Binding and endocytosis of cluster glycosides by rabbit hepatocytes. Evidence for a short-circuit pathway that does not lead to degradation.

Abstract

Synthetic cluster glycosides containing either one, two, or three galactosyl or lactosyl residues per ligand were used to test the effect of carbohydrate clustering on binding by the rabbit hepatic Gal/GalNAc-binding lectin using either isolated rabbit hepatocytes or the solubilized, affinity-purified lectin. The tris- and bis-glycosides were superior to the mono-glycosides for inhibition of 125I-asialoorosomucoid binding to rabbit hepatocytes at 0 degrees C. The concentrations of the tris-glycosides required for 50% inhibition of 125I-asialoorosomucoid binding (4-8 microM) to hepatocytes were 50-100 times lower than the concentrations of the corresponding mono-glycosides required for 50% inhibition (400-500 microM). The isolated lectin, however, did not effectively discriminate between the mono-, bis-, and tris-glycosides, possibly indicating an organizational difference between the lectin in the cell membrane and the isolated lectin. When the cluster glycosides were labeled with 125I-tyrosine, it was shown that the tris- and bis-galactosides were bound to the hepatocytes at 37 degrees C, and that binding was followed by a step that led to ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid resistance, probably internalization. The process could be specifically inhibited by the neoglycoprotein Gal44-AI-bovine serum albumin, or by IgG specific for the hepatic lectin, but not by preimmune IgG. Internalization of the cluster glycosides did not lead to accumulation of ligand inside the cell, nor to degradation. Instead, the ligands were quickly released from the cells.