Showing papers on "Bovine serum albumin published in 1991"

Moisture-induced aggregation of lyophilized proteins in the solid state.

Abstract: A critical problem in the storage and delivery of pharmaceutical proteins is their aggregation induced by moisture. A model system has been elaborated and investigated to elucidate the mechanism of this phenomenon. When 10 mg of bovine serum albumin lyophilized from an aqueous solution of pH 7.3 are wetted with just 3 μL of a buffered physiological saline solution and incubated in the solid state at 37°C, the protein progressively loses its solubility in water; e.g., after a 24 h incubation 97% of the protein becomes insoluble. This moisture-induced aggregation of albumin has been discovered to be due to an intermolecular S—S bond formation via the thiol-disulfide interchange reaction. The dependence of the extent of the solid-state aggregation on the amount and mode of addition of moisture and the atmosphere, additives, temperature, and history of the protein powder have been investigated. The moisture-induced solid-state aggregation has also been established and studied for three other lyophilized proteins: ovalbumin, glucose oxidase, and β-lactoglobulin. In all cases, the loss of solubility is caused by thiol-disulfide interchange either alone or in combination with a conformational (noncovalent) process. The aggregation can be minimized by lyophilizing the proteins from acidic aqueous solutions, by adding inorganic salts, by co-lyophilizing the proteins with water-soluble polymers, or by controlling the moisture content at optimal levels.

Effects of formaldehyde fixation on protein secondary structure: a calorimetric and infrared spectroscopic investigation.

Abstract: We investigated the effects of formaldehyde fixation on the secondary structure of isolated proteins (bovine serum albumin, ribonuclease A, and hemoglobin) using high-sensitivity differential scanning calorimetry and Fourier transform infrared spectroscopy. Whereas thermograms obtained by scanning calorimetry on unfixed purified proteins demonstrated denaturation transitions in the 70-90 degrees C temperature range, the thermograms showed no denaturation transitions in this temperature range when the proteins had been placed in formaldehyde solutions. Thus, fixation destroyed the denaturation transition of bovine serum albumin, ribonuclease A, and hemoglobin. Infrared spectra obtained on the unfixed and fixed proteins were essentially identical. This demonstrates that the "fixed" proteins retain the secondary structure present before fixation. We therefore conclude that the cross-linking of proteins that occurs in the process of formaldehyde fixation "locks in" the secondary structure of these protein molecules.

Two novel rat liver membrane proteins that bind advanced glycosylation endproducts: relationship to macrophage receptor for glucose-modified proteins.

Abstract: Advanced glycosylation endproducts (AGEs), the glucose-derived adducts that form nonenzymatically and accumulate on tissue proteins, are implicated in many chronic complications associated with diabetes and aging. We have previously described a monocyte/macrophage surface receptor system thought to coordinate AGE protein removal and tissue remodeling, and purified a corresponding 90-kD AGE-binding protein from the murine RAW 264.7 cell line. To identify AGE-binding proteins in normal animals, the tissue distribution of 125I-AGE rat serum albumin taken up from the blood was determined in rats in vivo. These uptake studies demonstrated that the liver was a major site of AGE protein sequestration. Using a solid-phase assay system involving the immobilization of solubilized membrane proteins onto nitrocellulose to monitor binding activity, and several purification steps including affinity chromatography over an AGE bovine serum albumin matrix, two rat liver membrane proteins were isolated that specifically bound AGEs, one migrating at 60 kD (p60) and the other at 90 kD (p90) on SDS-PAGE. NH2-terminal sequence analysis revealed no significant homology between these two proteins nor to any molecules available in sequence databases. Flow cytometric analyses using avian antibodies to purified rat p60 and p90 demonstrated that both proteins are present on rat monocytes and macrophages. Competition studies revealed no crossreactivity between the two antisera; anti-p60 and anti-p90 antisera prevented AGE-protein binding to rat macrophages when added alone or in combination. These results indicate that rat liver contains at least two novel and distinct proteins that recognize AGE-modified macromolecules, although p90 may be related to the previously described 90-kD AGE receptor isolated from RAW 264.7 cells. The constitutive expression of AGE-binding proteins on rat monocytes and macrophages, and the sequestration of circulating AGE-modified proteins by the liver, provides further evidence in support of a role for these molecules in the normal removal of proteins marked as senescent by accumulated glucose-derived covalent addition products, or AGEs.

