Direct photoaffinity labeling of leukotriene binding sites

Due to their conjugated double bonds the leukotrienes themselves are photolabile compounds and may therefore be used directly for photoaffinity labeling of leukotriene binding sites. Cryofixation eliminates unspecific labeling taking place in solution by photoisomers and photodegradation products of leukotrienes. After fixation of receptor ligand interactions by shock-freezing of the samples, irradiation-induced highly reactive excited states and/or intermediates can form covalent bonds with the respective binding site in the frozen state. After cryofixation of a solution of albumin incubated with [3H8]leukotriene E4, irradiation at 300 nm resulted in time-dependent incorporation of radioactivity into the protein. Photoaffinity labeling of rat as well as of human blood serum with [3H8]leukotriene E4 after cryofixation revealed that only one polypeptide with an Mr of 67,000 was labeled. This polypeptide was identified as albumin. Photoaffinity labeling of rat liver membrane subfractions enriched with sinusoidal membranes resulted in the labeling of a polypeptide with an apparent Mr of 48,000, whereas no polypeptide was predominantly labeled in the subfraction enriched with canalicular membranes. Photoaffinity labeling of isolated hepatocytes disclosed different leukotriene E4 binding polypeptides. In the particulate fraction of hepatocytes a polypeptide with an apparent Mr of 48,000 was labeled predominantly, whereas in the soluble fraction several polypeptides were labeled to a similar extent. One of these, with an apparent Mr of 25,000, was identified as subunit 1 of glutathione transferases by immunoprecipitation. The method of direct photoaffinity labeling in the frozen state after cryofixation using leukotrienes as photoactivatable compounds, as exemplified by leukotriene E4, may be most useful for the identification and characterization of various leukotriene binding sites, including receptors, leukotriene-metabolizing enzymes, and transport systems.     

Insulin-like growth factor-binding protein from human plasma. Purification and characterization

Insulin-like growth factor (IGF)-binding protein (BP) has been purified from Cohn fraction IV of human plasma by acidification, ion exchange to remove endogenous ligands, and affinity chromatography on agarose-IGF-II. The pure protein appeared as a single peak by high performance reverse-phase and gel permeation chromatography (molecular mass, 45-50 kDa), but on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gave a major band at 53 kDa and a minor band at 47 kDa, unreduced, or 43 and 40 kDa, respectively, reduced. The two bands stained for both protein and carbohydrate. After storage at 2 degrees C for 5 months at pH 3, two additional bands, at 26 and 22 kDa on unreduced gels, were also present. Autoradiography after affinity labeling with IGF-I or IGF-II tracer revealed a single labeled band of 61 kDa. BP, quantitated using a specific radioimmunoassay, was retained by agarose-immobilized IGF-I, IGF-II, concanavalin A, and wheat germ lectin, but not Helix pomatia lectin. Competitive binding curves using pure BP and human IGF-I and IGF-II as both labeled and unlabeled ligands indicated association constants of 2-3 X 10(10) liters/mol for both peptides, with a slightly higher affinity for IGF-II than IGF-I, and 0.9 binding sites for either peptide per 53-kDa protein. The exact relationship of this acid-stable IGF BP to the 150-kDa complex from which it is derived remains to be determined.     

The labeled antigen method of immunoenzymatic staining.

A two stage immunohistological technique (the "labeled antigen" procedure) has been assessed for the detection of a variety of human and animal cytoplasmic constituents in tissue sections. In this method specific antiserum is followed by antigen complexed to horseradish peroxidase or to alkaline phosphatase. The primary antibody acts bivalently, linking the labeled antigen to antigen in the tissue section. The major advantage of this technique is that nonantigen specific antibody in the primary antiserum cannot cause nonspecific staining since it has no affinity for the antigen:enzyme complex. Consequently the specificity of the reaction is assured, background staining is minimized and the total staining time (from wax section to mounted slide) can be reduced to as little as 30 min. Further advantages include the possibility of labeling Ig allotypes and the high efficiency of enzyme utilization. Covalent human IgG:horseradish peroxidase complexes can also be used in a triple sandwich in conjunction with human anti-viral or autoimmune antibodies.     

WGA-binding, mucin glycoproteins protect the apical cell surface of mouse uterine epithelial cells.

