[Albumin-based Drug Delivery System Targeting Mannose Receptors and Its Application to Medical Treatments]

The onset and progression of liver diseases and cancer have shown to be affected by over-active macrophages and fibroblasts. Therefore, developing methods to suppress the activation of these cells has become an urgent task. Prior to this study, a mannosylated-albumin (Man-HSA) that targets mannose receptors expressed in hepatic macrophages (Kupffer cells) or fibroblasts was created. Here, we report on the development of medical treatments based on Man-HSA. To target the reactive oxygen species or inflammation derived from Kupffer cells, we developed a nano-antioxidant, i.e., polythiolated (SH)-Man-HSA, by introducing thiol groups into Man-HSA, or a nano-anti-inflammatory drug, i.e., Man-HSA-IFNα2b, by fusing Man-HSA and IFNα2b. SH-Man-HSA or Man-HSA-IFNα2b attenuated Kupffer cell-derived oxidative stress or inflammation, respectively, resulting in the suppression of liver damage and overall improvement of the survival rate in mice with acute and chronic liver injuries. Tumor-associated macrophages (TAM) and cancer-associated fibroblasts (CAF), both of which are present in the stroma of intractable cancers, also express mannose receptors. Thus, mono-polyethylene glycol modified Man-HSA (monoPEG-Man-HSA) was synthesized as a novel drug delivery carrier targeting TAM/CAF. A complex of monoPEG-Man-HSA with paclitaxel suppressed tumor growth by decreasing the number of TAM/CAF and the stroma area. For the present study, we focused on the mannose receptors expressed in macrophages and fibroblasts, and developed drug delivery carriers that target these cells. Considering the excellent drug-carrying capacity and high biocompatibility of HSA, it is expected that this research will pave the way for innovative pharmacotherapy to treat unmet medical needs, i.e., intractable liver diseases and cancer.

Polymerized albumin receptor on rat liver cells

The polymerized albumin hypothesis was proposed for the mechanism of a hepatitis B virus (HBV) infection of human liver parenchymal cells on the basis that a receptor for polymerized albumin treated with glutaraldehyde was detected on isolated human liver parenchymal cells. However, some controversy exists regarding this hypothesis, because a receptor for formaldehyde-treated bovine serum albumin (f-BSA) has been found on liver non-parenchymal cells. Therefore, we characterized the uptake of polymerized rat serum albumin (p-RSA) and f-BSA by rat liver in vivo, and their bindings to liver cells in vitro. Most p-RSA and f-BSA was taken up by the liver after intravenous administration, and the uptake of p-RSA was inhibited by a 1,000-fold excess of f-BSA. In addition, more than 80% of p-RSA taken up by the liver was found in the non-parenchymal cells, and the remainder was found in the parenchymal cells. P-RSA as well as f-BSA could bind to isolated rat liver parenchymal and non-parenchymal cells. Furthermore, p-RSA and f-BSA could bind to isolated rat liver cell plasma membranes, and these bindings were completely inhibited by 1,000-fold excess of either f-BSA or p-RSA. These results indicate that there is a receptor, which can recognize both p-RSA and f-BSA, on not only rat liver non-parenchymal cells but also the parenchymal cells. It is also indicated that the receptor on the parenchymal cells as well as the non-parenchymal cells is involved in the in vivo uptake of p-RSA.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of polymerized-albumin receptor domain in the pre-S2 region of hepatitis B virus surface antigen M protein

The pre-S2-coding region in the hepatitis B virus surface antigen M (P31; pre-S2 + S) protein gene was modified to identify a polymerized-albumin receptor (PAR) domain by deleting restriction fragments or performing site-directed mutagenesis. The modified M protein genes (M-P31x; x = d, e, f, h and i) were cloned into the yeast generalized-expression vector pGLD 906-1 and expressed in Saccharomyces cerevisiae under the control of yeast glyceraldehyde-3-phosphate dehydrogenase gene promoter. The PAR activities of these gene products suggested that the PAR domain is located in the hydrophilic and highly conserved domain in the pre-S2 region (around Leu12 approximately Tyr21). Antibodies specific for a pre-S2 peptide (Phe8 approximately Pro34, subtype adr), which covers the PAR domain, were purified from sera of rabbits immunized with yeast-derived M protein particles having a natural PAR domain. Immune electron microscopy showed that the purified antibodies could aggregate HBV particles. Therefore, it was speculated that the PAR domain overlapped with the dominant virus-neutralizing and virus-protecting epitopes.

Albumin-binding proteins of endothelial cells: immunocytochemical detection of the 18 kDa peptide

Extracts of isolated microvascular endothelial cells (MEC) and cultured bovine aortic endothelial cells (BAEC) were subjected to sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), electrotransfer and incubation with albumin either radioiodinated or adsorbed to 5-nm gold particles. Both ligands reacted exclusively with two peptides of 18 and 31 kDa. To the 18 kDa peptide (excised from preparative SDS-PAGE), an antibody was raised in rabbits and purified by affinity on 18 kDa obtained from two-dimensional gel electrophoresis and immobilized on nitrocellulose paper. The specificity of the anti-18 kDa was assessed by immunoblotting and immunoprecipitation of endothelial cell extracts. To check whether the 18 kDa peptide is exposed on the endothelial cell surface and/or its components (uncoated pits, open plasmalemmal vesicles), the apical membrane of BAEC was radioiodinated, the solubilized proteins incubated with the anti-18 kDa, and the immune complexes formed were precipitated with protein A-Sepharose CL-4B. The ensuing SDS-PAGE and autoradiography revealed that from all radioiodinatable surface proteins, the 18 kDa was the only polypeptide immunoprecipitated by the anti-18 kDa antibody. To localize the 18 kDa peptide, we applied indirect immunofluorescence technique on cultured MEC and BAEC and immunoelectron microscopy (EM) on ultrathin cryosections of mouse heart. Nonpermeabilized whole MEC and BAEC incubated with anti-18 kDa followed by rhodamine-conjugated second antibody showed a relatively intense surface fluorescence often appearing as small dots. At the EM level, heart ultrathin cryosections exposed anti-18 kDa followed by gold-conjugated second antibody revealed that 18 kDa was primarily associated with the membrane of plasmalemmal vesicles of capillary endothelia.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential degradation of a recombinant albumin-binding receptor in Escherichia coli

The degradation in Escherichia coli of the recombinant serum-albumin-binding receptor derived from streptococcal protein G was investigated using a dual-affinity fusion approach. The proteolytic degradation of the receptor was characterized when fused to human proinsulin and human secretin. Several cleavages occurred at sequences not normally regarded as proteolytically sensitive, such as the dipeptide sequences Ile-Gly, Val-Ser and Ser-Ala. Depending on the fusion partner, large differences in the degradation of the albumin-binding domain were observed. Thus, susceptibility to proteolysis of a recombinant protein can be affected by a neighbouring domain.