Thermal gelation of globular proteins: influence of protein conformation on gel strength

Abstract: Circular dichroic analysis of protein fluid expressed from globular protein gels at high centrifugal force has been used to elucidate the structural state of globular proteins in gels. In the case of bovine serum albumin gels, gelation involved transconformation of α-helix and aperiodic structures into β-sheet conformation. In the case of soy proteins, only a reduction in th eβ-sheet content and a increase in aperiodic structure content were observed in the gel state.

Transfer of oxygen from an artificial protease to peptide carbon during proteolysis

Abstract: Site-specific cleavage of proteins with metal chelates is an approach for designing artificial proteolytic reagents that are directed by proximity to a peptide bond rather than by an amino acid residue type. In the presence of ascorbate and H2O2, an iron chelate attached to Cys-212 of the enzyme human carbonic anhydrase I quickly cleaved the protein between residues Leu-189 and Asp-190 to produce two discrete fragments. The transfer of an 18O atom from [18O]H2O2 (or [18O]O2) to the carboxyl group of Leu-189 was demonstrated by mass spectrometry. Quantitative experiments revealed that one molecule of H2O2 and one molecule of ascorbate afforded the hydrolysis of one peptide bond (1:1:1 stoichiometry) and that the reaction required ascorbate and H2O2. The process is catalytic, since related experiments on the protein bovine serum albumin revealed two cleavage events for each polypeptide chain cleaved. Hydroxyl radical scavengers had no significant effect. These results may be explained by generation of a highly nucleophilic oxygen species, such as peroxide coordinated to the iron chelate, that attacks a carbonyl carbon nearby.

Locations of the three primary binding sites for long-chain fatty acids on bovine serum albumin.

Abstract: Binding of 13C-enriched oleic acid to bovine serum albumin and to three large proteolytic fragments of albumin--two complementary fragments corresponding to the two halves of albumin and one fragment corresponding to the carboxyl-terminal domain--yielded unique patterns of NMR resonances (chemical shifts and relative intensities) that were used to identify the locations of binding of the first 5 mol of oleic acid to the multidomain albumin molecule. The first 3 mol of oleic acid added to intact albumin generated three distinct NMR resonances as a result of simultaneous binding of oleic acid to three heterogeneous sites (primary sites). Two of these resonances were seen upon addition of 1 or 2 mol of oleic acid to fragments representing either the carboxyl-terminal half (residues 307-582) or the carboxyl-terminal domain (residues 377-582); the third resonance was seen upon addition of 1 mol of oleic acid to the fragment representing the amino-terminal half (residues 1-306). The resonance patterns for the fourth and fifth moles of oleic acid added to albumin (secondary sites) could not be duplicated by addition of more oleic acid to individual fragments. These resonance patterns were generated, however, when the two complementary fragments were mixed in equimolar proportions to form an albumin-like complex with a reconstituted middle domain. Thus, two primary fatty acid binding sites are assigned to the carboxyl-terminal domain, one primary site is assigned to the amino-terminal half, and the secondary sites are assigned to the middle domain. This distribution suggests albumin to be a less symmetrical binding molecule than theoretical models predict. This work also demonstrates the power of NMR for the study of microenvironments of individual fatty acid binding sites in specific domains.

Oxygen concentration and protein source affect the development of preimplantation goat embryos in vitro

Abstract: The effect of oxygen concentration and the source of protein in culture medium on the development of 2- to 4-cell goat embryos in vitro was investigated. Embryos were collected from superovulated Angora-Cashmere-cross goats 48 h after ovulation and cultured for 6 days in synthetic oviduct fluid (SOF) medium under one of two oxygen concentrations (20% or 7%) and in the presence of one of five protein sources; Miles bovine serum albumin (Miles BSA), Commonwealth Serum Laboratory bovine serum albumin (CSL BSA), goat serum (GS), fetal calf serum (FCS) and human serum (HS). In the presence of 20% oxygen the percentage of embryos reaching the expanded and/or hatched blastocyst stage in SOF medium containing Miles BSA was 29%, with a mean cell number per embryo of 28.1 +/- 6.0 (+/- s.e.m.). Use of an oxygen concentration of 7% significantly increased the percentage of embryos reaching this stage (80%, P less than 0.01) and the mean number of cells per embryo (65.3 +/- 8.2, P less than 0.01). The mean number of cells of the early-cleavage-stage embryos was significantly lower when the medium contained CSL BSA, GS or FCS (42.7 +/- 5.6, 29.0 +/- 6.1 and 21.3 +/- 3.2, respectively) than with Miles BSA (92.8 +/- 6.4) or HS (104.8 +/- 17.2) (P less than 0.01). Under 7% oxygen and with Miles BSA or HS, embryos were morphologically comparable to those developed in vivo, but the mean cell numbers in vitro were only approximately half those obtained in vivo.

