Methodological aspects of purification and reconstitution of transport proteins from mammalian plasma membranes

Overview on solubilization and lipid reconstitution of Na,K-ATPase: enzyme kinetic and biophysical characterization

Na,K-ATPase is a membrane protein which plays a vital role. It pumps Na⁺ and K⁺ ions across the cellular membranes using energy from ATP hydrolysis, and is responsible for maintaining the osmotic equilibrium and generating the membrane potential. Moreover, Na,K-ATPase has also been involved in cell signaling, interacting with partner proteins. Cardiotonic steroids bind specifically to Na,K-ATPase triggering a number of signaling pathways. Because of its importance, many efforts have been employed to study the structure and function of this protein. Difficulties associated with its removal from natural membranes and the concomitant search for appropriate replacement conditions to keep the protein in solution have presented a challenge that had to be overcome prior to carrying out biophysical and biochemical studies in vitro. In this review, we summarized all of the methods and techniques applied by our group in order to obtain information about Na,K-ATPase in respect to solubilization, reconstitution into mimetic system, influence of lipid composition, stability, oligomerization, and aggregation.

Identification and purification of the rat liver Golgi membrane UDP-N-acetylgalactosamine transporter

Glycosylation of glycoproteins, proteoglycans, and glycosphingolipids occurs mainly in the lumen of the endoplasmic reticulum and the Golgi apparatus. Nucleotide sugars, donors of all the sugars involved in Golgi glycosylation reactions, are synthesized in the cytoplasm and require specialized transporters to be translocated into the lumen of the Golgi apparatus. By controlling the supply of sugar nucleotides in the lumen of the Golgi apparatus, these transporters directly regulate the glycosylation of macromolecules transiting the Golgi. We have identified and purified the rat liver Golgi membrane UDP-N-acetylgalactosamine transporter. The transporter was purified to apparent homogeneity by a combination of conventional and dye color chromatography. An approximately 63,000-fold purification (6% yield) was achieved starting from crude rat liver Golgi membranes and resulting in a protein with an apparent molecular mass of 43 kDa. The transporter was active when reconstituted into phosphatidylcholine vesicles and could be specifically photolabeled with P3-(4-azidoanilido)-uridine-5'-[P1-32P]triphosphate, an analog of UDP-N-acetylgalactosamine. Native functional size determination on a glycerol gradient suggested that the transporter exists as a homodimer within the Golgi membrane.

Effect of calcium ions on rat osseous plate alkaline phosphatase activity

Rat osseous plate alkaline phosphatase is a metalloenzyme with two binding sites for Zn2+ (sites I and III) and one for Mg2+ (site II). This enzyme is stimulated synergistically by Zn2+ and Mg2+ (Ciancaglini et al., 1992) and also by Mn2+ (Leone et al., 1995) and Co2+ (Ciancaglini et al., 1995). This study was aimed to investigate the modulation of enzyme activity by Ca2+. In the absence of Zn2+ and Mg2+, Ca2+ had no effects on the activity of Chelex-treated, Polidocanol-solubilized enzyme. However, in the presence of 10 microM MgCl2, increasing concentration of Ca2+ were inhibitory, suggesting the displacement of Mg2+ from the magnesium-reconstituted enzyme. For calcium-reconstituted enzyme, Zn2+ concentrations up to 0.1 microM were stimulatory, increasing specific activity from 130 U/mg to about 240 U/mg with a K0.5 = 8.5 nM. Above 0.1 microM Zn2+ exerted a strong inhibitory effect and concentrations of Ca2+ up to 1 mM were not enough to counteract this inhibition, indicating that Ca2+ was easily displaced by Zn2+. At fixed concentrations of Ca2+, increasing concentrations of Mg2+ increased the enzyme specific activity from 472 U/mg to about 547 U/mg, but K0.5 values were significantly affected (from 4.4 microM to 38.0 microM). The synergistic effects observed for the activity of Ca2+ plus magnesium-reconstituted enzyme, suggested that these two ions bind to the different sites. A model to explain the effect of Ca2+ on the activity of the enzyme is presented.