Comparison of albumin receptors expressed on bovine and human group G streptococci

The albumin receptor expressed by bovine group G streptococci was extracted and affinity purified. The protein was characterized for species reactivity, and monospecific antibodies were prepared to the purified receptor. The bovine group G albumin receptor was compared functionally, antigenically, and for DNA homology with the albumin-binding protein expressed by human group G streptococci. In agreement with previous reports, the albumin-binding activity of human strains was mediated by a unique domain of the type III immunoglobulin G-Fc-binding molecule, protein G. The albumin receptor expressed by bovine group G strains was found to lack any immunoglobulin G-binding potential but displayed a wider profile of species albumin reactivity than protein G. Both albumin receptors could inhibit the binding of the other to immobilized human serum albumin, and each displayed similar binding properties. Antigenic comparison of the two albumin receptors demonstrated a low level of cross-reactivity; however comparison at the DNA level, using an oligonucleotide probe specific for the albumin-binding region of protein G, demonstrated that the two albumin receptors expressed by human and bovine group G streptococcal strains do not display significant homology.

Albumin binding sites are expressed on the abluminal plasma membrane of capillary endothelium

The presence of albumin binding sites on the abluminal front of vascular endothelium was examined on the capillaries of the adipose tissue. The experimental procedure consisted in injecting interstitially within the rat epididymal fat and epicardial fat, albumin (alone or bearing oleic acid) either conjugated with gold particles (Alb-Au or Alb-OA-Au) or radioiodinated [( 125I]-Alb). In controls, polyethyleneglycol-gold complex (PEG-Au) and [125I]-IgG were used as tracers. The results revealed that: a) albumin binding sites are expressed on the abluminal front of endothelium especially concentrated on plasmalemmal vesicles; b) the retrotranscytosis of albumin conjugates from the interstitium to the capillary lumen is a poorly represented process; c) the binding of the tracers used appears to be time and concentration dependent; d) albumin conjugates do not bind significantly to plasmalemmal vesicles of adipocytes, pericytes and smooth muscle cells; e) PEG-Au and [125I]-IgG do not show a binding pattern similar to that of albumin conjugates.

Transthyretin receptors on human astrocytoma cells

Transthyretin (TTR), a transport protein for T4 and retinol-binding protein, is the principal T4-binding protein of cerebrospinal fluid. Its function in regard to the delivery of its ligands and in other respects is unclear. The binding of [125I] TTR to cultured human astrocytoma cells was studied to determine whether these cells carry receptors for TTR. Scatchard analysis was consistent with a single class of binding sites with a Kd of 3 nM. No significant cross-reactivity with transferrin or serum albumin was observed. Internalization of TTR was temperature dependent and proportional to receptor occupancy. Dilutions of cerebrospinal fluid displaced [125I]TTR in proportion to their content of radioimmunoassayable TTR and in parallel with purified TTR. The uptake and internalization of TTR increased in the presence of high T4 or T3 concentrations. These results demonstrate that TTR binds to specific high affinity receptors on human astrocytoma cells. Receptor binding of TTR provides a potential mechanism for the delivery of its ligands within the central nervous system.

Expression and secretion of preS containing hepatitis B surface antigen in vaccinia virus system

The expression and secretion of preS containing hepatitis B surface antigen in vaccinia virus system was investigated. The human TK- 143 cells were infected with the recombinant vaccinia viruses vTMS-1 or vTLS-1. Cells infected with vTMS-1, which contains the preS2 + S gene, produced preS2 containing middle HBsAg proteins. Similarly, cells produced preS1 containing large HBsAg proteins upon infection with vTLS-1, which carries the preS1 + preS2 + S gene. The expression products could be secreted and form 22 nm particles. They reacted specifically with anti-preS1 and/or anti-preS2 monoclonal antibodies, and exhibited pHSA-receptor (for polymerized human serum albumin) activity. In addition, the major S components of hepatitis B surface antigen were also present in the products expressed by vTMS-1 and vTLS-1.

Sarcolemmal fatty acid transfer in isolated cardiomyocytes governed by albumin/membrane-lipid partition

The mechanism of transfer of long chain fatty acids across the myocardial sarcolemmal membrane was investigated in isolated, calcium-resistant, rat cardiomyocytes. The initial rate of 14C-palmitate uptake was determined at constant and increasing palmitate/albumin ratios. The latter condition led to a saturable dependence of uptake rate on palmitate concentration. At a constant palmitate/albumin ratio however, there was an almost constant rate of uptake even though the absolute concentration of palmitate increased. The enhanced metabolic rate resulting from electrically induced contractions of the myocytes decreased the apparent Km of uptake from 62 to 23 microM. Thirty seconds after administration, there was no further increase in the [14C]palmitate content of the myocytes. Moreover, from experiments using ghost membrane vesicles the concentration of palmitate in membranes increased almost linearly with increasing palmitate/albumin ratios. This concentration remained virtually constant if vesicles were pre-treated with diamide. Our results do not support the concept of an albumin receptor-mediated uptake but rather suggest that fatty acids are incorporated into cardiomyocytes by a simple diffusion process which is not rate-limiting. The rate of uptake is influenced both by the metabolic rate and by the concentration of fatty acids in the membranes. The rate-limiting step of fatty acid uptake is probably either the formation of acyl-CoA catalyzed by the membrane associated acyl-CoA synthetase, or the transfer of fatty acid carnitine esters across the mitochondrial matrix membrane.