Expression of apical cell surface proteins and glycoproteins was examined in polarized primary cultures of mouse uterine epithelial cells (UEC). Lectin-gold cytochemistry revealed that wheat germ agglutinin (WGA) bound specifically to the components of the apical glycocalyx as well as intracellular vesicles. Double labeling with the pH sensitive dye 3-(2,4-dinitroanilino)-3'amino-N-methyldipropylamine (DAMP) demonstrated the acidic nature of the WGA-staining intracellular vesicles. The enzymatic and chemical sensitivities of the WGA binding sites on the apical cell surface were monitored both by WGA-gold staining as well as by 125I-WGA binding assays. In thin sections, a large fraction of these sites were removed by pronase; however, application of a wide variety of proteases, glycosidases, or chemical treatments to the apical surface of intact UEC failed to reduce WGA binding. In no case did treatments designed to remove sialic acids reduce 125I-WGA binding more than 12%. In contrast, endo-beta-galactosidase as well as a combination of beta-galactosidase with beta-hexosaminidase succeeded in removing 28% and 77% of these sites, respectively. These studies suggested that the majority of the apically disposed WGA binding sites involved N-acetylglucosamine residues rather than sialic acids and included lactosaminoglycans. Many of the proteins detected at the apical cell surface by lactoperoxidase-catalyzed radioiodination were WGA-binding glycoproteins. A major class of these glycoproteins displayed Mr > 200 kDa by SDS-PAGE and was heavily labeled metabolically by 3H-glucosamine or by vectorial labeling at the apical cell surface with galactosyl transferase and UDP-3H-galactose. Analyses of the 3H-labeled oligosaccharides labeled by either procedure indicated that a large fraction of the apically disposed WGA-binding oligosaccharides consisted of neutral, O-linked mucin-type structures with median MW of approximately 1,500. Oligosaccharides in this fraction were partially (15%) sensitive to endo-beta-galactosidase digestion and bound to Datura stramonium agglutinin (68%), demonstrating the presence of lactosaminoglycan sequences. UEC were an extremely effective barrier to attachment or invasion by either a highly invasive melanoma cell line, B16-BL6, or implantation-competent mouse blastocysts. In contrast, neither uterine stromal cells nor a non-polarizing UEC cell line, RL95, prevented B16-BL6 attachment.(ABSTRACT TRUNCATED AT 400 WORDS)     

Separate subunits for agonist and benzodiazepine binding in the gamma-aminobutyric acidA receptor oligomer

The gamma-aminobutyric acidA (GABAA) agonist muscimol can be photoactivated by 254 nm illumination to affinity label its binding site in the GABAA receptor. We have conducted this reaction on the pure receptor from bovine cerebral cortex in detergent solution, showing that [3H]muscimol can produce then a specific saturable labeling. In the detergent solution, the receptor alone is sensitive to 254 nm irradiation; this reduces the efficiency of incorporation to below that in the membranes, but the competing photoreaction with [3H]muscimol is sufficient and occurs at a representative set of the muscimol-binding sites, such that it can be employed for the photolabeling of those sites. The affinity of [3H]muscimol displayed in this irreversible reaction is indistinguishable from that of its reversible binding. gamma-Aminobutyric acid and bicuculline compete in the photolabeling reaction according to their known affinities at the gamma-aminobutyric acid-binding site. The labeling is shown to occur at the beta-subunit (apparent Mr 57,000) in the pure receptor. The binding sites for gamma-aminobutyric acid agonists, on the beta-subunits, and the benzodiazepine binding sites, on the alpha-subunits, are linked allosterically so that a strongly cooperative hetero-oligomeric structure of this receptor is deduced.     

Identification of cysteine 530 as the covalent attachment site of an affinity-labeling estrogen (ketononestrol aziridine) and antiestrogen (tamoxifen aziridine) in the human estrogen receptor