Factors affecting the stability of the renal sodium/phosphate symporter during its solubilization and reconstitution

Phosphate is reabsorbed across the brush-border membrane of the proximal tubule by a specific sodium-dependent symporter. Like the other brush-border membrane transport proteins of the kidney, the phosphate carrier remains to be isolated in a functional state. To establish a set of parameters that allow to preserve its biological activity, the phosphate carrier was solubilized under systematically varied conditions and reconstituted into proteoliposomes. Successful reconstitution was achieved only when the extraction buffer contained lipids extracted from the renal brush-border membrane. Glycerol, an osmolyte which reduces the water activity of the solution, was also required. It could however be replaced by 150 mM sodium or potassium phosphate. Below this concentration and in the presence of glycerol, the ionic strength of the solution had little effect on the stability of the transporter, but sodium phosphate could not be replaced by sodium chloride. Phosphate transport in reconstituted vesicles depended on the concentration of detergent and pH of the extraction buffer. Finally, transport activity was increased when solubilization was carried out in the presence of a reducing agent, dithiothreitol. These results should be helpful during the purification and further characterization of the renal phosphate symporter.

Purification of the Golgi adenosine 3'-phosphate 5'-phosphosulfate transporter, a homodimer within the membrane

Sulfation of proteoglycans, secretory and membrane proteins, and glycolipids occurs in the lumen of the Golgi apparatus. Adenosine 3'-phosphate 5'-phosphosulfate (PAPS), the sulfate donor in these reactions, must be transported from the cytosol, its site of synthesis, into the lumen of the Golgi apparatus. We have identified and purified to apparent homogeneity the rat liver Golgi membrane PAPS transporter by a combination of conventional and affinity chromatography as well as photoaffinity radiolabeling with adenosine 3',5'-bisphosphate, a competitive inhibitor of PAPS transport. The transporter, a 75-kDa protein, was purified 70,000-fold over homogenate (6% yield) and transported PAPS into phosphatidylcholine liposomes selectively and in a saturable manner (apparent Km of 1.7 microM). Radiation target-inactivation analyses of the transport activity in rat liver Golgi vesicles, together with the above described biochemical approaches, demonstrate that the PAPS transporter within the Golgi membrane is a homodimer.

Intestinal absorption of beta-lactam antibiotics and oligopeptides. Functional and stereospecific reconstitution of the oligopeptide transport system from rabbit small intestine

The H(+)-dependent uptake system responsible for the enteral absorption of oligopeptides and orally active beta-lactam antibiotics was functionally reconstituted into liposomes. Membrane proteins from rabbit small intestinal brush border membrane vesicles were solubilized with n-octyl glucoside and incorporated into liposomes using a gel filtration method. At protein/lipid ratios of 1:10 and 1:40, the uptake of the orally active alpha-amino-cephalosporin, D-cephalexin into proteoliposomes was stimulated by an inwardly directed H+ gradient and was protein-dependent. In these proteoliposomes the binding protein for oligopeptides and beta-lactam antibiotics of Mr 127,000 could be labeled by direct photoaffinity labeling with [3H]benzylpenicillin revealing an identical binding specificity as in the original brush border membrane vesicles. The uptake system for beta-lactam antibiotics and oligopeptides showed a remarkable stereospecificity; only D-cephalexin was taken up by intact brush border membrane vesicles, whereas the L-enantiomer was not taken up to a significant extent. This stereospecificity for uptake was also seen after reconstitution of solubilized brush border membrane proteins into liposomes demonstrating a functional reconstitution of the peptide transporter. Both enantiomers however, bound to the 127-kDa binding protein as was shown by a decrease in the extent of photoaffinity labeling of the 127-kDa protein in the presence of both enantiomers. After reconstitution of subfractions of brush border membrane proteins obtained by wheat germ lectin affinity chromatography into proteoliposomes, only liposomes containing the 127-kDa binding protein showed a significant uptake of D-cephalexin whereas the L-enantiomer was not transported. The uptake rates for D-cephalexin into proteoliposomes correlated with the content of 127-kDa binding protein in these liposomes as was determined by specific photoaffinity labeling with [3H]benzylpenicillin. The purified 127-kDa binding protein was also reconstituted into liposomes and its ability for specific binding of substrates as well as stereospecific uptake of cephalexin could be restored. These results indicate that the binding protein for oligopeptides and beta-lactam antibiotics of Mr 127,000 mediates the stereospecific and H(+)-dependent transport of orally active beta-lactam antibiotics across the enterocyte brush border membrane. We therefore suggest that this 127-kDa binding protein is the intestinal peptide transport system (or a component thereof).