Changes of pre-S1 and pre-S2 antigens in sera of patients with hepatitis B virus infection

Polypeptides encoded by the pre-S1 and pre-S2 genes of hepatitis B virus (HBV) (pre-S1 antigen and pre-S2 antigen) were detected by enzyme-linked immunosorbent assay (ELISA) in 137 serum samples of patients with HBV infection. The HBV-DNA level closely correlated with the titer of pre-S antigens. However, HBV-DNA levels more closely correlated with the titer of the pre-S1 antigen [HBV-asymptomatic carrier (ASC): n = 40, r = 0.800, P less than 0.01; chronic hepatitis B (CH): n = 60, r = 0.730, P less than 0.01] than with the titer of the pre-S2 antigen [ASC: r = 0.675, P less than 0.01; CH: r = 0.575, P less than 0.01]. Thirty patients with CH, in whom hepatitis e antigen (HBeAg) was cleared after acute exacerbation (AE) [alanine aminotransferase (ALT) level greater than 200 IU/L] and the ALT level normalized, were followed for 12 months and classified into two groups: Group 1, those in whom HBeAg reappeared with an elevated ALT level within 12 months, and Group 2, those in whom HBeAg was persistently cleared from the serum and a normal ALT level continued. Of the 30 patients, 24 (80%) were classified into Group 1 and 6 (20%) were classified into Group 2. Changes in serum levels of HBV markers a month before and after AE were observed. The HBV-DNA level and DNA-P activity became negative after AE in both groups. The titer of pre-S1 antigen also decreased after AE, and no significant differences were observed between Group 1 and Group 2.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between hepatitis B surface proteins and monomeric human serum albumin

HBsAg is known to bind to human serum albumin polymerized by glutaraldehyde, human serum albumin has been found in preparations of HBsAg by several investigators. However, it is not yet known whether natural human serum albumin binds to hepatitis B virus under physiological conditions. We studied the binding between natural or recombinant HBsAg and monomeric human serum albumin by immunological, biochemical and biophysical methods. The binding capacity of 20-nm HBs spheres was variable but ranged up to six molecules HSA/sphere. A reversible binding site for human serum albumin was exclusively localized in the preS2 domain, whereas the S domain was inactive in vitro. Human serum albumin copurified with HBsAg of human origin during gel chromatography or sucrose-gradient centrifugation. This human serum albumin was monomeric in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The preS2-bound part of the human serum albumin could be removed from HBsAg by high-salt, such as CsCl centrifugation, but another part could only be removed by treatment with a disulfide cleaving reagent. Most of this covalently bound human serum albumin was retained at the HBsAg particle after complete cleavage of medium-sized HBs protein with trypsin. This indicates a second way in which albumin binds irreversible to cysteine(s) of the small HBs protein (SHBs, P24 and GP27).

Structure and ligand binding properties of human serum albumin

1.

INTRODUCTION

Serum albumin possesses a unique capability to bind, covalently or reversibly, a great number of various endogenous and exogenous compounds. Several different transport proteins exist in blood plasma but albumin only is able to bind a wide diversity of ligands reversibly with high affinity. The subject of the present thesis is these binding properties. In 1981 the author proposed in a review a model for binding of ligands to serum albumin. In the model, binding of ligands to at least 6 distinct regions was considered. The purpose of the experimental work described here was to test the validity of the model. This was done by performing a large number of competition experiments. From these new data a revised model for ligand binding is presented. 2.

STRUCTURE AND CONFORMATIONAL CHANGES OF SERUM ALBUMIN

Human serum albumin consists of 585 amino acids forming a single polypeptide of known sequence. A number of well characterized genetic variants have been reported. The physico-chemical characteristics of the protein are well-established. By contrast, the complete secondary and tertiary structures are not known; information about major structural features only has been obtained. The albumin molecule seems to have an overall ellipsoidal shape (about 140 x 40 A) and to be composed of domains. On the basis of the amino acid sequence, Brown (1977a) proposed a 3-domain model for the protein. Each domain is believed to consist of 6 helices forming a hydrophobic channel with basic and hydrophobic amino acid residues placed at the ends. Experimental data, however, indicate that the domains cannot be identical. Long-chain fatty acid ions are proposed to bind with high affinity within the channels. The ability to fluctuate between isomeric forms in aqueous solution could assist in adapting the albumin molecule to bind ligands with a diverse nature with high affinity. This possibility is discussed on the basis of several physico-chemical techniques including hydrogen-deuterium exchanges. Also the importance of the N-B transition for ligand binding is considered. 3.

PRELIMINARY BINDING MODEL OF SERUM ALBUMIN

Single binding of ligands to serum albumin is usually described as high-affinity binding to one or two sites and weaker binding to a larger number of sites. In this chapter, the original binding model for high-affinity binding is elaborated. Region 1 seems to be specific for binding of one, or possibly two, ions of long-chain fatty acids. Region 2 is somewhat less specific and includes binding of octanoate, tryptophan, chlorazepate, thyroxine, p-iodobenzoate and possibly also chloride. Region 3 accommodates bilirubin, Phenol Red, Bromophenol Blue and iopanoate. Region 4 is a special site for strong binding of metal ions such as Cu++ and Ni++. Evidence is presented for placing the primary haemin site in a separate region (no. 5). The existence of additional binding regions, well-suited for high-affinity binding of drugs, is discussed...

Binding characteristics of reduced hepatic receptors for acetylated low-density lipoprotein and maleylated bovine serum albumin

The binding characteristics of reduced hepatic membrane proteins for acetylated low-density lipoprotein (acetyl-LDL) and maleylated bovine serum albumin (Mal-BSA) have been examined. Two receptor activities were extracted from hepatic membranes in the presence of octyl beta-D-glucoside and beta-mercaptoethanol, and were separated by chromatography on Mal-BSA-Sepharose 4B. The receptors were revealed by ligand blotting. The active binding proteins had apparent molecular masses of 35 and 15 kDa in SDS/polyacrylamide gels. Equilibrium studies with protein-phosphatidylcholine complexes indicated that the reduced 35 kDa protein expresses two binding sites for Mal-BSA and one for acetyl-LDL, whereas the 15 kDa protein-phosphatidylcholine complex binds 131I-Mal-BSA and 131I-acetyl-LDL with a 4:1 stoichiometry. 131I-Mal-BSA binding was linear with both proteins, with a Kd of 4.8 nM at the 35 kDa protein and a Kd of 5.6 nM at the 15 kDa protein. The 35 kDa protein displayed saturable binding of 131I-acetyl-LDL with a Kd of 5 nM; the 15 kDa binding protein bound 131I-acetyl-LDL with a Kd of 2.3 nM. A 85 kDa protein was obtained by Mal-BSA-Sepharose chromatography when the hepatic membranes had been solubilized with Triton X-100 in presence of GSH/GSSG. This protein displayed saturable 131I-Mal-BSA binding with a Kd of 30 nM and 131I-acetyl-LDL binding with a Kd of 6.5 nM. The 131I-Mal-BSA binding capacity was four times higher than that of 131I-acetyl-LDL. Competition studies with the 35 kDa, 15 kDa and 85 kDa proteins binding Mal-BSA, acetyl-LDL, formylated albumin and polyanionic competitors provide evidence for the existence of more than one class of binding sites at the reduced binding proteins.

