The galactose-recognizing system of rat peritoneal macrophages. Receptor-mediated binding and uptake of glycoproteins

Binding and phagocytosis of sialidase-treated cells by peritoneal macrophages is mediated by a galactose-specific receptor. So far, only cells or particles exposing terminal galactose residues were demonstrated to be ligands. We present results obtained with a newly developed radio-receptor assay, which proves both binding and uptake of glycoproteins mediated by the galactose-recognizing receptor of peritoneal macrophages. Requirement of Ca2+ for binding is used to distinguish between reversibly surface-bound and irreversibly internalized ligands. By using this approach, the uptake of the ligand is followed and its inhibition with phenylglyoxal and N-ethylmaleimide is demonstrated. Evidence was also obtained that internalization is followed by degradation of the ligand. Studies on the specificity show that only galactose is recognized but that the binding strength depends on the arrangement of galactose residues presented by the ligand.