Sequential use of detergents for solubilization and reconstitution of a membrane ion transporter

Solubilization and reconstitution of the cardiac sarcolemmal Na+/Ca2+ exchanger by use of the anionic detergent cholate and its application for reconstitution of the exchanger following solubilization with zwitterionic or nonionic detergents is described. Solubilization and reconstitution with cholate provided a 32.6-fold enrichment of Na+/Ca2+ exchange activity over sarcolemmal vesicles (5.2 to 170 nmol/mg/s) with 202% recovery of total activity. In combination with asolectin, the cholate dilution technique (H. Miyamoto and E. Racker, J. Biol. Chem. 255, 2656, 1980) offers a rapid and simple means for reconstitution and provides good recovery of total and specific Na+/Ca2+ exchange activity. However, the use of anionic detergents for solubilization precludes the use of certain chromatographic procedures for protein purification. Conversely, nonionic and zwitterionic detergents permit effective use of available chromatographic techniques, but can be troublesome during reconstitution. We have combined the advantages of solubilization with nonionic and zwitterionic detergents with the advantages of reconstitution by cholate dilution. Reconstitution of the exchanger, after solubilization with 3-[(3-cholamidopropyl)-dimethyl-ammonio]-1-propanesulfonate (Chaps) or n-octyl-beta-D-glucoside, was accomplished by the addition of a cholate/asolectin medium followed by dilution. Na+/Ca2+ exchange activity was enriched 30.7-fold with 196% recovery with Chaps and 34.1-fold with 204% recovery with n-octyl-beta-D-glucoside. The presence of Chaps was found to shift the optimal asolectin concentration for reconstitution from 15 mg/ml (cholate alone) to 25 mg/ml. In addition, pelleting of proteoliposomes subsequent to reconstitution resulted in greatest recovery of total activity when volumes were kept below 1.0 ml.(ABSTRACT TRUNCATED AT 250 WORDS)

Group-selective reagent modification of the sodium- and chloride-coupled glycine transporter under native and reconstituted conditions

Glycine transporter from rat brain stem and spinal cord is inactivated by specific sulfhydryl reagents. Modification of lysine residues also promotes a decrease of the transporter activity but in a lesser extent than that promoted by thiol group reagents. Mercurials showed a more marked inhibitory effect than maleimide derivatives. SH groups display a similar reactivity for p-chloromercuribenzenesulfonate (pCMBS) and mersalyl in synaptosomal membrane vesicles and proteoliposomes reconstituted with the solubilized transporter. However, different reactivity is observed with N-ethylmaleimide (MalNEt), the greatest effect being attained in membrane vesicles. The rate of inactivation by pCMBS and MalNEt is pseudo-first-order showing time- and concentration-dependence. pCMBS and MalNEt decrease the Vmax for glycine transport and to a lesser extent act on the apparent Km. Treatment with dithiothreitol (DTT) of the transporter modified by pCMBS results in a complete restoration of transporter activity indicating that the effect exercised by the reagent is specific for cysteine residues on the protein. It is concluded that SH groups are involved in the glycine transporter function and that these critical residues are mostly located in a relatively hydrophilic environment of the protein.