Changes in serum levels of hepatitis B virus markers after interferon treatment

The effect of interferon (IFN) treatment on serum levels of pre-S antigens [pre-S(1) antigen, pre-S(2) antigen, polymerized human serum albumin receptor (pAR)] which are coded by the pre-S region of hepatitis B virus DNA (HBV-DNA), and HBV-markers was analyzed in 23 patients with chronic hepatitis B. One year after IFN treatment, 4 patients (Group C) became HBeAg negative. Six patients (Group B) transiently became HBeAg-negative, but reverted to HBeAg positive. Thirteen patients (Group A) remained HBeAg positive. All of the patients remained HBsAg-positive. Initiation of IFN treatment was rapidly followed by reduction or loss of DNA-P in the serum whether the patients became HBeAg negative or remained positive, and whether serum transaminase (S-GPT) levels became normal or not after IFN treatment. Group C patients, in whom pre-S antigens decreased rapidly during IFN treatment and disappeared before S-GPT levels normalized, became HBeAg negative one year after IFN treatment. Anti-pAR was detected in three out of these 4 patients. In contrast, Group A and Group B patients, in whom pre-S antigens decreased slowly during IFN treatment and did not disappear in spite of those patients being transiently negative for HBeAg and DNA-P, remained HBeAg positive with elevated S-GPT levels one year after IFN treatment. Anti-pAR was almost undetectable. These results suggest that testing for pre-S antigens is more useful for determining the prognosis of patients with chronic hepatitis B treated with IFN than testing for HBsAg, HBeAg and DNA-P.

Albuminoid molecules: a novel, variability-generating cell-surface receptor system?

The mechanisms by which lymphoid cells produce infinitely variable molecules of the immunoglobulin protein superfamily have been recently elucidated. These molecules serve, in part, as the mediators of cell:cell recognition and interaction among lymphoid cells. However, the generality of those molecular mechanisms to occur in non-lymphoid cell types has not yet been established. In this paper, we propose that the serum albumin superfamily of proteins has the necessary characteristics to serve analogous functions in epithelioid cells, and we critique recent evidence which leads to this hypothesis.

Endothelial barrier function

The endothelial barrier in all organ beds allows the free exchange of water, but is restrictive to varying degrees to the transport of solutes such as albumin. For example, in the brain microvessels, the endothelial barrier restricts the transport of protein, whereas in fenestrated and continuous endothelial cells of the renal and lung endothelial cells, the endothelial barrier is semipermeable. The endothelial monolayer demonstrates selectivity, i.e., the permeation of molecules is inversely related to the molecular weight. Although the "pore" theory has been used to describe the transport across the endothelial barrier, the transport of solutes is also dependent on the charge of solutes and the endothelial cell, and the ability of the solute to bind to or be taken up by endothelial cells. Receptor-mediated trancytosis of albumin may contribute to albumin transport in addition to transport by paracellular pathways (i.e., through a so-called pore). Water permeability across the endothelium is determined by the interaction of albumin with glycocalyx and interstitial components of the endothelium (the "fiber matrix"). Ambient concentration of albumin serves to lower endothelial hydraulic conductivity. Increased endothelial permeability to solutes and water in inflammatory states is dependent on the shape and configuration of endothelial cells as determined by alterations in cytoskeletal elements, such as f-actin, and as regulated by intracellular second messengers such as free cytosolic calcium.

Transmembrane transport of fatty acids in the heart

Although fatty acid uptake by the myocardium is rapid and efficient, the mechanism of their transmembrane transport has been unclear. Fatty acids are presented to the plasma membrane of cardiomyocytes as albumin complexes within the plasma. Since albumin is not taken up by the cells, it was postulated that specific high affinity binding sites at the sarcolemma may mediate the dissociation of fatty acids from the albumin molecules, before they are transported into the cells. In studies with a representative long-chain fatty acid, oleate, it was in fact shown that fatty acids bind with high affinity to isolated plasma membranes of rat heart myocytes revealing a KD of 42 nM. Moreover, a specific membrane fatty acid-binding protein (MFABP) was isolated from these membranes. It had a molecular weight of 40 kD, an isoelectric point of 9.0, and lacked carbohydrate or lipid components. Binding to a specific membrane protein might represent the first step of a carrier mediated uptake process. Therefore, the uptake kinetics of oleate by isolated rat heart myocytes was determined under conditions where only cellular influx and not metabolism occurred. Uptake revealed saturation kinetics and was temperature dependent which were considered as specific criteria for a facilitated transport mechanism. For evaluation whether uptake is mediated by MFABP, the effect of a monospecific antibody to this protein on cellular influx of oleate was examined. Inhibition of uptake of fatty acids but not of glucose by the antibody to MFABP indicated the physiologic significance of this protein as transmembrane carrier in the cellular uptake process of fatty acids.(ABSTRACT TRUNCATED AT 250 WORDS)

The albumin receptor effect may be due to a surface-induced conformational change in albumin

To determine whether equilibrium binding between albumin and hepatocytes involves a cell surface receptor for albumin, we incubated freshly isolated rat hepatocytes with 125I-albumin and determined the amount of albumin associated with the cells as a function of the total albumin concentration. The resulting two-phase binding curve showed the rat albumin-hepatocyte interaction to consist of a saturable binding interaction with a dissociation constant of 1.1 microM and 2 X 10(6) sites/cell in addition to a weak, nonsaturable binding interaction. However, the saturable binding of albumin to hepatocytes did not appear to result from the presence of an albumin receptor on the cell surface; the interaction was the same for different species of albumin, for chemically modified albumins, and for fragments of albumin representing mutually exclusive domains of the molecule. The saturable binding was, instead, found to involve a subpopulation of albumin with an enhanced affinity for the cell surface. We show that this subpopulation of albumin is generated upon contact with either solid surfaces or cell surfaces and can be transferred from one surface to another. We propose that the two-phase Scatchard binding curve and the "albumin receptor effect" reflect two populations of albumin that bind to the cell surface with different affinities rather than one population of albumin that binds to two classes of binding sites.