Radiosequence analysis of peptide fragments of the estrogen receptor (ER) from MCF-7 human breast cancer cells has been used to identify cysteine 530 as the site of covalent attachment of an estrogenic affinity label, ketononestrol aziridine (KNA), and an antiestrogenic affinity label, tamoxifen aziridine (TAZ). ER from MCF-7 cells was covalently labeled with [3H]TAZ or [3H]KNA and purified to greater than 95% homogeneity by immunoadsorbent chromatography. Limit digest peptide fragments, generated by prolonged exposure of the labeled receptor to trypsin, cyanogen bromide, or Staphylococcus aureus V8 protease, were purified to homogeneity by high performance liquid chromatography (HPLC), and the position of the labeled residue was determined by sequential Edman degradation. With both aziridines, the labeled residue was at position 1 in the tryptic peptide, position 2 in the cyanogen bromide peptide, and position 7 in the V8 protease peptide. This localizes the site of labeling to a single cysteine at position 530 in the receptor sequence. The identity of cysteine as the site of labeling was confirmed by HPLC comparison of the TAZ-labeled amino acid (as the phenylthiohydantoin and phenylthiocarbamyl derivatives) and the KNA-labeled amino acid (as the phenylthiocarbamyl derivative) with authentic standards prepared by total synthesis. Cysteine 530 is located in the hormone binding domain of the receptor, near its carboxyl terminus. This location is consistent with earlier studies using sodium dodecyl sulfate-polyacrylamide gel electrophoresis to analyze the size of the proteolytic fragments containing the covalent labeling sites for TAZ and KNA and the antigen recognition sites for monoclonal antibodies. The fact that both the estrogenic and antiestrogenic affinity labeling agents react covalently with the same cysteine indicates that differences in receptor-agonist and receptor-antagonist complexes do not result in differential covalent labeling of amino acid residues in the hormone binding domain.     

Agonist-induced changes in the structure of the acetylcholine receptor M2 regions revealed by photoincorporation of an uncharged nicotinic noncompetitive antagonist.

To characterize structural changes induced in the nicotinic acetylcholine receptor (AChR) by agonists, we have mapped the sites of photoincorporation of the cholinergic noncompetitive antagonist 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine (]125I]TID) in the presence and absence of 50 microM carbamylcholine. [125I]TID binds to the AChR with similar affinity under both these conditions, but agonist inhibits photoincorporation into all subunits by greater than 75% (White, B. H., Howard, S., Cohen, S. G., and Cohen, J. B. (1991) J. Biol. Chem. 266, 21595-21607). [125I]TID-labeled sites on the beta- and delta-subunits were identified by amino-terminal sequencing of both cyanogen bromide (CNBr) and tryptic fragments purified by Tricine sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by reversed-phase high-performance liquid chromatography. In the absence of agonist, [125I]TID specifically labels homologous aliphatic residues (beta L-257, delta L-265, beta V-261, and delta V-269) in the M2 region of both subunits. In the presence of agonist, labeling of these residues is reduced approximately 90%, and the distribution of labeled residues is broadened to include a homologous set of serine residues at the amino terminus of M2. In the beta-subunit residues beta S-250, beta S-254, beta L-257, and beta V-261 are all labeled in the presence of carbamylcholine. This pattern of labeling supports an alpha-helical model for M2 with the labeled face forming the ion channel lumen. The observed redistribution of label in the resting and desensitized states provides the first direct evidence for an agonist-dependent rearrangement of the M2 helices. The efficient labeling of the resting state channel in a region capable of structural change also suggests a plausible model for AChR gating in which the aliphatic residues labeled by [125I]TID form a permeability barrier to the passage of ions. We also report increased labeling of the M1 region of the delta-subunit in the presence of agonist.     

Identification and localization of cholecystokinin-binding sites on rat pancreatic plasma membranes and acinar cells: a biochemical and autoradiographic study

Using the combined approaches of affinity labeling and light and electron microscopic autoradiography, we investigated the identification and localization of cholecystokinin (CCK)-binding sites on rat pancreatic acinar cells. To define the molecular properties of the CCK-binding site, we incubated rat pancreatic plasma membranes with 125-I-CCK-33 for 15 min at 23 degrees C followed by washing and cross- linking with disuccinimidyl suberate. Specific labeling of a major Mr 85,000 component was revealed as assessed by SDS PAGE under reducing conditions and autoradiography of the dried gels. Components of Mr greater than 200,000, Mr 130,000-140,000, and, Mr 55,000 were labeled under maximal cross-linking conditions. The labeling of all components was specifically inhibited by CCK-8 in a dose-dependent manner (Kd approximately 9 nM). The Mr 85,000 component had identical electrophoretic mobilities under reducing and nonreducing conditions indicating that it likely does not contain intramolecular disulfide bonds. The larger labeled species may be cross-linked oligomers of this binding protein or complexes between it and neighboring polypeptides. For studies on the distribution of CCK-binding sites, pancreatic acini were incubated with 125I-CCK-33 (0.1 nM) in the absence or presence of CCK-8 (1 microM) for 2 or 15 min at 37 degrees C, washed, and fixed in 2% glutaraldehyde. Quantitative autoradiographic analysis indicated that approximately 60% of the total grains were located within +/- 1 HD (1 HD = 100 nm) of the lateral and basal plasmalemma with little or no labeling of the apical plasmalemma. From these data, it was estimated that each acinar cell possesses at least 5,000-10,000 CCK-binding sites on its basolateral plasmalemma. The remaining grains showed no preferential concentration over the cytoplasm or nucleus. Together, these data indicate that CCK interacts with a Mr 85,000 protein located on the basolateral plasmalemma of the pancreatic acinar cell.     

