The binding site of rabbit hepatic lectin

The role of the liver in clearance of glycoproteins from the general circulation, with special reference to intestinal alkaline phosphatase

Glycoproteins represent a wide variety of macromolecules with important physiological functions. Characteristic variations in carbohydrate composition and plasma concentration of these proteins may occur during pathological conditions. Steady-state plasma concentrations are determined by release from normal or diseased tissues and simultaneous clearance from the general circulation. The liver occupies a central position in the production but also clearance and catabolism of such glycoproteins. A number of specialized receptor-mediated transport processes for different types of glycoproteins in this organ is reviewed. Membrane recognition is generally followed by absorptive endocytosis and vesicle transport to lysosomes, Golgi system and/or bile canaliculis. The charge of the protein, the nature of the terminal sugar residue or complex formation with other glycoproteins may determine the extent of uptake in the various cell types of the liver. By means of these transport processes the liver is able to remove potentially dangerous macromolecules such as denatured proteins, aggressive enzymes and immunocomplexes from the general circulation. Drugs can bind to some of these proteins or may interact with the hepatic transport or catabolism processes. Special attention is paid to the hepatic clearance of asialoglycoproteins with terminal galactose groups. Intestinal alkaline phosphatase is used as a model compound to characterize the pharmacokinetic profiles of hepatic uptake and biliary excretion in the rat in vivo and isolated perfused rat livers. Histochemical and electron-microscopic studies demonstrated a galactose-specific, receptor-mediated endocytotic process, mainly but not exclusively localized in centrolobular hepatocytes. Drug interactions with these processes will be the subject of further investigations.

A kinetic study of hepatic uptake of canine intestinal alkaline phosphatase in the rat

This study describes the "plasma" disappearance and biliary excretion of injected purified canine intestinal alkaline phosphatase in the isolated perfused rat liver and in the intact rat. The plasma disappearance curve of the enzyme showed, apart from an initial rapid phase, a secondary phase, which may result from "exocytosis" of interiorized canine intestinal alkaline phosphatase into the circulation or from exhaustion of receptors involved in uptake of the enzyme during the process of endocytosis. A small fraction of the injected enzyme was found to be excreted into the bile and it was shown that there is a "kinetic" relationship between the biliary excretion pattern and the "plasma" disappearance of the enzyme. In addition, the effect of various sugars, sugar polymers and glycoproteins on plasma disappearance and biliary excretion of the enzyme was studied. Intravenous administration of galactose, lactose, galactan and especially asialofetuin prior to injection of canine intestinal alkaline phosphatase, resulted in inhibition of plasma disappearance of the enzyme, while the biliary excretion pattern of the enzyme was also changed. It is concluded that the rapid removal of injected canine intestinal alkaline phosphatase is predominantly, or even exclusively, achieved by the liver and that the hepatic uptake of the enzyme is most likely mediated by the "galactose-specific lectin" present on the plasma membrane of the hepatocytes.

Galactose and N-acetylgalactosamine-specific endocytosis of glycopeptides by isolated rat hepatocytes

We have examined the kinetics of binding and uptake of iodinated glycoproteins and glycopeptides bearing terminal Gal or GalNAc moieties in an isolated rat hepatocyte system. Asparagine-linked, triantennary complex oligosaccharides with three terminal Gal residues are endocytosed with the same kinetics as asialo-orosomucoid, whereas biantennary, complex oligosaccharides with one or two terminal Gal residues are not endocytosed Glycopeptides bearing as few as four O-glycosidically-linked Gal beta 1, 3GalNAc or GalNAc moieties are also rapidly endocytosed, while glycopeptides with one or two more closely spaced moieties are not endocytosed. All the endocytosable glycoproteins and glycopeptides have similar apparent dissociation constants and a similar number of binding sites on the surface of the intact hepatocyte. The ligand-binding properties of the receptor in the plasma membrane of intact cells differ from those of the solubilized receptor, suggesting that interaction with other as yet undefined cellular components confers the ability to discriminate among closely related oligosaccharide structures. This is consistent with a model in which only glycopeptides bearing terminal Gal or GalNAc residues that fall within a restricted spatial relationship can induce a conformational alteration in the receptor which is required for uptake to occur. The endocytosis of a number of glycoproteins such as human asialo-ceruloplasmin can be accounted for by the presence of a single, complex oligosaccharide with the appropriate structure.

Investigation of the uptake of asialo-glycoproteins by isolated rat hepatocytes. Implications for its biological significance

The uptake of asialo-glycoproteins, asialo-fetuin and asialo-orosomucoid, was studied in isolated rat hepatocytes. The total binding capacity for either of these asialo-glycoproteins was 150 000 molecules per cell. The association constants were 0.98 x 10(8) M-1 (asialo-fetuin) and 4.8 x 10(8) M-1 (asialo-orosomucoid) at 10 degrees C. At 37 degrees C, the affinity of asialo-transferrin for the receptor was approximately 1/300 of that of asialo-fetuin. The uptake process was sensitive to neuraminidase, the effect being measurable at 0.5 mU/ml. At 10 mU/ml, the uptake was 15% of that in control cells. The uptake was at the same level when the cells were washed to remove neuraminidase before the addition of asialo-glycoprotein. Lowering the concentration of calcium ions below 2 mM reduced the affinity of the receptor, as addition of an excess of EGTA led to rapid release of asialo-glycoprotein from the cells. The fraction which was released was low when the cells had been incubated with asialo-glycoprotein at 37 degrees C before the addition of EGTA; nearly all of the cell-associated asialo-glycoprotein could be released if the cells had been incubated at 10 degrees C. At 10 degrees C as well as 37 degrees C, the uptake declined sharply when the concentration of calcium ions in the medium was lowered from 0.5 mM to 0.05 mM.