Purification and reconstitution of the high affinity choline transporter

The high-affinity choline transporter has been solubilized from synaptosomal membranes by various detergents. The solubilized carrier protein has been incorporated into liposomes after removal of the detergent by dialysis. Using the reconstitution of choline transport activity as an assay, the components catalyzing choline translocation were purified from the detergent extract by ion-exchange chromatography on a Mono-Q column followed by immunoaffinity chromatography. Monitoring the active fractions by sodium dodecylsulfate polyacrylamide gel electrophoresis and isoelectrofocussing gave one major protein with an apparent molecular weight of about 90,000 and an isoelectric point of pH 4.7. The isolated protein appeared to be heavily glycosylated as shown by lectin binding; upon treatment with endoglycosidase F the polypeptide was degraded to an apparent molecular weight of about 65,000. Accumulation of choline into liposomes reconstituted with the purified protein was driven by artificially imposed sodium gradients and inhibited by hemicholinium-3.

Immunological recognition of sodium/D-glucose cotransporter from renal brush border membranes by polyclonal antibodies

Antisera prepared in rabbit to a D-glucose-inhibitable phlorizin binding component of the pig kidney brush border membrane precipitated more than 90 percent of the D-glucose-inhibitable phlorizin binding activity from a Triton extract. These antibodies also stimulated D-glucose uptake by native brush border membranes at low D-glucose concentrations (1 mM) and inhibited it at higher D-glucose concentrations. Immunoblotting was used to locate polypeptide subunits of the glucose transporter in polyacrylamide gels of proteins extracted from the brush border membranes. The antibodies labelled the Mr 70,000 phlorizin-binding component in both reducing and non reducing conditions. Two additional polypeptides with relative molecular mass of 120,000 and 45,000 were also recognized under the same conditions; they might correspond, respectively, to another Na+/D-glucose cotransport unit and to a post mortem degradation product.

Two substrate sites in the renal Na(+)-D-glucose cotransporter studied by model analysis of phlorizin binding and D-glucose transport measurements

Time courses of phlorizin binding to the outside of membrane vesicles from porcine renal outer cortex and outer medulla were measured and the obtained families of binding curves were fitted to different binding models. To fit the experimental data a model with two binding sites was required. Optimal fits were obtained if a ratio of low and high affinity phlorizin binding sites of 1:1 was assumed. Na+ increased the affinity of both binding sites. By an inside-negative membrane potential the affinity of the high affinity binding site (measured in the presence of 3 mM Na+) and of the low affinity binding site (measured in the presence of 3 or 90 mM Na+) was increased. Optimal fits were obtained when the rate constants of dissociation were not changed by the membrane potential. In the presence of 90 mM Na+ on both membrane sides and with a clamped membrane potential, KD values of 0.4 and 7.9 microM were calculated for the low and high affinity phlorizin binding sites which were observed in outer cortex and in outer medulla. Apparent low and high affinity transport sites were detected by measuring the substrate dependence of D-glucose uptake in membrane vesicles from outer cortex and outer medulla which is stimulated by an initial gradient of 90 mM Na+ (out greater than in). Low and high affinity transport could be fitted with identical Km values in outer cortex and outer medulla. An inside-negative membrane potential decreased the apparent Km of high affinity transport whereas the apparent Km of low affinity transport was not changed. The data show that in outer cortex and outer medulla of pig high and low affinity Na(+)-D-glucose cotransporters are present which contain low and high affinity phlorizin binding sites, respectively. It has to be elucidated from future experiments whether equal amounts of low and high affinity transporters are expressed in both kidney regions or whether the low and high affinity transporter are parts of the same glucose transport molecule.