Drug binding properties of neonatal albumin

Neonatal and adult albumin was isolated by gel chromatography on Sephacryl S-300, from adult and umbilical cord serum, respectively. Binding of monoacetyl-diamino-diphenyl sulfone, warfarin, sulfamethizole, and diazepam was studied by means of equilibrium dialysis and the binding data were analyzed by the method of several acceptable fitted curves. It was found that the binding affinity to neonatal albumin is less than to adult albumin for monoacetyl-diamino-diphenyl sulfone and warfarin. Sulfamethizole binding to the neonatal protein is similarly reduced when more than one molecule of the drug is bound per albumin molecule, and binding of the first sulfamethizole molecule is possibly reduced as well. Diazepam binds with equal affinity to the fetal and adult proteins. Among the two main albumin drug-binding functions, for warfarin and diazepam, the former is thus compromised in the newborn infant while the diazepam binding function is at the adult level.

Role of the maleyl-albumin receptor in activation of murine peritoneal macrophages in vitro

It has been previously demonstrated that maley-lated-BSA (maleyl-albumin) induces functional activation in murine peritoneal macrophages. Furthermore, maleyl-albumin has been shown to interact with two distinct sites on human monocytes; one site is the scavenger receptor, a 260-kDa oligomeric protein which recognizes modified forms of low density lipoprotein (LDL), and the second is a lower affinity site which has yet to be structurally characterized. In the present study, we wished to quantitatively assess the number and character of maleyl-albumin-binding sites on murine peritoneal macrophages and to determine which site or sites are involved in signaling the macrophage to undergo extensive functional development. Binding studies. demonstrate at least two distinct receptors for maleyl-albumin on murine peritoneal macrophages. Scatchard analyses of the binding isotherms reveal two sites characterized by dissociation constants (Kd) of 17.6 nM and 4.9 microM and maximal binding of 1.2 x 10(5) and 1 x 10(6) sites/cell, respectively. The contribution of the scavenger receptor, determined by binding analyses of malondialdehyde-LDL, is described by two sites with Kd of 39.4 pM and 9.6 nM, and maximal binding of 2.7 x 10(3) and 1.9 x 10(4) sites/cell, respectively. Maleyl-albumin blocks binding of malondialdehyde-LDL, whereas modified LDL fails to inhibit binding of maleyl-albumin. Maleyl-albumin, at concentrations producing lower affinity binding, stimulates tumor cytolysis, expression of mRNA encoding TNF, and suppression of INF-gamma-induced expression of Ia Ag. Malondialdehyde-LDL fails to elicit these responses. We conclude that macrophage activation produced by maleyl-albumin is mediated by interaction with the low affinity, high capacity binding site for maleyl-albumin rather than the scavenger receptor.

'Albumin-receptor' uptake kinetics do not require an intact lobular architecture and are not specific for albumin

When freshly isolated well-stirred single cell suspensions of rat hepatocytes were incubated with 5-600 microM [3H]oleate or [35S]sulfobromophthalein (BSP) in the presence of 150 microM bovine serum albumin (BSA), uptake of both ligands increased as a linear function of the total ligand concentration in the medium. By contrast, when the same ligand concentrations were incubated as 1:1 complexes with BSA, apparent saturation of ligand uptake was observed. Analogous results were obtained in incubations employing beta-lactoglobulin instead of BSA. In none of these studies did ligand uptake velocity correlate in simple fashion with the concentration of unbound ligand in the incubation medium. These studies establish that the basis for the kinetic observations termed the 'albumin receptor phenomenon' does not require an intact hepatic lobular architecture or space of Disse, and is not specific for albumin.

A receptor for formaldehyde-treated serum albumin on human placental brush-border membrane

Formaldehyde-treated serum albumin (f-Alb) is known to be taken up and degraded by sinusoidal liver cells via receptor-mediated endocytosis. We report that 125I-labeled f-Alb (125I-f-Alb) binding to human placental brush-border membranes also occurs. This binding reached equilibrium within 40 min at 37 degrees C. Kinetic studies demonstrated the presence of saturable binding with an apparent Kd of 2.1 micrograms of f-Alb/ml and a maximal binding of 2.3 micrograms/mg of membrane protein at pH 7.5. Maximal binding was observed at between pH 7.5 and 8.0. 125I-f-Alb binding to the membranes was little inhibited by a 1000-fold molar excess of ovalbumin, human apo-transferrin and native bovine serum albumin. No binding was observed with membranes which had been pretreated with proteinase or trypsin. This f-Alb receptor was extremely heat-stable, since the binding was not abolished even by pretreatment of the membranes at 78 degrees C for 30 min. EDTA, Ca2+ and Mg/4 had no effect on 125I-f-Alb binding, so the binding was independent of divalent cations. These data suggest that a receptor specific for f-Alb exists on human placental brush-border membranes of syncytial trophoblasts.

Endocytic uptake of nonenzymatically glycosylated proteins is mediated by a scavenger receptor for aldehyde-modified proteins

Long term incubation of proteins with glucose, named the Maillard reaction (Maillard, L. C. (1912) C. R. Acad. Sci. (Paris) 154, 66-68), gives rise to advanced glycosylation end product (AGE) with fluorescence, color, as well as cross-linked properties. The receptor-mediated endocytosis of AGE-proteins by macrophages was reported (Vlassara, H., Brownlee, M., and Cerami, A. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 5588-5592). The present study on the binding of AGE-bovine serum albumin (BSA) to rat peritoneal macrophages and sinusoidal liver cells demonstrated the presence of a saturable, high affinity receptor for AGE-BSA with Kd = 2.4 x 10(-7) M (macrophages) and 2.1 x 10(-7) M (sinusoidal cells). The cellular binding of AGE-BSA and its endocytic uptake by these cells were competitively inhibited by BSA preparations modified with aliphatic aldehydes such as formaldehyde or glycolaldehyde, ligands known to be specific for a scavenger receptor for aldehyde-modified proteins (Horiuchi, S., Murakami, M., Takata, K., and Morino, Y. (1986). J. Biol. Chem. 261, 4962-4966). These ligands also had a profound in vivo effect on the plasma clearance of 125I-AGE-BSA as well as its hepatic uptake. Thus, endocytic uptake of AGE-proteins by macrophages appeared to be mediated by a scavenger receptor for aldehyde-modified proteins. This provides evidence for the biological importance of the scavenger receptor in eliminating senescent macromolecules from the circulation.