Immunoaffinity isolation of urinary 8-hydroxy-2'-deoxyguanosine and 8-hydroxyguanine and quantitation of 8-hydroxy-2'-deoxyguanosine in DNA by polyclonal antibodies.

Abstract: An immunoaffinity column is described that facilitates the analysis of oxidative DNA damage. DNA adducts excised from DNA are excreted in urine and can be assayed as a measure of DNA damage in individuals. Polyclonal antibodies that recognize 8-hydroxy-2'-deoxyguanosine (oh8dG), a biomarker of oxidative damage to DNA, have been produced and their binding properties characterized. The antibodies, raised in rabbits following immunization with protein carrier-hapten conjugates prepared by covalently linking periodate-treated 8-hydroxyguanosine (oh8G) to bovine serum albumin (BSA) or casein, bind oh8dG with high affinity and selectivity, as measured by a competitive radioimmunoassay (RIA). Antibodies obtained from the rabbits immunized with the casein conjugate exhibited a binding affinity for oh8dG of 6.9 x 10(8) M-1. Studies on the relative binding affinities of these polyclonal antibodies for oh8dG, unmodified nucleosides, or derivatives of guanine indicate that the antibodies are suitable for the preparation of immunoaffinity columns that permit us to rapidly isolate oh8dG and 8-hydroxyguanine (oh8Gua) from urine. The high selectivity of the antibodies for oh8dG and oh8G reduces the amount of urinary contaminants previously observed in samples prepared by solid phase extraction, thus greatly facilitating the isolation of these damage products from urine. The relative binding affinity of these antibodies for oh8Gua and 2'-deoxyguanosine were approximately 7.6 x 10(3) and 7.4 x 10(4) fold lower respectively, than the binding affinity for oh8dG. The antibody can be used to quantitate oh8dG in enzymatic hydrolyzates of DNA with values comparable to those obtained by HPLC with electrochemical detection (HPLC-EC).

Characteristics of the adhesive determinants of Lactobacillus fermentum 104.

Abstract: The adhesion of Lactobacillus fermentum 104-R and the variant strain 104-S to porcine gastric squamous epithelium was investigated. An epithelium-specific adhesion was detected for strain 104-S; however, strain 104-R expressed enhanced adhesion capacity to the control surfaces of polystyrene and bovine serum albumin. To characterize the adhesive determinants, the bacterial cells were exposed to various treatments. The adhesion pattern of bacterial cells in buffers of pH values ranging from 2 to 7 was determined. The adhesion of strain 104-S to epithelium was greater in a buffer with a higher pH value. On the other hand, adhesion of strain 104-R to the epithelium was rather unaffected by a change in pH. To the control surfaces of polystyrene or bovine serum albumin, the adhesion of both strains was greatest at pH 2 to 4. Treatment of strain 104-S with metaperiodate did not affect the adhesion to epithelium or polystyrene; however, protease treatment dramatically decreased the adhesion of both strains, thus suggesting that the determinants responsible for the adhesion were proteinaceous. Carbohydrates may be partially involved in the adhesion of 104-R because metaperiodate-treated cells adhered more poorly than control, iodate-treated cells. The adhesion-promoting components are most probably tightly bound to the cell wall, because washing with low-pH buffer (pH 1.2) or sodium dodecyl sulfate had no major effect on the adhesion.

The biological activity of undenatured dietary whey proteins: role of glutathione.

Abstract: This study compared the effects of different sources of whey protein concentrate (20 g/100 g diet) and of casein on the spleen, liver, and heart glutathione content of C3H/HeJ mice, and on the immune response of their spleen cells to sheep red blood cells. Body weight curves were similar in all dietary groups. Our data indicate that the humoral immune response is highest in mice fed a dietary whey protein concentrate exhibiting the highest solubility (undenatured conformation) and a greater relative concentration of the thermolabile bovine serum albumin and immunoglobulins. In addition, the mice fed this type of whey protein concentrate exhibit higher levels of tissue glutathione. The presence in the serum albumin fraction of glutamylcysteine groups (rare in food protein) and the specific intramolecular bond as related to the undenatured conformation of the molecule are considered to be key factors in the glutathione-promoting activity of the protein mixture.

