Effects of thiol modification and Ca++ on agonist-specific state transitions of nicotinic acetylcholine receptor from Torpedo californica electroplax.

The agonist binding affinity of nicotinic acetylcholine receptor (nAChR) from $\text{Torpedo}$$\text{californica}$ electroplax, as inferred from ability of agonist to inhibit specific curaremimetic neurotoxin binding to nAChR, is sensitive to the duration of exposure to agonist. The concentration of carbachol necessary to prevent one-half of toxin binding over a 30 min incubation with nAChR (K₃₀) is 10 μM when toxin and carbachol are simultaneously added to membrane-bound nAChR, and 3 μM when nAChR are pretreated with carbachol for 30 min prior to the addition of toxin. These alterations in agonist affinity may be mimicked by modification of nAChR thiol groups. Affinity of nAChR for carbachol is decreased following treatment with dithiothreitol (DTT). Dithio-bis-nitrobenzoic acid treatment of DTT-reduced membranes yields K₃₀ values of 5 μM for carbachol, while N-ethylmaleimide treatment of DTT-reduced nAChR produces nAChR with reduced affinity for carbachol, reflected in K₃₀ values of about 400 μM. In the absence of Ca⁺⁺, K₃₀ values for carbachol binding to native and DTT-reduced nAChR are diminished 3–6 fold. These affinity alterations are not observed with d-tubocurarine (antagonist) binding to nAChR. Thus, Ca⁺⁺ and the oxidation state of nAChR thiols appear to affect the affinity of nAChR for agonists (but not antagonists), and may therefore be related to agonist-mediated events in receptor activation and/or desensitization.     

Independent sites of low and high affinity for agonists on Torpedocalifornica acetylcholine receptor

It is demonstrated that two classes of binding site for acetylcholine are present on $\text{Torpedo}\text{californica}$ acetylcholine receptor. One class is the well documented site on each of the two subunits of 40,000 daltons, which can be covalently modified by bromocetylcholine. Both in the absence and in the presence of bromoacetylcholine another binding site is shown to exist by virtue of acetylcholine dependent fluorescence changes in the receptor covalently modified by 4-[N-(iodoacetoxy)ethyl-N-methyl]-amino-7-Nitrobenz-2-oxa-1,3 diazole (IANBD). This site has a low affinity for acetylcholine (K_d ～ 80 μM) that corresponds closely with the known concentration dependence of acetylcholine mediated activation of this receptor and we conclude that it may represent a site of association that participates in channel opening in this system.     

Purification and molecular properties of the acetylcholine receptor from Torpedo electroplax

The acetylcholine receptor of Torpedo electroplax is purified by affinity adsorption using cobra toxin (Naja naja siamensis) covalently attached to Sepharose 4B. Desorption by 10 mm benzoquinonium produces a protein that binds α-[¹²⁵I]bungarotoxin but not [³H]acetylcholine or other reversible cholinergic ligands. On the other hand, desorption by 1 m carbamylcholine produces an acetylcholine receptor protein that binds [³H]acetylcholine, [³H]decamethonium, [³H]nicotine, [¹⁴C]dimethyl-d-tubocurarine, and α-[¹²⁵I]bungarotoxin. The batch method of affinity adsorption employed gives recoveries of acetylcholine receptor (as measured by acetylcholine binding) averaging 69.2 ± 14.6%. The purity of the isolated acetylcholine receptor protein is estimated to be at best 87% as judged by disc gel electrophoresis and electrofocusing.The purified acetylcholine receptor binds 7.8 nmoles acetylcholine/mg protein based on estimation of protein concentration by a spectrophotometric method. Of these, 2.7 nmoles exhibit high affinity (K_D = 0.02 μM) and 5.1 nmoles a lower affinity (K_D = 1.97 μM. If the protein concentration used is that obtained by amino acid analysis, the total specific activity would be 10.4 nmoles acetylcholine bound per milligram protein. The subunit carrying one acetylcholine binding site is estimated to range between 83,000 and 112,000 daltons. In contrast to the membrane-bound or Lubrol-solubilized acetylcholine receptor, the purified acetylcholine receptor shows no autoinhibition with acetylcholine concentrations up to 10 μm. Binding of acetylcholine was totally inhibited by α-bungarotoxin or cobra toxin and was partially blocked by four nicotinic drugs, but not by two muscarinic ones. The amino acids of the acetylcholine receptor are analyzed and compared to those of acetylcholinesterase.     