Evidence in liver for a disulphide-linked scavenger receptor containing a binding site for acetylated low-density lipoprotein and maleylated bovine serum albumin

Membranes from rat liver were analysed under reducing conditions. The components of the soluble membranes responsible for the binding of acetylated low density lipoprotein (acetyl-LDL) and maleylated bovine serum albumin (Mal-BSA) were chromatographed on a polyethyleneimine-cellulose column and subsequently separated by gel electrophoresis. For both ligands a major binding protein (Mr = 35,000) was revealed by ligand blotting. A minor protein (Mr greater than 67,000) exhibited little binding. The Scatchard plot of the 131I-Mal-BSA binding data of the 35 kDa protein was linear, with a Kd of 17.3 nM. High concentrations of acetyl-LDL competed for half of the 131I-Mal-BSA binding. Excessive Mal-BSA competed for all the visible acetyl-LDL binding. The findings indicate the existence, in the reduced hepatic membrane, of a 35 kDa protein that has two binding sites for 131I-Mal-BSA and one binding site for acetyl-LDL.

Isolation and characterization of a specific receptor for human albumin on a group L Streptococcus

Certain group L streptococci demonstrate surface receptors for human albumin. Binding of 125I-albumin to group L streptococci could be inhibited by unlabelled albumin preparations from humans, dogs, mice and bovines, but not by albumin from rabbits. The albumin-binding proteins (ABP) could be solubilized from the streptococcal surface by hot acid treatment of the bacteria and isolated by affinity chromatography on human-albumin sepharose. ABP and specific antisera produced against ABP inhibited 125I-albumin binding to group L streptococci. The molecular weight of ABP determined by SDS-PAGE and Western blotting, was approximately 48,000 Dalton. ABP preparations of group G streptococci isolated from bovines and humans demonstrated cross reactivity with antiserum produced against group L streptococcal ABP.

What's new in the hepatocellular uptake mechanism of bilirubin and fatty acids?

Although hepatocellular uptake of bilirubin and fatty acids represents a major function of the liver, little was known about the molecular mechanism of their translocation across the plasma membrane. This review provides evidence that these classes of albumin-bound organic anions enter hepatocytes via specific membrane associated carrier systems. After dissociation of the albumin-ligand complexes at the cell surface, bilirubin and related cholephilic organic anions bind to a 55 kDa membrane glycoprotein, while fatty acids bind to a 40 kDa membrane protein. Both proteins function as carrier of their respective ligand across the plasma membrane. Although the driving forces for the influx of bilirubin are still unknown, it could be demonstrated that the translocation of fatty acids across the plasma membrane is driven by an active, potential-sensitive, sodium-dependent transport system.

Lack of linear correlation between hepatic ligand uptake rate and unbound ligand concentration does not necessarily imply receptor-mediated uptake

Previous studies of the hepatic uptake of several albumin-bound ligands, using constant and variable albumin concentrations, were interpreted as being inconsistent with the traditional mechanism of uptake, defined as an uptake rate directly proportional to unbound ligand concentration, and led to the formulation of the albumin receptor theory of hepatic uptake. Because other experimental designs have failed to confirm the albumin receptor theory, we reexamined, using the isolated perfused rat liver preparation, the traditional uptake mechanism under the conditions used in the original studies, of constant and variable albumin concentration. Livers (n = 6) were perfused in randomized sequence with 10 different solutions containing 24-14C-taurocholate in a single-pass design. Five solutions contained fixed albumin (0.1 mM) and variable taurocholate (3-48 microM) concentrations, and five maintained the taurocholate-albumin ratio fixed at 0.06; absolute concentrations of taurocholate varied from 3-48 microM, and of albumin from 0.05-0.08 mM. At constant albumin concentration in hepatic inflow, elimination rate of taurocholate was linearly related to both total (Cin) and unbound (Cin,u) taurocholate concentration in hepatic inflow, indicating first-order elimination kinetics. When taurocholate and albumin were increased in hepatic inflow in a fixed molar ratio, taurocholate uptake rate was not linearly related to Cin,u but was still consistent with the traditional uptake mechanism. Moreover, the apparent saturation of taurocholate uptake by added albumin was consistent with the reduction in unbound fraction (fu) in accordance with the traditional uptake mechanism. This study shows that although the traditional uptake mechanism dictates that ligand uptake rate is linearly related to unbound ligand concentration within the liver, uptake rate need not necessarily be linearly related to Cin,u. Therefore, experiments in which lack of a linear relationship between uptake rate and Cin,u is found do not necessarily imply receptor-mediated uptake.

[Detection of pre-S gene products in HBsAg carriers and in hepatitis B vaccines]

The immunogenicity and efficacy of different commercially available vaccines against hepatitis B virus (HBV) has been demonstrated in a number of controlled vaccination trials throughout the world. However, the significance of pre-S proteins in hepatitis B vaccines is still controversially discussed. Therefore, a radioimmunoassay (RIA) was developed to detect pre-S proteins in HBsAg-carriers as well as in different hepatitis B vaccines. This study confirmed the high amount of pre-S proteins as well as poly-albumin receptors in purified Dane particles and sera of patients with acute hepatitis B. The currently available plasma derived vaccine H-B-Vax and the recombinant hepatitis B vaccines Gen H-B-Vax and Engerix B yielded negative results in pre-S- and poly-albumin-receptor-RIAs. The plasma derived vaccine Hevac B Pasteur included pre-S encoded proteins. Future vaccination trials in man have to evaluate the role of pre-S antigens and antibodies in the pathogenesis and the protection of hepatitis B infection including recombinant pre-S vaccines which are under development.

[Quantitative and qualitative findings of hepatitis B virus-associated antigens, receptors and antibodies in the course of the acute phase of hepatitis B infection]

Receptors for pHSA may be the links between HBV and hepatocytes. Therefore, we evaluated the correlation between serum binding activity for pHSA during the acute phase of hepatitis B in relation to the decrease of surface antigen concentration.

METHODS

130 sera from 22 patients with acute (HBeAg-positive) hepatitis B were obtained weekly during an average interval of 7 weeks after the onset of the disease. R-pHSA were tested by RIA in 4 titre steps (1:50, 1:500, 1:5000, 1:50,000). HBsAg was analysed quantitatively by RIA according to the standard serum of the Paul-Ehrlich-Institute.

