Effect of transferrin on the degradation of glycoproteins bearing a hybrid or high-mannose glycan by alveolar macrophages.

A subfraction of hen ovalbumin and a special form of rat transferrin, possessing a hybrid glycan, were studied with respect to their binding to, and degradation by macrophages isolated from the rat lung. Both ligands were found to be degraded partly through the mannose receptor pathway and partly by another mechanism, presumably adsorptive pinocytosis. Catabolism of both proteins was markedly increased by adding standard (i.e., normally glycosylated) transferrin from various species to the medium. This increase was not diminished, or even augmented, when Ca2+ was depleted in the medium, thus implying involvement of the pinocytic pathway rather than the mannose receptor. Binding to the cell surface of both ligands was altered in the presence of transferrin. A hypothesis is advanced to suggest that when transferrin is bound to a component of macrophage plasmalemma, thought to be the glycosaminoglycan of heparan sulfate, its conformation may change in such a way that it attracts other proteins. Protein molecules temporarily captured by adsorbed transferrin would then be taken up by a "piggyback" process and degraded.