Solubilization of membrane-bound matrix-induced alkaline phosphatase with polyoxyethylene 9-lauryl ether (polidocanol): purification and metalloenzyme properties

1. Matrix-induced alkaline phosphatase prepared from rat osseous plate was solubilized with polidocanol and purified on a Sephacryl S-300 column. 2. Purified solubilized alkaline phosphatase has a molecular weight of ca 115,000 and bind one magnesium and two zinc ions. At least 110 detergent molecules are bound to each enzyme molecule. 3. Solubilization and purification procedures did not destroy the ability of the enzyme to hydrolyze adenosine-5'-triphosphate, p-nitrophenylphosphate, pyrophosphate and bis p-nitrophenylphosphate. 4. Magnesium, manganese and cobalt ions are stimulators of PNPPase activity of solubilized enzyme whereas calcium and zinc ions are inhibitors.

Identification of an Mr 75000 component of the H+/D-glucose cotransporter from Zea mays with monoclonal antibodies directed against the mammalian Na+/D-glucose cotransporter

Monoclonal antibodies which interact with the mammalian Na+/D-glucose cotransporter and bind to Mr 75,000 and Mr 47,000 polypeptide components of this transporter have been described (Koepsell, H., Korn, K., Raszeja-Specht, A., Bernotat-Danielowski, S. and Ollig, D. (1988) J. Biol. Chem., 263, 18419-18429). The interaction of these antibodies with plasma membranes from Zea mays L. coleoptiles containing an H+/D-glucose cotransporter was studied. Four monoclonal antibodies cross-reacted with Mr 75,000 and Mr 33,000 polypeptides. One of these antibodies, which inhibits Na+/D-glucose cotransport in the kidney and stimulates Na+/D-glucose cotransport in intestine, stimulates electrogenic uptake of 3-O-methyl-D-[14C]glucose in plant membrane vesicles. The data indicate common epitopes in the mammalian Na+/D-glucose cotransporter and the H+/D-glucose cotransporter of plants and suggest that both transporters contain an Mr 75000 polypeptide component.

Solubilization and reconstitution of the sodium-and-chloride-coupled glycine transporter from rat spinal cord

Synaptic membranes from rat spinal cord were solubilized in the presence of 2% sodium cholate, phospholipids and 15% ammonium sulphate. The soluble extract was incorporated into liposomes consisting of asolectin and crude rat brain lipids. Reconstitution of the functional transporter protein was achieved by removal of detergent by gel filtration. Several parameters proved to be important for optimal reconstitution efficiency: (a) the lipid composition of the liposomes, (b) the type of detergent, and (c) the phospholipid/protein and detergent/protein ratio during reconstitution. In the reconstituted system, the transport of glycine showed a specific activity about twice that of native vesicles. The ionic dependence of the transport, the inhibitory effect of nigericin in the presence of external sodium and the stimulatory effect of valinomycin in the presence of internal potassium on glycine transport were preserved and more clearly observed in the reconstituted system. These results indicate that, in this preparation, the glycine transporter protein retains the same features displayed in the synaptic plasma membrane vesicles, namely dependence on sodium and chloride, electrogenicity and inhibitor sensitivity.

Functional reconstitution of prokaryote and eukaryote membrane proteins

A new strategy for the functional reconstitution of membrane proteins is described. This approach introduces a new class of protein stabilizing agents--osmolytes--whose presence at high concentration (10-20%) during detergent solubilization prevents the inactivations that normally occur when proteins are extracted from natural membranes. Osmolytes that act in this way include compounds such as glycerol and higher polyols (erythritol, xylitol, sorbitol), sugars (glucose, trehalose), and certain amino acids (glycine, proline, betaine). The beneficial effects of osmolytes are documented by reconstitution of a variety of prokaryote and eukaryote membrane proteins, including several proton- and calcium-motive ATPases, cation- and anion-linked solute carriers (symport and antiport), and a membrane-bound hydrolase from endoplasmic reticulum. In all cases, the presence of 20% glycerol or other osmolyte during detergent solubilization led to 10-fold or more increased specific activity in proteoliposomes. These positive effects did not depend on use of any specific detergent for protein solubilization, nor on any particular method of reconstitution, but for convenience most of the work reported here has used octylglucoside as the solubilizing agent, followed by detergent-dilution to form proteoliposomes. The overall approach outlined by these experiments is simple and flexible. It is now feasible to use reconstitution as an analytical tool to study the biochemical and physiological properties of membrane proteins.

