HEPATOCYTE HORSERADISH-PEROXIDASE UPTAKE IS SATURABLE AND INHIBITED BY MANNOSE-TERMINAL GLYCOPROTEINS

In the liver, horseradish peroxidase (HRP) is thought to be taken up via mannose receptor-mediated endocytosis by non-parenchymal cells (NPC) and via fluid-phase endocytosis by hepatocytes. When we attempted to inhibit NPC uptake of HRP with mannan in the whole perfused rat liver, >80% of HRP uptake was eliminated. Liver cell fractionation revealed that mannan not only inhibited HRP uptake by NPC (91%) but also by hepatocytes (81%). In isolated hepatocytes, HRP uptake was linear over 60 min and saturable in the range of 0 to 200 mg/l (V(max) = 4.3 ng.mg protein-1.min-1; K(m) = 8.3 mg/1). Mannan inhibited uptake competitively (K(i) = 2.0-2.5 mg/1). At high concentrations of HRP, a nonsaturable component of HRP uptake became evident (k = 2.8 pg.mg protein-1.min-1.mg HRP-1.l-1). Hepatocyte uptake of HRP was inhibited by other glycoproteins and glycopeptides with mannose-terminal groups, as well as by mannan, but not by asialofetuin (ASF) or bovine serum albumin. Hepatocyte uptake of I-125-labeled ASF, which is taken up via the asialoglycoprotein receptor, was saturable and not inhibited by mannan. HRP binding to hepatocytes, determined at 4-degrees-C, was also inhibited by mannan. Quantification of contamination of the parenchymal cell fraction by NPC by cell counting and by pronase digestibility suggested our results could not be explained by contamination of hepatocytes by NPC. At concentrations used for most morphological studies (1,000-10,000 mg/l), fluid-phase endocytosis accounts for much of HRP uptake. However, at low concentrations, a saturable low-capacity mechanism is responsible for most HRP uptake by the hepatocyte. Inhibition of uptake by mannan and mannose-terminal glycoproteins suggests that mannose is a determinant of uptake.