Effects of glycosylation inhibitors on the expression of polyalbumin receptors by hepatitis B surface antigen produced in vitro.
(17/31)
In this study we examined the role of secondary modifications in the expression of polyalbumin receptors by hepatitis B surface antigen (HBsAg) produced in vitro. Several clones isolated from 3T3 mouse fibroblasts after transfection with the hepatitis B virus genome produced HBsAg with marked variation in the expression of polyalbumin receptors. Treatment of the cells with glycosylation inhibitors (tunicamycin, glucosamine) resulted in loss of the 27 000 mol. wt. HBsAg glycopolypeptide, concomitantly with a marked increase in polyalbumin receptors. These data suggest that glycosylation regulates the activity of polyalbumin receptors associated with native HBsAg. (+info)
In vitro and in vivo synthesis of the hepatitis B virus surface antigen and of the receptor for polymerized human serum albumin from recombinant human adenoviruses.
(18/31)
We have developed an adenovirus vector to express foreign proteins under the control of the adenovirus E1a promoter. Two recombinant plasmids, harbouring either the S gene or the pre-S2 region and the S gene of hepatitis B virus under the control of the E1a promoter, were used to construct two recombinant adenoviruses. These two viruses direct the synthesis of hepatitis B virus surface antigen (HBsAg) particles during the time course of an infectious cycle. When the pre-S2 region is present in the constructed virus, the synthesis of particles carrying the receptor for polymerized human serum albumin (pHSA) is observed. Moreover, the inoculation of rabbits with this latter purified recombinant adenovirus elicits the production of antibodies that react with both HBsAg and pHSA receptor. (+info)
Scavenger receptor for aldehyde-modified proteins.
(19/31)
This paper describes an unexpectedly broad ligand specificity of a scavenger receptor of sinusoidal liver cells that is responsible for endocytic uptake of formaldehyde-treated bovine serum albumin (f-Alb). Binding of 125I-f-Alb to the isolated cells was effectively inhibited by bovine serum albumin (BSA) modified with aliphatic aldehydes such as glycolaldehye, DL-glyceraldehyde, and propionaldehyde whereas albumin preparations modified by aromatic aldehydes such as pyridoxal, pyridoxal phosphate, salicylaldehyde, and benzaldehyde did not affect this binding process. Binding of 125I-glycolaldehyde-treated BSA to the cells exhibited a saturation kinetics with an apparent Kd = 3.3 micrograms of the ligand/ml. This binding process was inhibited by unlabeled f-Alb as well as by the antibody raised against the f-Alb receptor. Indeed, 125I-glycolaldehyde-treated BSA underwent a rapid plasma clearance (t1/2 approximately 2 min) which was markedly retarded by unlabeled f-Alb. Upon treatment by these aldehydes, other proteins such as ovalbumin, soybean trypsin inhibitor, and hemoglobin were also converted to active ligands for the f-Alb receptor, while no ligand activity was generated with gamma-globulin and RNase A. These results clearly show that the f-Alb receptor, originally described as being specific for f-Alb, exhibits a broad ligand specificity in terms of both aldehydes and proteins and, hence, should be described as a scavenger receptor for aldehyde-modified proteins. (+info)
Specific binding sites for albumin restricted to plasmalemmal vesicles of continuous capillary endothelium: receptor-mediated transcytosis.
(20/31)
The interaction of homologous and heterologous albumin-gold complex (Alb-Au) with capillary endothelium was investigated in the mouse lung, heart, and diaphragm. Perfusion of the tracer in situ for from 3 to 35 min was followed by washing with phosphate-buffered saline, fixation by perfusion, and processing for electron microscopy. From the earliest time examined, one and sometimes two rows of densely packed particles bound to some restricted plasma membrane microdomains that appeared as uncoated pits, and to plasmalemmal vesicles open on the luminal front. Morphometric analysis, using various albumin-gold concentrations, showed that the binding is saturable at a very low concentration of the ligand and short exposure. After 5 min, tracer-carrying vesicles appeared on the abluminal front, discharging their content into the subendothelial space. As a function of tracer concentration 1-10% of plasmalemmal vesicles contained Alb-Au particles in fluid phase; from 5 min on, multivesicular bodies were labeled by the tracer. Plasma membrane, coated pits, and coated vesicles were not significantly marked at any time interval. Heparin or high ionic strength did not displace the bound Alb-Au from vesicle membrane. No binding was obtained when Alb-Au was competed in situ with albumin or was injected in vivo. Gold complexes with fibrinogen, fibronectin, glucose oxidase, or polyethyleneglycol did not give a labeling comparable to that of albumin. These results suggest that on the capillary endothelia examined, the Alb-Au is adsorbed on specific binding sites restricted to uncoated pits and plasmalemmal vesicles. The tracer is transported in transcytotic vesicles across endothelium by receptor-mediated transcytosis, and to a lesser extent is taken up by pinocytotic vesicles. The existence of albumin receptors on these continuous capillary endothelia may provide a specific mechanism for the transport of albumin and other molecules carried by this protein. (+info)
Receptor recognition of maleyl-albumin induces chemotaxis in human monocytes.