A radioimmunoassay to screen for antibodies to native conformational antigens and analyse ligand-induced structural states of antigenic proteins

A radioimmunoassay is described in which antigenic protein was immobilized by incubating nitrocellulose filters of defined diameter with antigen-containing solutions. The amount of adsorbed antigen increased in a linear fashion over a wide range of antigen concentrations. The antigen-antibody reactions and the indicator reactions were performed by incubating the filters with appropriate solutions. During the test any contact of the antigen with air was avoided. Antigenic sites which are sensitive to protein denaturation by drying could be detected with the assay. The assay was also used to screen hybridoma supernatants for antibodies directed against Na+ cotransport proteins from renal brush-border membranes. Monoclonal antibodies were selected which showed different binding characteristics depending on whether or not substrates of Na+ cotransporters were present. Since binding of several antibodies was altered by two different substrates and not by non-transported control substances, these monoclonal antibodies were believed to interact with more than one transport system. One of the antibodies, which showed different antibody binding after addition of D-glucose or L-lactate, bound to a polypeptide component of the renal Na+-D-glucose cotransporter and was able to inhibit Na+ gradient-dependent D-glucose uptake in brush-border membrane vesicles (Koepsell, Korn, Raszeja-Specht, Bernotat-Danielowski, Ollig, 1988, J. Biol. Chem., in press). To investigate the effects of D-glucose and L-lactate on the binding of this antibody concentration dependence was measured. High and low affinity binding sites for D-glucose and L-lactate were characterized thereby demonstrating that the radioimmunoassay permits investigations of the properties of high and low affinity substrate binding sites.

Expression of neurotransmitter transport from rat brain mRNA in Xenopus laevis oocytes

To permit a molecular characterization of neurotransmitter transporter proteins, we have studied uptake activities induced in Xenopus laevis oocytes after injection of adult rat forebrain, cerebellum, brainstem, and spinal cord poly(A)+ RNA. L-Glutamate uptake could be observed as early as 24 hr after injection, was linearly related to the quantity of mRNA injected, and could be induced after injection of as little as 1 ng of cerebellar mRNA. Transport of radiolabeled L-glutamate, gamma-aminobutyric acid, glycine, dopamine, serotonin, and choline could be measured in single microinjected oocytes with a regional profile consistent with the anatomical distribution of particular neurotransmitter synthesizing soma. Forebrain L-glutamate and dopamine uptake, as well as cerebellar L-glutamate transport, were found to be Na+-dependent. Cerebellar mRNA-induced L-glutamate transport was both time and temperature-dependent, was saturable by substrate, suggesting a single activity with an apparent transport Km of 14.2 microM and a Vmax of 15.2 pmol/hr per oocyte, and was sensitive to inhibitors of brain L-glutamate transport. Thus, the oocyte L-glutamate transport induced by injection of adult rat cerebellar mRNA appears essentially identical to the high-affinity, Na+-dependent L-glutamate uptake found in brain slices and nerve terminals. Experiments with size-fractionated cerebellar mRNA reveal single, comigrating peaks for cerebellar L-glutamate and gamma-aminobutyric acid transport, with peak activity obtained in fractions of approximately 2.7 kilobases, suggesting the presence of single or similarly sized mRNAs encoding each of these activities.