A fluorescence probe of acetylcholine receptor conformation and local anesthetic binding

The fluorescent dye ethidium bromide binds to the acetylcholine receptor with an apparent K_d of $\text{3 μ}\text{M}$ and a stoichiometry of 1 molecule of ethidium per α-bungarotoxin site. Time dependent fluorescent increases were observed upon addition of carbamylcholine, the amplitude and half-time of which were dependent on the Carb¹ concentration. It appeared that these fluorescence increases resulted from a lowering of the K_d for ethidium as the AcChR-Carb complex underwent an isomerization from low to high affinity form(s) for carb, and more ethidium was bound. Titration with the local anesthetic procaine led to ethidium fluorescence increases at low procaine concentrations, followed by a fluorescence decrease at higher procaine concentrations to that level induced by saturating α-bungarotoxin. Thus it appeared that the ethidium binding site either interacted with or was identical with local anesthetic binding site(s).     

The identification of an invertebrate acetylcholine receptor

The results of a series of experimental studies have culminated in the identification of an acetylcholine receptor from the invertebrate $\text{Limulus polyphemus}$. The binding ligand α-bungarotoxin was used to identify a specific protein in the central nervous system tissue of this organism. The specific interaction of α-bungarotoxin with an acetylcholine receptor has been confirmed by physiological, competitive binding, subcellular fractionation and autoradiographic techniques. The toxin binding protein was solubilized and exhibited properties consistent with the nature of a nicotinic cholinergic receptor. Therefore, the identified protein is proposed as an acetylcholine receptor protein from the central nervous system of this invertebrate species.     

Steganacin: An inhibitor of HeLa cell growth and microtubule assembly invitro

The plant derivative steganacin, an antitumor compound, blocks the replication of HeLa cells in mitosis. Steganacin inhibits microtubule assembly $\text{in}\text{vitro}\text{(ID}{}_{50}\text{= 1.5 μM)}$ and is a competitive inhibitor of colchicine binding to purified tubulin (K_i = 3.1 μM). The structure-activity relationships of steganacin and a series of analogues are reported.     

Binding of glycoproteins of microsomal and Golgi membranes to lectins.

Four subunits of the acetylcholine receptor molecule, obtained from the electric organ of $\text{Torpedo ocellata}$, have been isolated using polyacrylamide gel electrophoresis, and assayed by titration with a fluorescent lanthanide, terbium, and by affinity-labeling with $\text{p}$-($\text{N}$-maleimido)benzyl [trimethyl-³H] ammonium iodide. The site with which the activator-analogue affinity label reacts, as well as the terbium-binding sites, are mainly associated with the smallest of the subunits of an apparent molecular weight of 40,000. Calcium competes with terbium for these binding sites. The affinity for terbium is the same in the intact molecule as in the subunit (K_Tb ? 19 ± 1 μM), but the affinity for calcium decreases by a factor of 4 (K_Ca ? 4 mM) in the subunit. Hydrolysis of the receptor, catalyzed by trypsin and chymotrypsin, to peptides with an apparent molecular weight of 8000 or less, does not affect the terbium-binding sites. These experiments indicate that the binding sites for neural activators and for calcium are associated with the same subunit, and that the terbium- and calcium-binding sites reflect structural properties of the polypeptide chain rather than the three-dimensional structure of the protein.     

Hydrodynamic properties of rat hepatic prolactin receptors

The hydrodynamic properties of rat hepatic prolactin receptors have been determined by a combination of gel chromatography and ultracentrifugation. Prolactin receptors were detergent extracted from partially purified plasma membranes prepared from female rat livers. Fifteen different nonionic detergents were tested for solubilizing prolactin receptors, including Triton X-100, Polyoxyethylene W-1, Lubrol WX, detergents of the Tween and Brij series, and digitonin. When the receptors were detergent solubilized after ligand was bound to the receptor, 1% Triton X-100 had the highest efficacy of solubilization. However, if the receptors were solubilized prior to exposure to ligand, maximum binding was to receptors solubilized with 0.25% Triton X-100. The K_d of 43.2–74.5 pM for binding to the soluble receptor was three to fivefold lower than the K_d for the membrane receptor. Gel chromatography (Bio-Gel A-1.5m, 2.5 × 50 cm) of the soluble receptor indicated a Stokes radius (R_s) of 5.0 nm for the hormonereceptor-detergent complex. The hydrodynamic properties of the receptor-detergentligand complex were determined by centrifugation in 5–20% sucrose gradients in H₂O and in D₂O. They are $\text{v}\text{?}\text{= 0.7; s}{}_{\mathrm{<mtext>20,</mtext><mtext>w</mtext>}}\text{= 4.7;}\text{f}\text{f}{}_0\text{= 1.49; M}{}_{\mathrm{<mtext>r</mtext>}}\text{= 118,000}$ for the complex, 73,000 for the receptor alone. Approximately 0.22 mg of Triton X-100 is estimated bound per milligram of protein. This represents about 25 mol detergent/mol receptor.     