(21/31)
We demonstrate here that the exceptionally active maleyl-albumin receptor of human monocytes functions in vitro as a chemoattractant receptor. Chemotaxis of human monocytes occurs at an effective median dose of 3-4 microM maleyl-albumin, a concentration representing 1% of the total albumin in the adult human. Computerized analyses by LIGAND of the saturable binding of maleyl-albumin to human monocytes reveal two classes of binding sites, described by dissociation constants of 37 nM and 5.3 microM with maximal binding of 1.6 and 23 pmol maleyl-albumin/mg cellular protein, respectively. Chemotaxis of human monocytes thus occurs at concentrations of maleyl-albumin promoting binding to the lower-affinity sites. We propose that conformational isomers of albumin that are chemotactic may form in vivo and that albumin, in addition to receptor-independent plasma transport functions, may also play an important role in the receptor-mediated recruitment and accumulation of phagocytic cells at sites of inflammation and injury. (+info)
Renal plasma membrane receptors for certain modified serum albumins. Evidence for participation of a heparin receptor.
(22/31)
Binding of formaldehyde-treated (f-alb), reduced-carboxymethylated (ac-alb) or reduced-acetamidated (am-alb) bovine serum albumins to purified rat renal plasma membranes was studied. Radioiodinated f-alb or ac-alb bound to kidney membranes while am-alb neither bound significantly nor competed with f-alb binding to kidney membranes. The binding was specific, saturable and heat- and proteinase-sensitive. Competition studies showed that f-alb and ac-alb sites may be the same on these membranes. To determine the role played by charge in binding, competition experiments with polyanions were performed. Polyanions such as nucleic acid or glycosaminoglycans were effective competitors of f-alb binding to cell membranes. Heparin was especially inhibitory, being several-fold more so than chondroitin sulphate. Completely reduced and carboxymethylated albumin was a better competitor than its partially modified counterpart. Furthermore, f-alb was a significant competitor of [35S]heparin binding to kidney membranes. Also, partially purified heparin receptor demonstrated specific binding of 125I-f-alb. These data suggest that a heparin receptor is responsible for binding and internalization of intravenously injected f-alb. A Scatchard plot revealed two classes of receptors with dissociation constants of 3.2 X 10(-6) M and 4.7 X 10(-5) M. (+info)
Uptake of oleate from albumin solutions by rat liver. Failure to detect catalysis of the dissociation of oleate from albumin by an albumin receptor.
(23/31)
The hepatic removal of albumin-bound substances from plasma requires that they dissociate from albumin. Using indirect methods, we and others have proposed that dissociation may be catalyzed by interaction of albumin with the liver cell surface. This study looked for direct evidence of catalysis by comparing the rate of dissociation of oleate from albumin in vitro with the rate observed within the sinusoids of perfused rat liver. No evidence for catalysis was found. The rate of hepatic oleate removal from dilute albumin solutions did not exceed but instead closely paralleled the rate predicted from the in vitro dissociation rate constant (0.14s-1). These results suggest that under some conditions the liver can remove unbound material from the sinusoids faster than it can be replenished by dissociation from albumin, resulting in dissociation-limited removal. However, dissociation of oleate does not appear to be catalyzed by the liver. (+info)
Surface receptors for human serum albumin in Peptococcus magnus strains.
(24/31)
Eighty-one bacterial strains representing 16 anaerobic species were tested in a sensitive binding assay for uptake of 125I-labelled human serum proteins. Fifteen of 36 Peptococcus magnus strains (42%) bound significant amounts of human serum albumin (HSA). None of the other bacterial species showed any affinity for HSA. All strains studied were incapable of uptake of human fibrinogen, fibronectin, haptoglobin or aggregated beta 2-microglobulin. P. magnus strain Ra 4 was tested for binding of purified serum albumin from 11 animal species, and showed a binding profile similar to human group-C and -G streptococci, but different from Streptococcus pyogenes, Strep. zooepidemicus and Strep. dysgalactiae. Kinetic experiments showed that albumin binding was a rapid displaceable, time-dependent process, that could take place over a wide range of pH or salt concentrations. The albumin-binding component of P. magnus strain Ra 4 was resistant to heat and to periodate treatment, but sensitive to proteolytic enzymes. (+info)