Effect of protein precipitating agents on the recovery of plasma free amino acids.

Abstract: The effect of the protein precipitants acetone, acetonitrile, perchloric acid and trichloroacetic acid on free amino acid concentrations in supernatants from ovine plasma and bovine serum albumin s...

Factors affecting the ability of glycosylphosphatidylinositol-specific phospholipase D to degrade the membrane anchors of cell surface proteins

Abstract: Mammalian serum and plasma contain high levels of glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD). Previous studies with crude serum or partially purified GPI-PLD have shown that this enzyme is capable of degrading the GPI anchor of several purified detergent-solubilized cell surface proteins yet is unable to act on GPI-anchored proteins located in intact cells. Treatment of intact ROS17/2.8, WISH or HeLa cells (or membrane fractions prepared from them) with GPI-PLD purified from bovine serum by immunoaffinity chromatography gave no detectable release of alkaline phosphatase into the medium. However, when membranes were treated with GPI-PLD in the presence of 0.1% Nonidet P-40 substantial GPI anchor degradation (as measured by Triton X-114 phase separation) was observed. The mechanism of this stimulatory effect of detergent was further investigated using [3H]myristate-labelled variant surface glycoprotein and human placental alkaline phosphatase reconstituted into phospholipid vesicles. As with the cell membranes the reconstituted substrates exhibited marked resistance to the action of purified GPI-PLD which could be overcome by the inclusion of Nonidet P-40. Similar results were obtained when crude bovine serum was used as the source of GPI-PLD. These data indicate that the resistance of cell membranes to the action of GPI-PLD is not entirely due to the action of serum or membrane-associated inhibitory factors. A more likely explanation is that, in common with many other eukaryotic phospholipases, the action of GPI-PLD is restricted by the physical state of the phospholipid bilayer in which the substrates are embedded. These data may account for the ability of endothelial and blood cells to retain GPI-anchored proteins on their surfaces in spite of the high levels of GPI-PLD present in plasma.

Effects of Meat and Selected Food Components on the Valence of Nonheme Iron during In Vitro Digestion

Abstract: Our objective was to determine whether meat and other dietary factors have the capacity to reduce nonheme ferric iron to the ferrous form during digestion. Beef, selected organic acids, selected purified proteins, red blood cells, whole blood, or blood plasma was mixed with FeCIJ and subjected to simulated gastrointestinal digestion. Ferrozine was used to monitor the formation of dialyzable Fe(H). Ascorbic acid, glutathione and cysteine produced large increases in Fe(H) while meat (raw and cooked), hemoglobin and red blood cells yielded smaller increases. Casein, plasma, bovine serum albumin and egg albumin did not affect Fe(I1) formation. Our Results suggest dietary factors which enhance iron absorption in vivo promote Fe(W) reduction in the intestinal lumen.

Interactions of annexins with membrane phospholipids.

Abstract: The annexins are proteins that bind to membranes and can aggregate vesicles and modulate fusion rates in a Ca2(+)-dependent manner. In this study, experiments are presented that utilize a pyrene derivative of phosphatidylcholine to examine the Ca2(+)-dependent membrane binding of soluble human annexin V and other annexins. When annexin V and other annexins were bound to liposomes containing 5 mol % acyl chain labeled 3-palmitoyl-2-(1-pyrenedecanoyl)-L-alpha-phosphatidylcholine, a decrease in the excimer-to-monomer fluorescence ratio was observed, indicating that annexin binding may decrease the lateral mobility of membrane phospholipids without inducing phase separation. The observed increases of monomer fluorescence occurred only with annexins and not with other proteins such as parvalbumin or bovine serum albumin. The extent of the increase of monomer fluorescence was dependent on the protein concentration and was completely and rapidly reversible by EDTA. Annexin V binding to phosphatidylserine liposomes was consistent with a binding surface area of 59 phospholipid molecules per protein. Binding required Ca2+ concentrations ranging between approximately 10 and 100 microM, where there was no significant aggregation or fusion of liposomes on the time scale of the experiments. The polycation spermine also displaced bound annexins, suggesting that binding is largely ionic in nature under these conditions.