Identification of albumin-binding proteins in capillary endothelial cells

Isolated fat tissue microvessels and lung, whose capillary endothelia express in situ specific binding sites for albumin, were homogenized and subjected to SDS-gel electrophoresis and electroblotting. The nitrocellulose strips were incubated with either albumin-gold (Alb-Au) and directly visualized, or with [125I]albumin (monomeric or polymeric) and autoradiographed. The extracts of both microvascular endothelium and the lung express albumin-binding proteins (ABPs) represented by two pairs of polypeptides with major components of molecular mass 31 and 18 kD. The ABP peptides have pIs 8.05 to 8.75. Rabbit aortic endothelium, used as control, does not express detectable amounts of ABPs. The ABPs subjected to electrophoresis bind specifically and with high affinity (Kd = approximately 60 X 10(-9)M) both monomeric and polymeric albumin: the binding is saturable at approximately 80 nM concentration and 50% inhibition is reached at 5.5 micrograms/ml albumin concentration. Sulfhydryl-reducing agents beta-mercaptoethanol and dithiothreitol do not markedly affect the ABPs electrophoretic mobility and binding properties. As indicated by cell surface iodination of isolated capillary endothelium followed by electroblotting, autoradiography, and incubation with Alb-Au, the bands specifically stained by this ligand are also labeled with radioiodine.

Isolation and characterization of a 14-kDa albumin-binding fragment of streptococcal protein G

Protein G, a streptococcal cell wall protein, has separate binding sites for human albumin and IgG. Streptococci expressing protein G were treated with the bacteriolytic agent mutanolysin. Several IgG- and human serum albumin (HSA)-binding peptides were identified in the material thus solubilized and one of these, a 14-kDa peptide, was found to bind HSA but not IgG in Western blot experiments. This molecule was purified by affinity chromatography on Sepharose coupled with HSA followed by gel filtration on Sepharose 6B and a final affinity chromatography on IgG-Sepharose, by which low Mr W(15 to 20 kDa)IgG-binding peptides were removed. In different binding experiments the purified 14-kDa peptide bound exclusively HSA and the equilibrium constant between the peptide and HSA was determined to be 3.4 X 10(8) M-1. The relation between the 14-kDa molecule and protein G was studied by analyzing the N-terminal amino acid sequence of the peptide and comparing it with the previously determined protein G sequence. The 40 N-terminal amino acids were found to be identical with an amino acid sequence starting at position 62 in the protein G molecule. These and previous data enabled us to locate the albumin binding to the repetitively arranged domains in the N-terminal half of the protein G molecule.

High sensitivity towards monensin of receptor-mediated endocytosis of formaldehyde treated albumin by liver endothelial cells

Endocytosis of formaldehyde-treated serum albumin (f-albumin) in isolated liver sinusoidal endothelial cells was studied. Uptake occurs via the scavenger receptor and was found to be very sensitive to the ionophore monensin. Binding at 4 degrees C of f-albumin was reduced to 50% of control values by preincubation for 2 min with 2 microM monensin. Both uptake and degradation of f-albumin were more sensitive to monensin. No lag-phase in the inhibitory effect on uptake and degradation was detected. A concentration of 0.1 microM monensin reduced uptake of f-albumin by 50%. Degradation of internalized f-albumin was reduced by 50% in the presence of 0.2 microM monensin. Since uptake and degradation of f-albumin were very sensitive to monensin, the effect of introducing the drug during endocytosis of the ligand was tested. All processing of f-albumin stopped instantly upon addition of monensin; hence, there seems to be no step in the endocytic process beyond which monensin is ineffective. The data suggest that the scavenger receptor of liver endothelial cells is internalized and recycled very rapidly.

Uptake and metabolism of polymerized albumin by rat liver. Role of the scavenger receptor

Hepatitis B virus binds avidly to albumin polymers, which in turn may mediate viral attachment to liver cells. This hypothesis is critically dependent on prior results obtained using glutaraldehyde-polymerized human serum albumin as a model for naturally occurring albumin species. We used the perfused rat liver to characterize the uptake, cellular distribution, and metabolism of glutaraldehyde-polymerized human albumin. 125I-glutaraldehyde-polymerized human albumin was efficiently removed from the perfusate by the liver (29% extraction). However, few autoradiographic grains were located over hepatic parenchymal cells (6%). Instead, most glutaraldehyde-polymerized human albumin appeared to be removed by endothelial (59%) or Kupffer (31%) cells. Hepatic uptake was strongly inhibited by formaldehyde-treated monomeric albumin, a known ligand of the endothelial scavenger receptor for chemically modified proteins. After uptake, most glutaraldehyde-polymerized human albumin was rapidly degraded and released into the perfusate (74% within 60 min). This process was blocked by chloroquine and leupeptin, suggesting that it involves lysosomal acid hydrolases. We conclude that glutaraldehyde-polymerized albumin is efficiently cleared and degraded by the endothelial scavenger pathway. Glutaraldehyde-polymerized albumin therefore appears to be a poor model for predicting the hepatic handling of naturally occurring albumin species bound to hepatitis B virions. Even if viral particles were to follow this pathway, few would enter parenchymal hepatocytes.

[Hepatocyte receptors for polymerized albumin. Experimental study on isolated hepatocytes and in vivo animals]

In the present work receptors for polymerized rat serum albumin (P-RSA) were studied by incubation of radiolabelled polymers with rat hepatocytes isolated with mechanical techniques. Isolated rat hepatocytes showed a receptor activity for polymerized albumin and such a membrane receptor seems to be controlled by the cellular cytoskeleton. During rat total body scintigraphy receptor expression was detected exclusively in the liver. These observations support the hypothesis of a selective receptor activity of the liver cells for polymerized albumin. To confirm our results, further studies were performed on rat bile, after 125I-P-RSA infusion in vivo.

Receptors for polymerized human serum albumin in plasma derived hepatitis B vaccines

The presence of polymerized human serum albumin receptors (pHSA-R) in two hepatitis B virus (HBV) plasma derived vaccines (HB-Vax, Merck Sharp and Dohme; Hevac-B, Pasteur) was detected by three methods, using pHSA polystyrene coated beads and 125I-anti-HBs (method 1) and polyclonal (method 2) or monoclonal (method 3) peroxidase conjugated anti-HBs. Only a very weak reaction was found for pHSA-R in HB-Vax vaccine when the tests were performed in undiluted vaccine. No reactivity in 1/100 dilution (normally used to test pHSA-R in serum samples) was observed. In contrast, Hevac-B vaccine contained pHSA-R activity in 1/100 dilution as tested by any of the three methods. Furthermore, the level of pHSA-R detected in Hevac-B vaccine is similar to that observed in asymptomatic HBsAg carriers with the same HBsAg concentration. In summary, Hevac-B vaccine contains pHSA-R, whilst HB-Vax shows only weakly reacting pHSA-R, probably insufficient to develop anti-pHSA-R antibodies.