Mapping of the acetylcholine binding site of the nicotinic acetylcholine receptor: [3H]nicotine as an agonist photoaffinity label

The agonist [3H]nicotine was used as a photoaffinity label for the acetylcholine binding sites on the Torpedo nicotinic acetylcholine receptor (AChR). [3H]nicotine binds at equilibrium with Keq = 0.6 microM to the agonist binding sites. Irradiation with 254-nm light of AChR-rich membranes equilibrated with [3H]nicotine resulted in covalent incorporation into the alpha- and gamma-subunits, which was inhibited by agonists and competitive antagonists but not by noncompetitive antagonists. Inhibition of labeling by d-tubocurarine demonstrated that the alpha-subunit was labeled via both agonist sites but the gamma-subunit was labeled only via the site that binds d-tubocurarine with high affinity. Within the alpha-subunit, 93% of the labeling was contained within a 20-kDa Staphylococcus aureus V8 proteolytic fragment beginning at Ser-173. Sequence analysis of this peptide indicated that approximately 80% of the incorporation was into Tyr-198, approximately 13% was into Cys-192, and approximately 7% was into Tyr-190. Chymotryptic digestion of the alpha-subunit confirmed that Tyr-198 was the principal amino acid labeled by [3H]nicotine. This confirmation required a novel radio-sequencing strategy employing omicron-phthalaldehyde, since the efficiency of photolabeling was low (approximately 1.0%) and the labeled chymotryptic peptide was not isolated in sufficient quantity to be identified by mass. [3H]Nicotine, which is the first photoaffinity agonist used, labels primarily Tyr-198 in contrast to competitive antagonist affinity labels, which label primarily Tyr-190 and Cys-192/Cys-193.     

Binding studies and photoaffinity labeling identify two classes of phencyclidine receptors in rat brain

Binding and photoaffinity labeling experiments were employed in order to differentiate 1-(1-phenylcyclohexyl)piperidine (PCP) receptor sites in rat brain. Two classes of PCP receptors were characterized and localized: one class binds [3H]-N-[1-(2-thienyl)cyclohexyl]piperidine [( 3H]TCP) with high affinity (Kd = 10-15 nM) and the other binds the ligand with a relatively low affinity (Kd = 80-100 nM). The two classes of sites have different patterns of distribution. Forebrain regions are characterized by high-affinity sites (hippocampus greater than frontal cortex greater than thalamus greater than olfactory bulb greater than hypothalamus), but some parts (e.g., hippocampus, hypothalamus) contain low-affinity sites as well. In the cerebellum only low-affinity sites were detected. Binding sites for [3H]PCP and for its photolabile analogue [3H]azido-PCP showed a regional distribution similar to that of the [3H]TCP sites. The neuroleptic drug haloperidol did not block binding to either the high- or the low-affinity [3H]TCP sites, whereas Ca2+ inhibited binding to both. Photoaffinity labeling of the PCP receptors with [3H]AZ-PCP indicated that five specifically labeled polypeptides of these receptors (Mr 90,000, 62,000, 49,000, 40,000, and 33,000) are unevenly distributed in the rat brain. Two of the stereoselectively labeled polypeptides (Mr 90,000 and 33,000) appear to be associated with the high- and low-affinity [3H]TCP-binding sites; the density of the Mr 90,000 polypeptide in various brain regions correlates well with the localization of the high-affinity sites, whereas the density of the Mr 33,000 polypeptide correlates best with the distribution of the low-affinity sites.(ABSTRACT TRUNCATED AT 250 WORDS)     

