Tissue locations for the turnover of radioactively labeled rat orosomucoid in vivo

Sensitive chemiluminescence immunoassay for the determination of rat serum alpha1-acid glycoprotein

We present the establishment of a sensitive immunoassay for the determination of alpha1-acid glycoprotein (orosomucoid, AGP) in rat serum. The assay is based upon antigen capture by surface-immobilized specific polyclonal rabbit anti-AGP antibodies with biotinylated rat AGP (rAGP) as the tracer, and formatted as competitive enzyme immunoassay. Signaling is performed by streptavidin-conjugated horseradish peroxidase. Enzyme activity is quantified by an enhanced chemiluminometric method, allowing the sensitive detection of rAGP serum levels in small sample volumes.

Clearance of circulating gamma-glutamyltransferase by the asialoglycoprotein receptor. Enzyme forms with different sialic acid content are eliminated at different clearance rates and without apparent desialylation

gamma-Glutamyltransferase is eliminated from the circulation via the asialoglycoprotein receptor in liver. After purifying the enzyme from human liver, a subfractionation into differently sialylated forms was obtained using MonoQ ion exchange chromatography. The uptake of such forms from rat circulation was studied and the slowest rate was measured for the most sialylated form. To test if the uptake of the sialylated enzymes was dependent on prior desialylation in the circulation the enzyme was recovered from liver after uptake and from serum after inhibiting the uptake with asialofetuin. Analysis of these recovered forms showed no apparent alteration in charge. The enzyme is apparently eliminated without prior desialylation through available galactose units which bind with low affinity to the receptor.

The major subunit of the asialoglycoprotein receptor is expressed on the hepatocellular surface in mice lacking the minor receptor subunit

The mammalian asialoglycoprotein receptor (ASGPR) is located on the sinusoidal membrane of hepatocytes where it binds and endocytoses galactose-terminated glycoproteins (asialoglycoproteins). ASGPR is composed of two highly homologous subunits, termed hepatic lectin 1 and 2. Despite numerous studies the contribution of both subunits to biosynthesis and functional activity of ASGPR in vivo has remained controversial. Mice lacking the murine hepatic lectin (MHL)-2 subunit are viable and fertile without obvious phenotypic abnormalities. In the absence of MHL-2, knockout mice express MHL-1 protein at reduced levels. Here, we examine the intracellular fate and function of this remaining subunit. The results show that MHL-1 reaches the hepatocellular surface in knockout mice but is unable to effectively remove any one of three different radiolabeled ligands within 30 min. A small but detectable residual ligand clearance in knockout mice at 4 h is apparently not mediated by remaining MHL-1. Serum concentrations of galactose-terminating glycoproteins are not elevated in these ASGPR-deficient mice. However, competitive in vitro degradation experiments suggest that other endogenous ASGPR ligands, the nature of which remain to be determined, accumulate in serum of knockout animals.

Biodistribution of neoglycoproteins in mice bearing solid Ehrlich tumor

Synthetic neoglycoproteins were employed for the specific detection of their corresponding cellular sugar receptors, such as endogenous lectins, by specific protein-carbohydrate interaction. A panel of 16 radioiodinated probes with defined carbohydrate content, attached to the carrier protein bovine serum albumin, disclosed marked differences in the expression of corresponding sugar receptors on Ehrlich ascites tumor cells in vitro as an exemplary tumor model. To quantify tumor uptake in the more complex in vivo situation and to assess binding to individual organs attributable to the various types of carbohydrates we determined the biodistribution of the radiolabelled neoglycoproteins 48 h after injection into tumor-bearing mice. The individual pattern of retention of radioactivity demonstrated distinct properties of the different organs that need to be accounted for in drug-targeting and tumor-imaging studies, based on protein-carbohydrate interactions. Combined tumor-to-blood and tumor-to-muscle ratios of neoglycoprotein accumulation were well above 1 after covalent attachment of xylose or glucuronic acid, respectively, to the carrier protein. These data constitute the basis for further refinements of the carbohydrate part of suitable neoglycoproteins to allow a potentially rational application of neoglycoproteins as drug-targeting vehicles and tumor-imaging radiopharmaceuticals.

Glycoconjugates as noninvasive probes of intrahepatic metabolism: II. Application to measurement of plasma alpha 1-acid glycoprotein turnover during inflammation

We describe a new method for measuring plasma glycoprotein turnover in vivo using carbohydrate-moiety labeling compared with precursor UDP-sugar monitoring, which allows estimation of the specific activity of the precursor at the biosynthetic site of the glycoprotein. This method has been applied to plasma alpha 1-acid glycoprotein (AGP) kinetics, and has allowed direct quantitation of absolute synthesis rates of AGP in a non-steady state. Following turpentine-induced acute inflammation, AGP was found to undergo a maximum increase in plasma level of eight-fold with a 20- to 25-fold induction in absolute synthesis rate peaking at 25 to 50 h, and a concurrent 2.0 to 2.5-fold increase in fractional degradation rate. The changes in absolute synthesis rates were quite comparable both temporally and quantitatively to changes in hepatic AGP mRNA levels and gene transcription rates reported by others following turpentine inflammation, thus suggesting that AGP synthesis in vivo is predominantly regulated by the level of its mRNA. The carbohydrate moiety labeling method can be applied to other plasma glycoproteins to measure their kinetic parameters in the intact animal or human subject.

Uptake and degradation of 125I-labeled rat asialoorosomucoid by the perfused rat liver

The uptake and degradation of a homologous rat serum asialoglycoprotein, 125I-asialoorosomucoid, and the effects on this metabolism by leupeptin, a proteinase inhibitor, were studied in the perfused rat liver. 125I-Asialoorosomucoid was rapidly taken up by the liver (t1/2 = 5.7 min) and acid-soluble degradation products began to appear in the circulating perfusate medium after 20-30 min. These products accounted for 60-65% of the initially added radioactivity after 90 min of perfusion. The early events in the galactose-mediated uptake of 125I-asialoorosomucoid were unchanged by the presence of leupeptin. However, the appearance of acid-soluble degradation products was greatly reduced when livers had been pretreated with the inhibitor (1.0 mg for 60 min). This effect corresponded with an increase in acid-precipitable material being located within the lysosomal-rich fraction from homogenates of leupeptin-treated livers. Leupeptin inhibited degradation of 125I-asialoorosomucoid by approx. 85% relative to control values over 90 min of perfusion. Inhibition of asialoorosomucoid degradation was also demonstrated in vitro. Leupeptin (1.0 mM) reduced hydrolysis of this glycoprotein substrate by greater than 50% during a 24 h incubation with isolated lysosomal enzymes. The thiol proteinases, cathepsin B, H and L, which are known to be inhibited by leupeptin, are apparently involved in initiating digestion of rat 125I-asialoorosomucoid within liver lysosomes. As a result of inhibition by leupeptin both in the perfused liver and in vitro very limited changes occurred in the native molecular weight of the starting glycoprotein.