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Biomedical subjects

G Ashwell

Publications and source records attributed to G Ashwell.

At least 55 records · Page 3Linked to original sources
PubMed1979

Subcellular membrane topology and turnover of a rat hepatic binding protein specific for asialoglycoproteins.

Examination of the topological distribution of the rat liver binding protein specific for desialylated serum glycoproteins has revealed a subcellular distribution wherein this receptor is located largely, if not entirely, on the luminal membrane surface of the Golgi complex and smooth microsomes. In contrast, the receptor protein has been identified on the external, or cytosolic, surface of the lysosomal membranes. Recovery of the purified binding protein from whole rat livers established an average half-life survival time of approximately 88 h. Subsequently, injection in vivo of the readily catabolized ligand, asialo-orosomucoid, was found to be without effect upon the turnover rate of the receptor purified from isolated plasma membranes. These observations are interpreted as suggesting the existence of a recycling mechanism wherein the receptor is spared destruction in the lysosomes and remains available for subsequent reinsertion into the hepatocyte plasma membrane as part of a continuing process for the clearance of galactose-terminated ligands from the circulation.

Animals↗
PubMed1979

An electron microscope autoradiographic study of the carbohydrate recognition systems in rat liver. I. Distribution of 125I-ligands among the liver cell types.

Electron microscope autoradiography was used to study the cellular localization of seven glycoproteins rapidly cleared from the circulating plasma of rats and taken up by the liver. 1 and 15 min after intravenous administration of the 125I-glycoproteins, livers were fixed in situ by perfusion and processed for autoradiography. Autoradiographic grains in the developed sections were found to represent the intact 125I-ligand. A quantitative analysis of the distribution and concentration (density) of autoradiographic grains over the three major cell types of the liver was then performed. Three molecules, asialo-fetuin, asialo-orosomucoid, and lactosaminated RNase A dimer, the oligosaccharide chains of which terminate in galactose residues, were bound and internalized almost exclusively (greater than 90%) by hepatocytes. Conversely, four molecules, the oligosaccharide chains of which terminate in either N-acetyl-glucosamine (agalacto-orosomucoid) or mannose (ahexosamino-orosomucoid, preputial beta-glucuronidase, and mannobiosaminated RNase A dimer), were specifically bound and internalized by cells lining the blood sinusoids--that is, by Kupffer cells and endothelial cells. Endothelial cells were two to six times more active (on a cell volume basis) than were Kupffer cells in the internalization of these four 125I-ligands. Mannose and N-acetylglucosamine-terminated glycoproteins competed with each other for uptake into either endothelial cells or Kupffer cells, indicating that a single system recognized mannose or N-acetyl-glucosamine residues. Finally, agalacto-orosomucoid and ahexosamino-orosomucoid were also associated with hepatocytes, but competition experiments utilizing excess asialo-orosomucoid demonstrated that residual galactosyl residues were responsible for this association.

Animals↗
PubMed1977

Isolation and characterization of an avian hepatic binding protein specific for N-acetylglucosamine-terminated glycoproteins.

An hepatic receptor which recognizes and binds specifically to serum glycoproteins bearing terminal, nonreducing N-acetylglucosamine residues has been purified to homogeneity by affinity chromatography from chicken liver. The isolated binding protein has been characterized as a water-soluble glycoprotein in which sialic acid, galactose, mannose, and glucosamine comprise 8% of the total molecule. The binding reaction is a saturable process and is proportional to receptor concentration. Evidence has been adduced to indicate the presence of a single high affinity binding site with a dissociation constant of 1.4 x 10(-9) M. A single subunit has been identified by polyacrylamide gel electrophoresis in sodium dodecyl sulfate with an estimated molecular weight of 26,000. The chemical and physical properties of the avian protein have been evaluated with respect to the analogous hepatic protein, of mammalian origin, which exhibits a binding specificity for galactose-terminal serum glycoproteins.