Photoaffinity labeling of rat alpha-fetoprotein

Two photosensitive estrogen derivatives, 16-diazoestrone and 4-azidoestradiol, have been studied as photoaffinity-labeling agents for the estrogen-binding site of rat alpha-fetoprotein (AFP). 16-Diazoestrone has a high affinity for AFP (121%, relative to 17 beta-estradiol), and photolysis of the 16-diazo[3H]estrone . AFP complex for 30 min at 300 nm results in the covalent attachment of 19% of the ligand bound reversibly to the estradiol site at the time of irradiation. The photocovalent attachment appears to result from both a "chromophore-dependent" process (photoaffinity labeling), whose time course follows the photolytic consumption of the diazoketone chromophore and is not susceptible to scavenging by nucleophiles, and a "chromophore-independent" process (pseudophotoaffinity labeling) that results from covalent attachment of an electrophilic photoproduct and can be intercepted by 20 mM mercaptoethanol. AFP covalently labeled with 16-diazo[3H]estrone has the same electrophoretic mobility as unlabeled AFP on normal and sodium dodecyl sulfate-polyacrylamide gels; labeled AFP has an apparent molecular weight of 69,400 and is distinguishable from albumin (which is also labeled by 16-diazo[3H]estrone, but not in a site-specific manner). While 4-azido[3H]estradiol undergoes extensive photoinduced covalent attachment to AFP, little of this is site-specific.     

Association of protein C23 with rapidly labeled nucleolar RNA

The association of nucleolar phosphoprotein C23 with preribosomal ribonucleoprotein (RNP) particles was examined in Novikoff hepatoma nucleoli. RNA was labeled with [3H]uridine for various times in cell suspensions, and RNP particles were extracted from isolated nucleoli and fractionated by sucrose gradient ultracentrifugation. The majority of protein C23 cosedimented with fractions containing rapidly labeled RNA (RL fraction). To determine whether there was a direct association of RNA with protein C23, the RL fraction was exposed to ultraviolet (UV) light (254 nm) for short periods of time. After 2 min of exposure there was a 50% decrease in C23 as measured by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analyses, with no significant further decrease at longer times. When UV-treated fractions were subjected to phenol/chloroform extractions, as much as 30% of the labeled RNA was found in the phenol (protein) layer, indicating that RNA became cross-linked to protein. Similarly, there was an increase in protein C23 extracted into the water layer after irradiation. By SDS-PAGE analyses the cross-linked species migrated more slowly than protein C23, appearing as a smear detected either by [3H]uridine radioactivity or by anti-C23 antibody. With anti-C23 antibodies, up to 25% of the labeled RNA was precipitated from the RL fraction. Dot-blot hybridizations, using cloned rDNA fragments as probes, indicated that the RNA in the RL fraction and the immunoprecipitated RNA contained sequences from 18S and 28S ribosomal RNA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Identification of heparan sulfate proteoglycan as a high affinity receptor for acidic fibroblast growth factor (aFGF) in a parathyroid cell line.

We have characterized two high affinity acidic fibroblast growth factor (aFGF) receptors in a rat parathyroid cell line (PT-r). Affinity labeling with 125I-aFGF showed that these two receptors, apparent molecular masses, 150 and 130 kDa, respectively, display higher affinity for aFGF than for bFGF. The 150-kDa receptor bears a heparan sulfate chain(s), demonstrated by a decrease in size of 15-20 kDa with heparitinase digestion after affinity labeling. Heparitinase digestion before affinity labeling markedly reduced the intensity of the 150 kDa species. Scatchard analysis showed two different high affinity binding sites (Kd of 3.9 pM with 180 sites/cell and Kd of 110 pM with 5800 sites/cell). The higher affinity site was completely eliminated by digestion with heparitinase before adding labeled aFGF; the lower affinity site was unaffected. In ion exchange chromatography after metabolic labeling of the cells with [3H]glucosamine and affinity labeling with 125I-aFGF, the larger receptor-ligand complex, 165 kDa, eluted with approximately 0.5 M NaCl, typical eluting conditions for heparan sulfate proteoglycans. Both of the receptor-ligand complexes were smaller on sodium dodecyl sulfate-polyacrylamide gel electrophoresis than two major heparan sulfate proteoglycans, HSPG I and II, which we characterized in this cell line previously (Yanagishita, M., Brandi, M. L., and Sakaguchi, K. (1989) J. Biol. Chem. 264, 15714-15720). Both receptors have similar N-linked oligosaccharide and sialic acid contents, shown by analysis of affinity-labeled receptors upon digestion with glycopeptidase F and with neuraminidase. All together, these results suggest that PT-r cells bear two distinct high affinity receptors for aFGF, a 150-kDa receptor which is a heparan sulfate proteoglycan and another that is a glycoprotein. The heparan sulfate glycosaminoglycan moiety of the 150- kDa receptor is critical for high affinity binding of aFGF. These findings contrast with current concepts derived from other systems, suggesting that heparan sulfate glycosaminoglycans/proteoglycans function as a reservoir source for FGF or as a group of low affinity binding sites.     