Chemiluminescence detection of proteins from single cells.

Abstract: The analysis of proteins from single cells requires techniques of supreme sensitivity. Although radiochemical procedures are capable of detecting small amounts of electrophoretically separated proteins, their sensitivity falls short of that required for routine detection of minor components of single cells. Utilizing the avidin-biotin interaction and the alkaline phosphatase substrate 3-(4-methoxyspiro[1,2-dioxetane-3,2'- tricyclo-[3.3.1.1(3,7)]decan]-4-yl)phenyl phosphate (AMPPD), we have developed an alternative, chemiluminescence-based method for protein detection whose sensitivity exceeds that of other methods. Applying this method to a purified protein, we could detect as little as 63 fg (0.9 amol) of biotinylated bovine serum albumin. The sensitivity of the method was demonstrated by the detection of proteins from individual photoreceptor outer segments, including proteins constituting approximately 1% of the total. Chemiluminescence detection also proved extremely sensitive for immunoblotting: a comparison of five methods for detection of antibody-antigen interactions showed that the AMPPD technique was more sensitive than detection with a colorimetric alkaline phosphatase substrate, 125I-labeled protein A, 125I-labeled anti-mouse IgG, or colloidal gold-conjugated anti-mouse IgG.

Modulation of Endotoxic Activity of Lipopolysaccharide by High-Density Lipoprotein

Abstract: Unlike agonists such as cytokines or hormones, the biological activity of bacterial lipopolysaccharide (LPS) is substantially modified by serum proteins. One such interaction in serum is with high-density lipoprotein (HDL) forming LPS-HDL complexes. LPS-HDL complexes have been previously shown to have reduced endotoxic activity, for example pyrogenicity, when compared to other forms of LPS in animal models. In this study, we report results of studies comparing the potency of LPS-HDL complexes with uncomplexed LPS as agonists for interleukin-1 (IL-1) production by two different sources of monocytes. LPS-HDL complexes were purified by ultracentrifugation in sodium bromide gradients. The human monocytic cell line THP-1 and the freshly isolated human monocytes, purified by adherence or elutriation from venous blood from healthy donors, were exposed to medium alone containing 1 mg/ml bovine serum albumin, HDL, LPS (parent LPS) and LPS-HDL complexes. mRNA level was analyzed on Northern blot, and cell-associated protein and supernatants were tested for IL-1 production using immunologic and biologic assays. LPS stimulates substantially more IL-1 mRNA and cell-associated IL-1 protein when the monocytes are stimulated with LPS alone versus LPS-HDL. These data suggest that LPS-HDL complexation may contribute to a reduction in endotoxic activities in vivo by preventing LPS (lipid A) from generating important transmembrane signals after binding to cells.

Alboaggregin-B: a new platelet agonist that binds to platelet membrane glycoprotein Ib.

Abstract: A new protein, called alboaggregin-B (AL-B), has been isolated from Trimeresurus albolabris venom by ion-exchange chromatography. It agglutinated platelets without the need for Ca2+ or any other cofactor. The purified protein showed an apparent molecular mass on SDS-PAGE and gel filtration of about 23 kDa under nonreducing conditions. Ristocetin did not alter the binding of AL-B to platelets or affect AL-B-induced platelet agglutination. Agglutinating activity was not dependent on either proteolytic or lectin-like activity in AL-B. Binding analysis showed that AL-B bound to platelets with high affinity (Kd = 13.6 +/- 9.3 nM) at approximately 30,800 +/- 14,300 binding sites per platelet. AL-B inhibited the binding of labeled bovine von Willebrand factor (vWF) to platelets. Monoclonal antibodies against the 45-kDa N-terminal domain of platelet glycoprotein Ib inhibited the binding both of AL-B and of bovine vWF to platelets, and also inhibited platelet agglutination induced by AL-B and bovine vWF. Specific removal of the N-terminal domain of GPIb by treatment of the platelets with elastase or Serratia marcescens protease reduced the binding of labeled AL-B and bovine vWF to platelets and blocked platelet agglutination caused by both agonists. Monoclonal antibodies to glycoprotein IIb/IIIa, to bovine vWF, and to bovine serum albumin did not show any effect on the binding of AL-B to platelets. Our results indicate that the binding domain for AL-B on platelet GPIb is close to or identical with the one for vWF. This new protein may be a very useful tool for studying the interaction between platelets and vWF.

