH range for binding. The apparent dissociation constant for fucosyl bovine serum albumin, the standard ligand used in these studies, is 1.4 X 10(-10) M. The ligand binding specificity was determined by measurement of the inhibition of binding of fucosyl bovine serum albumin by various glycoproteins and saccharides. D-Mannose, L-fucose, and N-acetyl-D-glucosamine were the most effective inhibitors, and D-galactose was much poorer. The equatorial hydroxyl groups on the C-3 and C-4 of the mannose ring are important in the lectin-ligand interaction, and the axial hydroxyl group on the C-2 contributes to a lesser extent. Immunocytological studies revealed that the lectin isolated from alveolar macrophages is widely distributed in other rat tissues. Hepatocytes are devoid of the lectin, but hepatic Kupffer cells and endothelial cells contain significant amounts. This was confirmed by isolation of the lectin from liver. Spleen and skeletal muscle also contain lectin, but much smaller amounts were found in brain, kidney, and heart muscle.