Catalytic and noncatalytic nucleotide binding sites of the Escherichia coli F1 ATPase. Amino acid sequences of beta-subunit tryptic peptides labeled with 2-azido-ATP

Under appropriate conditions tight, noncovalent binding of 2-azido-adenine nucleotides to either catalytic or noncatalytic binding sites on the E. coli F1-ATPase occurs. After removal of unbound ligands, UV-irradiation results primarily in the covalent incorporation of nucleotide moieties into the beta-subunit in both catalytic and noncatalytic site labeling experiments. Minor labeling of the alpha-subunit was also observed. After trypsin digestion and purification of the labeled peptides, microsequencing studies identified two adjacent beta-subunit tryptic peptides labeled by 2-azido-ADP or -ATP. These beta-subunit peptides were labeled on tyrosine-331 (catalytic sites) and tyrosine-354 (noncatalytic sites) in homology with the labeling patterns of the mitochondrial and chloroplast enzymes.     

FRET-based identification of mRNAs undergoing translation

We present proof-of-concept in vitro results demonstrating the feasibility of using single molecule fluorescence resonance energy transfer (smFRET) measurements to distinguish, in real time, between individual ribosomes programmed with several different, short mRNAs. For these measurements we use either the FRET signal generated between two tRNAs labeled with different fluorophores bound simultaneously in adjacent sites to the ribosome (tRNA-tRNA FRET) or the FRET signal generated between a labeled tRNA bound to the ribosome and a fluorescent derivative of ribosomal protein L1 (L1-tRNA FRET). With either technique, criteria were developed to identify the mRNAs, taking into account the relative activity of the mRNAs. These criteria enabled identification of the mRNA being translated by a given ribosome to within 95% confidence intervals based on the number of identified FRET traces. To upgrade the approach for natural mRNAs or more complex mixtures, the stoichiometry of labeling should be enhanced and photobleaching reduced. The potential for porting these methods into living cells is discussed.     

Characterization and purification of the fusicoccin-binding complex from plasma membranes of Commelina communis

The fungal phytotoxin fusicoccin binds with high affinity to plasma membranes of the monocotyledonous plant, Commelina communis L. The sites bind the toxin with an apparent Kd of 5.2 nM and a pH optimum of 6.0. They occur at a level of approximately 6-8 pmol/mg plasma membrane protein. Photoaffinity labeling with the biologically active fusicoccin derivative 9'-nor-8'-(4-azido[3,5-3H]benzoyl) diaminoethylfusicoccin identified a polypeptide of 31.5 kDa on SDS/PAGE which was strongly labeled. A second 32.5-kDa band was also consistently labeled, although not to the same extent. The binding sites were solubilized in functional form and a purification scheme was developed based on affinity and ion-exchange procedures. The purified fraction contains two polypeptides of apparent molecular masses of 30.5 kDa and 31.6 kDa. A detailed molecular analysis of the fusicoccin-binding complex is now possible.     

The effect of propylbenzilylcholine mustard on contraction and radioligand binding parameters of muscarinic receptors in guinea pig ileum