Acetylglucosamine↗
PubMed1977

Lymphocyte mitogenesis induced by a mammalian liver protein that specifically binds desialylated glycoproteins.

A purified rabbit liver membrane protein that binds desialylated glycoproteins has been shown to be a mitogen for human peripheral lymphocytes. The mitogenic activity is specific for desialylated thymus-derived (T)-cells. The loss of mitogenicity upon exposure of the binding protein to neuraminidase and the inhibitory potency of asialo-orosomucoid support the conclusion that the site involved in the binding of asialoglycoproteins is also responsible for the mitogenic effect on desialylated lymphocytes. The potential relevance of this phenomenon to the etiology of hepatocellular necrosis is considered.

Animals↗
PubMed1976

Subcellular distribution of a mammalian hepatic binding protein specific for asialoglycoproteins.

Evidence is presented to support the identification of a unique membrane binding protein in a variety of subcellular organelles including the Golgi complex, the lysosomes, the smooth microsomes, and the external plasma membranes prepared from rat liver homogenates. The binding protein, specific for desialylated serum glycoproteins, was isolated from each of the above components and purified by affinity chromatography on a column of Sepharose 4B to which asialoorosomucoid had been covalently linked. In each case, the final preparation exhibited an apparently complete identity of binding properties as well as similar subunit structure and immunological specificity. In contrast to the binding role previously ascribed to the plasma membranes as a prelude to transport and catabolism, the function of this activity in intracellular processes is currently unknown.

Animals↗
PubMed1976

Carbohydrate structure of glycopeptides isolated from an hepatic membrane-binding protein specific for asialoglycoproteins.

Two variant glycopeptide constituents of an hepatic membrane protein responsible for the clearance of circulating asialoglycoproteins have been isolated and characterized. Their relative abundance has been estimated to be 14 and 2 mol, respectively, per mol of intact protein. The larger component, glycopeptide I, is comprised of sialic acid, galactose, mannose, glucosamine, and aspartic acid in a molar ratio of 3:3:2:5:1. The smaller neutral fraction contains only two sugars and the molar relationship of mannose, glucosamine, and aspartic acid is 8:2:1. Based on the results of sequential digestion with specific glycosidases and periodate degradation, a carbohydrate sequence for both glycopeptides is proposed.

Amino Acids↗
PubMed1976

Chemical and physical properties of an hepatic membrane protein that specifically binds asialoglycoproteins.

The state of aggregation found in water-soluble preparations of an hepatic membrane protein responsible for the clearance of serum asialoglycoproteins has been shown to result from the self-associating properties of a single oligomeric protein. The smallest functional unit identifiable in aqueous solution possessed an estimated molecular weight of 500,000 with each of the successive components increasing in size by an equal amount to form an oligomeric series bearing an integral ratio of 1:2:3:4:5. The tendency towards self-association was promptly and completely reversed by the addition of Triton X-100 with the concomitant appearance of a single component. Extensive treatment with sodium dodecyl sulfate permitted the identification and isolation of two subunits with estimated molecular weights of 48,000 and 40,000, respectively. Amino acid and carbohydrate analyses revealed both subunits to be glycoproteins with a closely similar, but not identical, composition.

Amino Acids↗
PubMed1976

A hepatic receptor of avian origin capable of binding specifically modified glycoproteins.

Evidence is reported to support the following conclusions: (i) The presence of a specific hepatic receptor capable of recognizing and binding galactose-terminated glycoproteins is responsible for maintaining near zero levels of circulating asialoglycoproteins in mammalian serum. (ii) The absence of this galactose-specific binding protein in avian, and presumably reptilian, liver is correlated with high levels of circulating asialoglycoproteins in the serum of these species. (iii) The evolutionary significance of these observations is reinforced by the recognition of an avian hepatic binding protein specific for terminal, N-acetyl-glucosamine residues on glycoproteins.

Acetylglucosamine↗