Evidence for receptor-mediated binding of glycoproteins, glycoconjugates, and lysosomal glycosidases by alveolar macrophages.

TLDR

Alveolar macrophages bind glycoproteins and synthetic glycoconjugates that display mannose, N‑acetylglucosamine, or glucose at the exposed nonreducing end. An inhibition assay was developed using radioiodinated glucose‑albumin conjugate, agalacto‑orosomucoid, β‑glucuronidase, and RNase B as ligands to probe receptor‑mediated binding. Binding is selective for mannose or glucose termini, is blocked by excess yeast mannan, is temperature‑ and pH‑sensitive, is diminished by trypsin, and is inhibited by several glycoproteins (e.g., horseradish peroxidase, agalacto‑orosomucoid, β‑glucuronidase, ovalbumin, agalacto‑fetuin, RNase B), indicating a cell‑surface receptor on alveolar macrophages for these sugars.

Abstract

Alveolar macrophages have been shown to bind glycoproteins and synthetic glycoconjugates (neoglycorpoteins) that have mannose, N-acetylglucosamine, or glucose in the exposed, nonreducing position. Galactose-terminal glycoproteins are not bound. Binding of radiolabeled ligands to cells is nearly completely impaired by the presence of an excess of yeast mannan. Binding is temperature sensitive and proceeds optimally at pH 7.0. Prior treatment of the cells with trypsin severely decreases their capacity to bind ligands. An inhibition assay has been developed, using radioiodinated glucose-albumin conjugate, agalacto-orosomucoid, beta-glucuronidase, and RNase B as ligands. Various glycoproteins have been shown to be effective inhibitors of ligand binding including horseradish peroxidase, agalacto-orosomucoid, beta-glucuronidase, ovalbumin, agalacto-fetuin, and RNase B. RNase A and asialo-fetuin are ineffective as antagonists. The results suggest the presence of a cell surface receptor on alveolar macrophages that binds glycoproteins having terminal sugars with the mannose or glucose configuration.