Guanine nucleotides modulate cell surface cAMP-binding sites in membranes from Dictyostelium discoideum

D. discoideum contains kinetically distinguishable cell surface cAMP binding sites. One class, S, is slowly dissociating and has high affinity for cAMP (K_d = 15 nM, $\text{t}\text{1}\text{2}\text{= 15 s}$). A second class is fast dissociating ($\text{t}\text{1}\text{2}$ about 1 s) and is composed of high affinity binding sites H (K_d ≈ 60 nM), and low affinity binding sites L (K_d = ≈ 450 nM) which interconvert during the binding reaction. Guanine nucleotides affect these three binding types in membranes prepared by shearing D.discoideum cells through Nucleopore filters. The affinity of S for cAMP is reduced by guanine nucleotides from 13 nM to 25 nM, and the number of S-sites is reduced about 50%. The number of fast dissociating sites is not altered by guanine nucleotides, but these sites are mainly in the low affinity state. Half-maximal effects are obtained at about 1 μM GTP, 2 μM GDP and 10 μM Gpp(NH)p(guanyl-5′-yl-imidodiphosphate); ATP and ADP are without effect up to 1 mM. These results indicate that D.discoideum cells have a functionally active guanine nucleotide binding protein involved in the transduction of extracellular cAMP signals via cell surface cAMP receptors.     

Danazol inhibits human adrenal 21-and 11β-hydroxylation invitro

The effects of danazol on steroidogenesis $\text{in}$$\text{vitro}$ in the 16–20 week old human fetal adrenal were examined by studying: 1) danazol binding to adrenal microsomal and mitochondrial cytochrome P-450, and 2) enzyme kinetics of danazol inhibition of the adrenal microsomal 21-hydroxylase and the mitochondrial llβ-hydroxylase. The addition of danazol to preparations of adrenal microsomes or mitochondria elicited a type I cytochrome P-450 binding spectrum. Danazol bound to microsomal cytochrome P-450 with a high affinity apparent spectral dissociation constant (Kg) of 1 μM and with a lower affinity K's of 10 μM. Danazol bound to mitochondrial cytochrome P-450 with a Kg of 5 μM. In addition, danazol competitively inhibited the microsomal 21-hydroxylase (apparent enzymatic inhibition constant K_I = 0.8 μM) and the mitochondrial 11β-hydroxylase (K_I = 3 μM). These findings demonstrate that low concentrations of danazol directly inhibit steroidogenesis in the human fetal adrenal $\text{in}$$\text{vitro}$.     

Carbamylcholine-induced rapid cation efflux from reconstituted membrane vesicles containing purified acetylcholine receptor.

Membrane preparations containing essentially only the four polypeptides considered to constitute the acetylcholine receptor are purified from $\text{Torpedo}\text{californica}$ electroplax. Treatment of these membranes with 2% ($\text{w}\text{v}$ aqueous sodium cholate followed by removal of all insoluble matter results in a solubilized purified receptor preparation that can be reassociated with phospholipids during dialysis to remove the detergent. Such reconstituted receptor is shown to retain the capability of translocating ²²Na⁺ across the membrane in response to carbamylcholine binding in a highly reproducible manner. The dose response for this effect is similar to that observed for the original electroplax membrane preparation and the carbamylcholine induced signal is completely blocked by α-bungarotoxin.     

Phospholipase A inhibition of acetylcholine receptor function in Torpedo californica membrane vesicles

A protein isolated from $\text{Naja naja siamensis}$ venom on the basis of its phospholipase A activity inhibits acetylcholine receptor function in post-synaptic membrane vesicles from $\text{Torpedo californica}$. Specifically, the phospholipase A prevents the large increase in sodium efflux that can normally be induced by carbamylcholine, a receptor agonist. The phospholipase A inhibition shows the following properties: 1) it occurs at concentrations 50 times lower than the concentrations required for inhibition by α-neurotoxins; 2) the phospholipase A has no effect on the binding properties of the receptor; 3) the inhibition is abolished by removal of calcium ions; and 4) some phospholipid hydrolysis accompanies inhibition. It is suggested that the phospholipase A acts enzymatically to uncouple ligand binding from ion permeability in the receptor containing membrane vesicles.     