Liposomal Delivery of Purified Heat Shock Protein hsp70 Into Rat Pancreatic Islets as Protection Against Interleukin 1β–Induced Impaired β-Cell Function

Abstract: Recently it has been demonstrated that heat shock protein 70 (hsp70) is induced in pancreatic islet cells during prolonged exposure to interleukin 1 beta (IL-1 beta). It is unclear whether this represents a cellular defense against the noxious action of IL-1 beta or whether hsp70 is involved in the suppressive action of the cytokine. To assess the role for hsp70 in isolated islets exposed to IL-1 beta, hsp70 was purified and introduced into cells of isolated rat pancreatic islets via the liposome technique. Delivery of hsp70 was efficient according to immunoblot analysis, but delivered hsp70 disappeared within 16 h. Hsp70-containing liposomes did not affect protein synthesis, insulin secretion, or islet insulin mRNA content. However, when hsp70 liposome-incubated islets were further exposed to IL-1 beta (25 U/ml) for 16 h, these islets released more insulin in response to glucose stimulation and contained more insulin mRNA than islets incubated with control liposomes and subsequently exposed to the cytokine. No protective effect of liposomes containing bovine serum albumin or ovalbumin were observed. We conclude that hsp70 may protect against IL-1 beta-induced impairment of pancreatic beta-cell function.

Phenolic glycolipid-1 of Mycobacterium leprae binds complement component C3 in serum and mediates phagocytosis by human monocytes.

Abstract: Previous studies from this laboratory have demonstrated that Mycobacterium leprae, an obligate intracellular bacterial parasite, enters human mononuclear phagocytes via complement receptors on these host cells and bacterium-bound C3. The present study investigates the role of M. leprae surface molecules in C3 fixation and phagocytosis. By enzyme-linked immunosorbent assay, C3 binds selectively to phenolic glycolipid-1 (PGL-1), a major surface molecule of the leprosy bacillus. C3 fixation to PGL-1 is serum concentration dependent and is abolished in heat-inactivated serum or serum containing ethylenediaminetetraacetic acid. C3 fixation is also abolished in serum containing ethyleneglycol-bis (beta-aminoethyl ether)N,N,N'-tetraacetic acid and MgCl2 indicating that isolated PGL-1 fixes C3 via the classical complement pathway. The capacity of PGL-1 to fix C3 is dependent upon its terminal trisaccharide since sequential removal of monosaccharide units of the trisaccharide results in a stepwise reduction in C3 fixation. Deacylation of PGL-1 also abolishes C3 fixation. C3 fixes to the trisaccharide of PGL-1 that is chemically linked to bovine serum albumin via the chemical carrier, 8-methoxycarbonyloctanol. PGL-1 mediates C3 fixation to polystyrene microspheres, and PGL-1 and C3 together mediate ingestion of polystyrene microspheres by human monocytes, wherein these inert test particles reside in membrane-bound phagosomes. Thus, complement receptors on mononuclear phagocytes, complement component C3, and PGL-1 comprise a three-component receptor-ligand-acceptor molecule system for mediating phagocytosis of M. leprae.

Isolation and characterization of a mannose receptor from human pigment epithelium.

Abstract: Recent work demonstrated that a mannose receptor is involved in the phagocytosis of rod outer segments by the rat retinal pigment epithelium (RPE). In this study the binding of soluble mannose-containing ligands by human RPE explants is described. In addition, the authors report the isolation of a mannose receptor from human RPE and describe its relationship to the macrophage mannose receptor. Epithelial explants bound the soluble ligand 125I-mannose bovine serum albumin (BSA) by a mannose-specific process. The protein involved in mannose recognition was extracted from human tissue and purified using ligand-affinity chromatography. The protein that bound to the affinity column had a molecular weight of 175 kD by sodium dodecyl sulfate gel electrophoresis and migrated with the same mobility as the human macrophage mannose receptor. Antibodies directed against the macrophage receptor crossreacted with the mannose receptor from human RPE by immunoblot analysis. Binding specificity studies demonstrated that mannose and mannan inhibited ligand binding to the purified receptor by 65% and 90%, respectively; galactose had no effect. Using immunogold labeling of human RPE cells in explant culture, antimacrophage mannose receptor was localized at the apical plasma membrane. These results suggest that human RPE expresses a mannose receptor on its apical surface (as does the rat RPE) and that this receptor is similar to the human macrophage mannose receptor.