The receptor occupancy-biological effect relationship for muscarinic receptors in guinea pig ileal smooth muscle has been studied by comparison of radioligand binding and contractile response. Muscarinic receptors in homogenates of ileal smooth muscle were labeled with [3H]-1-Quinuclidinyl benzilate. Treatment with propylbenzilylcholine mustard (PrBCM), to inactivate irreversibly muscarinic receptors, caused a large dose dependent rightward shift of the dose-response curve to three agonistic furtrethonium derivatives with a concomitant decrease in maximal response. Using those data, the fraction of receptors remaining unoccupied (q-values) and "true affinity constants" (-log KA-values) were calculated. Exposure to 20 or 60 nM PrBCM for 15 minutes resulted in a 39% and a 61% reduction in specific [3H]-1-Quinuclidinyl benzilate binding sites respectively to be compared with a 62% and a 85% decrease expected from calculated q-values. KA-values for the methyl and ethyl derivative agreed well with the dissociation constants for the high affinity agonist sites determined from displacement of [3H-]-1-Quinuclidinyl benzilate. The KA-value for the propylfurtrethonium corresponds to the low affinity agonist dissociation constant. The fraction of receptors in the high affinity agonist state differs considerably for the three furtrethonium derivatives investigated. Neither the fraction of receptors in the high affinity agonist state, nor the ratio of dissociation constants for these states is affected by the alkylation of 85% of the functional muscarinic receptors. The inactivation of components of the effector system by PrBCM seems unlikely.     

Facilitated glucose transporter of human erythrocyte: proteolytic mapping of the [3H]cytochalasin B photoaffinity-labeled transporter polypeptide

The human erythrocyte D-glucose transporter is an integral membrane glycoprotein with an heterogeneous molecular mass spanning a range 45-70 kDa. The protein structure of the transporter was investigated by photoaffinity labeling with [3H]cytochalasin B and fractionating the labeled transporter according to molecular mass by preparative SDS-polyacrylamide gel electrophoresis. Each fraction was digested with either papain or S. aureus V8 proteinase, and the labeled proteolytically derived peptide fragments were compared by SDS polyacrylamide gel electrophoresis. Papain digestion yielded two major peptide fragments, of approx. molecular mass 39 +/- 2 and 22 +/- 2 kDa; treatment with V8 proteinase resulted in two fragments, with mass of 24 +/- 2 and 15 +/- 2. Proteolysis of each transporter fraction produced the same pattern of labeled peptide fragments, irrespective of the molecular mass of the original fractions. The binding characteristics of [3H]cytochalasin-B-labeled transporter to Ricinis communis agglutinin lectin was examined for each transporter molecular mass fraction. It was found that higher-molecular-mass fractions of intact transporter had a 2-fold greater affinity for the lectin than lower-molecular-mass fractions (i.e., 67 kDa greater than 45 kDa fraction). However, proteolytically derived labeled peptide fragments from each fraction had minimal affinity for the lectin. These results suggest that the labeled peptide fragments have been separated from the glycosylated regions of the parent transporter protein. The present findings indicate that, although transporter proteins have an apparently heterogeneous molecular mass, some regions of the protein share a common peptide. Furthermore, the glycosylated regions appear to be located some distance from the [3H]cytochalasin-B-labeled site(s).     

Comparative binding of 125I-and 99mTc-d-labeled native and glycated low-density lipoprotein to human microvascular endothelial cells-potential for atherosclerosis imaging?

Native (n), glycated (g), and glycoxidated (go) low-density lipoproteins (LDL) were labeled with 125I or 99mTc, and the labeling efficiency and binding were assessed for potential use of these LDL compounds in imaging analysis of atherosclerotic lesions (PPAR-gamma receptors) by determining the number of specific receptors for nLDL, gLDL or goLDL on human microvascular endothelial cells as well as the KDs using either 125I-or 99mTc-labeled LDLs. The specific activity of labeled gLDL and goLDL was much higher (for goLDL 20 times higher) than that of nLDL. Gel filtration of labeled LDLs revealed, however, that 99mTc-g/goLDL is significantly degraded by the labeling reaction. No fragmentation was observed for 99mTc-nLDL and all the 125I-labeled LDL forms. Binding studies using both 125I-and 99mTc-nLDL indicated a weak binding affinity (KD 10- 7mol/L) to human microvascular endothelial cells. The binding affinity of 125I-g/goLDL to these cells was significantly higher (KD 10- 9mol/L) and could be increased further by preactivation of the endothelial cells using TNFalpha. Incubation with 99mTc-goLDL, however, did not result in specific binding of the ligand, possibly as a consequence of the fragmentation of the lipoprotein during the labeling. Scatchard transformation of the binding data with 99mTc-gLDL revealed the presence of only a few binding sites. This was in contrast to the results obtained with 125I-labeled gLDL, which revealed a much higher membrane density of scavenger receptors for this ligand. We conclude that for in vitro binding studies as well as for potential in vivo imaging, only 125I-labeled goLDL should be used, whereas nLDL may be applied as 125I-or 99mTc-labeled ligand.