Binding of albumin fragments to surface receptor in A, C, and G streptococci

Fragments of human and bovine serum albumin were produced by treatment with trypsin at pH 8.15 and with pepsin at pH 3.5 and 3.7 in the presence of octanoic acid. A large fragment which included the C-terminal part of the native molecule was produced by trypsin treatment. The tryptic digest was subsequently treated with pepsin, resulting in smaller fragments. The ability of the fragments to bind to albumin-receptors on streptococci was investigated. According to Western blots only fragments with a mol. wt. of 23 kDa or more were able to bind to albumin-binding structures obtained from streptococci. The 23 kDa fragment was radiolabelled and tested for binding to whole bacteria. The fragment was capable of binding to albumin-reactive structures on group A, C, and G streptococci with the same species-specificity as native human and bovine serum albumin, respectively. Both the large 45 kDa tryptic fragment and the small 23 kDa fragment could bind to streptococci and could be dissociated by 2 M KSCN.

Albumin has no role in the uptake of copper by human fibroblasts

The mechanism of copper uptake by cells has been the subject of controversy for some time. This paper examines the possibility of a role for albumin in the uptake of copper by fibroblasts. Although the cells could accumulate copper from a copper-albumin complex, there was no evidence for either copper-albumin or albumin receptors on the cell surface. The possibility of a surface exchange mechanism for copper was examined. While copper uptake showed saturation with increasing concentrations of labelled copper-albumin, adding unlabelled copper to the incubation medium did not inhibit uptake. Adding albumin or histidine to the copper-albumin complex resulted in an inhibition of copper uptake. The results can only be explained by the cell taking up free copper from the incubation medium, with the albumin then releasing its copper to maintain the equilibrium between free and bound metal. Since, in vivo there is essentially no free copper in serum, it is concluded that albumin is most unlikely to play a role in the uptake of copper by fibroblasts.

Antibodies to polymerized human serum albumin in acute and chronic liver disease

Since antibodies to polyalbumin have been noted to occur in patients with hepatitis or cirrhosis, we investigated sera from 219 patients with a variety of acute and chronic liver diseases with and without HBsAg using an ELISA. The prevalence of circulating polyalbumin antibodies was shown to be significantly increased over that of healthy controls (6.7%) in patients with autoimmune chronic active hepatitis (67%), acute viral hepatitis (41%) and fulminant hepatic necrosis (38%), but such antibodies were absent in chronic hepatitis B patients and other liver diseases. Serial studies in acute hepatitis showed evidence of antibodies early in the course of illness with disappearance prior to full recovery. In acute hepatitis B, the presence of polyalbumin antibodies was significantly associated with female sex (p less than 0.01), 3-fold higher transaminase levels and shorter duration of clinical illness (less than 4 weeks in all cases). Polyalbumin antibodies appear to be associated with diseases characterized by active hepatocyte necrosis. Since they are evident early in acute hepatitis B when complexes of polyalbumin and virus are likely, these antibodies may play an adjunctive role to other hepatitis B-related antibodies in the clearance of hepatitis B virus infection.

In vivo binding and uptake of low-density lipoprotein-gold- and albumin-gold conjugates by parenchymal and sinusoidal cells of the fetal rat liver

To elucidate the participation of fetal rat liver cells in the receptor-mediated internalization of low-density lipoproteins (LDL), rat fetuses were injected with either LDL-gold or albumin-gold conjugates. The degree of binding and uptake of LDL-gold and albumin-gold by parenchymal and sinusoidal cells of the fetal rat liver differs markedly. Endothelial cells exhibit low LDL-gold uptake. In contrast, parenchymal cells internalize LDL-gold more actively (45 +/- 8 LDL conjugates/100 micrometers2 cytoplasm within 60 min). Kupffer cells exceed this value by a factor of 20. The uptake of albumin-gold by endothelial and Kupffer cells is high, whereas it is extremely low in parenchymal cells. Estradiol pretreatment causes a significant doubling (p less than 0.05) of the LDL-gold particle density/100 micrometers2 cytoplasm both in parenchymal and Kupffer cells, whereas estradiol has no effect on the albumin uptake. The results strongly indicate that LDL uptake by parenchymal and Kupffer cells in the fetal rat liver is mediated by estrogen-inducible receptors, which may correspond to B, E receptors in the adult liver.

Effect of phagocytosis on receptor distribution and endocytic activity in macrophages

Phagocytosis requires the internalization of a significant fraction of the plasma membrane and results in the intracellular deposition of large particles. We evaluated the effect of phagocytosis on the cellular distribution of recycling receptors and uptake of ligand to determine whether phagocytosis affects receptor behavior. Phagocytosis of zymosan, latex particles, or IgG-coated red blood cells by rabbit alveolar macrophages did not decrease the number of cell surface receptors for transferrin, alpha 2-macroglobulin X protease complexes, maleylated proteins, or mannosylated proteins. The number of surface receptors for transferrin was also unaltered in J774 cells, a macrophage-like cell line. In both cell types extensive phagocytosis did not affect the rate of receptor-mediated endocytosis or the distribution of receptors between the endosome and the cell surface. However, fluid phase pinocytosis was reduced by phagocytosis. The major reduction appeared to be not in the rate of internalization but rather in the delivery of fluid to the lysosome. These results demonstrate that internalization of a significant amount of the plasma membrane during phagocytosis does not diminish the number of receptors on the cell surface and has no effect on receptor-mediated ligand uptake.

Isolation of a specific albumin receptor from a group G streptococcal strain

The albumin receptor, a bacterial cell-wall protein with affinity for albumin, has been isolated from a bovine group G streptococcal strain (DG-8). Bacterial surface proteins were solubilized by boiling in 0.6M HCl for 5 min. The albumin receptor was isolated by sequential use of ion-exchange chromatography on DEAE-Sephadex A-50 and affinity chromatography on albumin coupled Sepharose 4B. The presence of albumin receptor in various pools and fractions during the isolation was followed by their ability to inhibit the binding of radiolabelled albumin to DG-8 bacteria. A highly purified albumin receptor was obtained according to Western blot analysis. The albumin binding band obtained showed a molecular weight of about 30,000. The purified receptor did not possess any IgG binding capacity. 50 micrograms albumin receptor was prepared from each gram of bacteria.