alpha 3-Galactosylated glycoproteins can bind to the hepaticasialoglycoprotein receptor

In mammals, clearance of desialylated serum glycoproteins to the liver is m ediated by a galactose-specific hepatic lectin, the 'asialoglycoprotein rec eptor'. In humans, serum glycoprotein glycans are usually capped with siali c acid, which protects these proteins against hepatic uptake. However, in m ost other species, an additional noncharged terminal element with the struc ture Gal alpha 1 --> 3Gal beta 1 --> 4R is present on glycoprotein glycans. To investigate if alpha 3-galactosylated glycoproteins, just like desialyl ated glycoproteins, could be cleared by the hepatic lectin, the affinities of alpha 3-galactosylated compounds towards this lectin were determined usi ng an in vitro inhibition assay, and were compared with those of the parent compounds terminating in Gal beta 1 --> 4R. Diantennary, triantennary and tetraantennary oligosaccharides that form part of N-glycans were alpha 3-ga lactosylated to completion by use of recombinant bovine alpha 3-galactosylt ransferase. Similarly, desialylated alpha(1)-acid glycoprotein (orosomucoid ) was alpha 3-galactosylated in vitro. The alpha 3-galactosylation of a bra nched, Gal beta 1 --> 4-terminated oligosaccharide lowered its affinity for the membrane-bound lectin on whole rat hepatocytes 50-250-fold, and for th e detergent-solubilized hepatic lectin 7-50-fold. In contrast, alpha 3-gala ctosylation of asialo-alpha(1)-acid glycoprotein caused only a minor decrea se in affinity, increasing the IC50 from 5 to 15 nm. Fully alpha 3-galactosylated alpha(1)-acid glycoprotein, intravenously inje cted into the mouse, was rapidly cleared from the circulation, with a clear ance rate close to that of asialo-alpha(1)-acid glycoprotein (t(1/2) of 0.4 2 min vs. 0.95 min). Its uptake was efficiently inhibited by pre-injection of an excess asialo-fetuin. Organ distribution analysis showed that the inj ected alpha(1)-acid glycoprotein accumulated predominantly in the liver. Ta ken together, these observations suggest that serum glycoproteins that are heavily alpha 3-galactosylated will be rapidly cleared from the bloodstream via the hepatic lectin. It is suggested that glycosyltransferase expressio n in murine hepatocytes is tightly regulated in order to prevent undesired uptake of hepatocyte-derived, circulating glycoproteins.