Studies of nicotinic acetylcholine receptor protein from rat brain: II. Partial purification

The pharmacological specificity of the binding of ¹²⁵I-labeled α-bungarotoxin to a 1% Emulphogene BC-720 extract of a rat brain particulate fraction has been investigated. The extract contains a component which possesses the binding characteristics of a nicotinic acetylcholine receptor protein. The crude soluble acetylcholine receptor protein was purified by affinity chromatography utilizing the α-neurotoxin of Naja naja siamensis as ligand and 1.0 M carbamylcholine chloride as eluant. A single, batch-wise, affinity chromatography procedure yields an average purification of 510-fold. When this purified material is treated a second time by affinity chromatography, purification as high as 12 600-fold has been obtained. Binding of ¹²⁵I-labeled α-bungarotoxin to this purified acetylcholine receptor protein is saturable with a K_d of 1·10^?8 M. Nicotine and acetylcholine iodide at concentrations of 10^?5 M inhibit ¹²⁵I-labeled toxin-acetylcholine receptor protein complex formation by 41 and 61% respectively. At 10^?4 M, carbamylcholine chloride and (+)-tubocurarine chloride give respectively 52 and 82% inhibition. Eserine sulfate and atropine sulfate have no effect on complex formation at a concentration of 10^?4 M. These data support the isolation of partially purified nicotinic acetylcholine receptor protein.     

Interaction of A-nor A.19-dinor,and A-homo-5α-androstane derivatives with the androgen receptor and the epididymal androgen-binding protein

The equilibrium affinity constant for rat prostate androgen receptor and epididymal androgen binding protein (ABP) has been determined for thirty-four potential progestogens. Three A-nor-, four A,19-dinor-, and one A-homo-5α-androstane derivative bind to the androgen receptor (K_D<0.5 μM). Five of these compounds also bind to ABP with an affinity of the same order of magnitude. “Anordrin” (compound $\text{24}$) and “Dinordrins” (compounds $\text{10, 14, 15, 16, 17}$), which are potential female contraceptives, do not bind with high affinity to the androgen receptor or to ABP. The following modifications in A-nor derivatives favour binding to the receptor as compared to ABP: 19-nor substitution (compound $\text{1}$), C-18 methyl homologation (compound $\text{5}$), 2α-ethinylation (compound $\text{22}$). One 2α-allenyl A-nor derivative (compound $\text{25}$) and one A-homo derivative (compound $\text{34}$) bind almost exclusively to ABP. The interaction with either binding protein is decreased by oxidation or esterification of the hydroxyl group at C-17, and by addition of a 17 α-ethinyl group. The latter modifications are likely to increase the specificity of androstane derivatives for receptors other than androgen binding proteins, such as the progesterone receptor.     

A novel radioiodinated high affinity ligand for the D2-dopamine receptor: Characterization of its binding in bovine anterior pituitary membranes

A novel high affinity dopaminergic ligand, N-(p-aminophenethyl)spiroperidol, has been synthesized and radioiodinated to a specific radioactivity of 2175 $\text{Ci}\text{mmol}$. Binding of this ligand to bovine anterior pituitary membranes is: (i) rapid (40–60 min to equilibrium at 25°C) and reversible t_{$\text{1}\text{2}$} = 1 h at 25°C); (ii) saturable and of high affinity (K_D ~ 20 pM) and (iii) displays a typical D₂-dopaminergic specificity. The ligand, which identifies the same number of receptor sites as other tritiated antagonist ligands, can be used in different tissues and preparations to delineate the characteristics of the D₂ receptor. Thus, this high affinity, high specific radioactivity ligand (N-(p-amino-m-[¹²⁵I]iodophenethyl)spiroperidol) represents a tool which until now had not been available for the characterization of the D₂-dopamine receptor.     

Rapid cation flux from Torpedo californica membrane vesicles: comparison of acetylcholine receptor enriched and selectively extracted preparations.