Reconstitution of the L-lactate carrier from rat and rabbit erythrocyte plasma membranes

1. Rat and rabbit erythrocyte plasma-membrane proteins were solubilized with decanoyl-N-methylglucamide and reconstituted into liposomes. The procedure includes detergent removal by gel filtration, followed by a freeze-thaw step. 2. The rate of [1-14C]pyruvate uptake into these vesicles was inhibited by approx. 70% by alpha-cyano-4-hydroxycinnamate and p-chloromercuribenzenesulphonate. The extent of uptake at equilibrium was not affected by the presence of these inhibitors, but was dependent on the osmolarity of the suspending medium. 3. Reconstituted bovine erythrocyte membranes, which have no lactate carrier, showed a much slower time course of pyruvate uptake, with no inhibitor-sensitive component. 4. L- but not D-lactate competed for alpha-cyano-4-hydroxycinnamate-sensitive [1-14C]pyruvate uptake.

Hepatocellular uptake of aflatoxin B1 by non-ionic diffusion. Inhibition of bile acid transport by interference with membrane lipids

Aflatoxin B1 permeates isolated rat hepatocytes by non-ionic diffusion. Its uptake is neither saturable nor influenced by metabolic energy and not inhibited by treatment of cells with proteases. The initial rate of aflatoxin B1 uptake measured at 7 degrees C is between 40 and 50% compared to that at 37 degrees C. However, after an incubation period of 7 minutes identical equilibrium uptake is reached at both temperatures. The apparent activation energies, calculated for aflatoxin B1 uptake by Arrhenius diagrams ranged between 1.69 and 4.5 kcal/mol. A Q10 value of 1.34 was calculated for a temperature interval of 7-17 degrees C but decreased to 1.05 for the interval of 27-37 degrees C. Liposomes or lipoproteins added to the cell suspension inhibited the aflatoxin B1 uptake into hepatocytes. Liposomes mainly composed of unsaturated fatty acids bind twice as much aflatoxin B1 as those composed of saturated ones, indicating that the lipophilicity of the mycotoxin is crucial in the determination of its uptake into liver cells. At concentrations above 5 micrograms/ml, aflatoxin B1 inhibited the carrier-mediated uptake of cholic acid and of phalloidin into hepatocytes. This effect was reversible and abolished by washing the cells after preincubation with aflatoxin. In concentrations below 5 micrograms/ml the uptake of phallotoxin and cholic acid was however stimulated by 15-25%. These results indicate, that a carrier-mediated uptake into hepatocytes via the multispecific bile salt transporter is not responsible for the organoselective clearance of aflatoxins by the liver. On the other hand, the cholestatic effect of aflatoxin B1 results at least partially from the inhibition of the multispecific bile acid transport system. This inhibition may arise from affinity of aflatoxins to lipid domains of the cell membrane.

Reaction of monoclonal antibodies with plasma membrane proteins after binding on nitrocellulose: renaturation of antigenic sites and reduction of nonspecific antibody binding

The immunochemical reaction of monoclonal antibodies directed against native membrane proteins was investigated after their separation in sodium dodecyl sulfate polyacrylamide gels and electrotransfer to nitrocellulose. Nonspecific binding of antibodies to membrane proteins, which was increased by beta-mercaptoethanol treatment or heat denaturation of the antibodies, could be significantly reduced if 1 M D-glucose plus 10% (v/v) glycerol was added during the incubation with the antibodies. It was found that specific antibody binding was drastically reduced by SDS treatment of the membrane proteins. During the electrotransfer to nitrocellulose and the simultaneous removal of SDS, some increase in antibody binding was observed. Considerable renaturation of antigenic sites in the blotted proteins could be induced if the nitrocellulose blots were incubated for 16 h at 37 degrees C in phosphate-buffered saline. With the introduction of both modifications, the renaturation step, and the addition of D-glucose and glycerol to reduce nonspecific antibody binding, the immunoblot technique may be successfully applied to detect conformational antibodies against membrane proteins.