Rapid efflux of ²²Na from within closed vesicles derived from $\text{Torpedo}\text{californica}$ electroplax membranes has been studied as an $\text{in}\text{vitro}$ assay of acetylcholine receptor functionality. The most highly purified membrane preparations contained major polypeptides of M.W. 43 and 90 × 10³ daltons in addition to the four peptides characteristic of the acetylcholine receptor (40, 50, 60, 65 × 10³ daltons). Removal of these extra peptides by base extraction did not significantly alter the characteristics of carbamylcholine induced ²²Na efflux: the agonist dose response curve was similar, preequilibration with agonist caused desensitization, the irreversible antagonist α-Bungarotoxin blocked the efflux and the reversible blockade by the neurotoxin perhydrohistrionicotoxin was also retained. The dose response curve for perhydrohistrionicotoxin corresponded closely to its known binding characteristics for base extracted membranes.     

Nuclear and cytoplasmic binding components for vitamin D metabolites.

Specific binding of 1α,25-dihydroxyvitamin D₃ to macromolecular components of small intestinal nuclei and cytosol is demonstrated. The nuclear 1α,25-dihydroxyvitamin D₃ complex can be extracted from chromatin by 0.3 M KCl and sediments at 3.7S in sucrose density gradients. The cytoplasmic 1α,25-dihydroxyvitamin D₃-binding components also sediment at 3.7S, identically to the nuclear complex under the ultracentrifugation procedures employed.Macromolecular binding components with a high affinity for 25-hydroxyvitamin D₃ (K_d = 4.5 × 10⁻⁹ M) were also identified in intestinal cytosol which differ from the 1α,25-hydroxyvitamin D₃ receptor in that: 1) they sediment at 5–6S in sucrose gradients, 2) they are observed in organs other than the intestine, and 3) while they do bind 1α,25-dihydroxyvitamin D₃ at higher concentrations than 25-hydroxyvitamin D₃, they are not observed to transfer either 25-hydroxyvitamin D₃ or 1α,25-dihydroxyvitamin D₃ to the nucleus, $\text{in vitro}$.     

Affinity-labeling of purified acetylcholine receptor from Torpedo californica

The receptor for acetylcholine purified from electric tissue of $\text{Torpedo californica}$ has been assayed both by affinity-alkylation and by neurotoxin binding. The specific activity by the latter method is about twice that by the former. Four major components of apparent molecular weights of 39,000, 48,000, 58,000 and 64,000 are separated by dodecyl sulfate-acrylamide gel electrophoresis. Reduction and affinity-alkylation of the receptor with a tritiated quaternary ammonium maleimide derivative results in the exclusive labeling of the 39,000 dalton subunit. This subunit, it is concluded, contains all or part of the acetylcholine binding site.     

Interaction of progesterone with amphibian melanosomes

Melanosomes purified by sucrose gradient centrifugation from different Amphibian tissues bind progesterone (K_D were respectively 8.2 10^?8M, 4.5 10^?8M and 4.4 10^?8M for $\text{Rana}$ skin, retina and ovary), and to a lesser extent other steroid hormones. HCl-hydrolysis removes melanoproteins and increases the progesterone binding capacity of melanin. This result suggests that progesterone binds directly to the melanin polymer. Effects of protein modifiers (SH and SS reagents, proteolytic enzymes, ionic strength) argue for the protein dependence of binding process. Proflavine and Atabrine strongly inhibit progesterone binding to melanosomes and melanin.     

Effects of the intracerebroventricular injection of antinociceptive doses of acetylcholine on the stereospecific binding of 3H-dihydromorphine

The antinociceptive effects of intraventricularly administered acetylcholine (ACh) and its congeners have been demonstrated by previous investigators. The opiate receptor binding concept was used in this study to investigate possible correlations between ACh antinociception and its effects on opiate stereospecific binding. ACh $\text{in vitro}$ decreased the stereospecific binding of ³H-dihydromorphine in mouse brain homogenates. Such decrease was also observed in the brain homogenates of mice which had been treated with ACh intracerebroventricularly (i.v.t.). The decrease in the stereospecific binding of ³H-dihydromorphine induced by (i.v.t.) acetylcholine was inhibited by naloxone, atropine, cyclazocine and pentazocine. The $\text{d}$-isomers of cyclazocine and pentazocine were more potent than the $\text{l}$-isomers in antagonizing the inhibitory effects of i.v.t. acetylcholine upon the stereospecific binding of ³H-dihydromorphine to mouse brain homogenates. The same stereospecificity of these two narcotic analgesics in blocking acetylcholine had been previously observed in the tail-flick test. It is suggested that the antinociceptive effects of acetylcholine are related to the inhibition of opiate stereospecific binding, the mechanism of which is yet to be understood.