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.     

Structural studies of a membrane-bound acetylcholine receptor from Torpedo californica

We investigated the differential repair of DNA lesions induced by bifunctional mitomycin C, monofunctional decarbamoyl mitomycin C and ultraviolet irradiation in normal human, Xeroderma pigmentosum and Fanconi's anemia cells using assays for the survival of clone-forming ability, alkaline sucrose sedimentation and hydroxyapatite chromatography of DNA. Four FA cell lines exhibited about 5 to 15 times higher sensitivity to MC killing, despite normal resistance to u.v. and DMC, than did normal human cells. The XP cells, however, were highly sensitive to u.v. and DMC killings due to their deficiency in excision repair, but the cells unexpectedly had an almost normal capacity for surviving MC and repairing the MC interstrand cross-links.In experiments to determine the sedimentation velocity of the DNA in alkaline sucrose gradients, normal and XP cells showed evidence for single-strand cutting following MC treatment. The sedimentation velocity of the DNA covalently cross-linked by MC in an FA strain was 2.5 times faster than that of the untreated control, and remained unaltered during post-incubation due to the lack of half-excision⁴ of cross-links. However, FA cells, but not XP cells, had the normal ability to incise DNA with the DMC monoadducts. Hydroxyapatite chromatography revealed the reversibly bihelical property of MC cross-linked DNA after denaturation. Normal and XP cells lost such reversibility during post-MC incubation as the result of cross-link removal with first-order kinetics (half-life = 2 h). The three FA lines studied exhibited two- to eightfold reduced rates of cross-link removal than normal and XP cells, indicating a difference in the repair deficiency of the FA strain. Thus we have been led to conclude that FA cells may have different levels of deficiency in half-excision repair of interstrand cross-links induced by MC, despite having normal mechanisms for repair of u.v.-induced pyrimidine dimers and DMC monoadducts, and vice versa in XP cells.     

Studies on the binding of nucleotides by rat brain hexokinase

Various nucleoside di- and triphosphates have been compared with respect to their ability to protect rat brain hexokinase (ATP: d-hexose 6-phosphotransferase, EC 2.7.1.1) activity against inactivation by chymotrypsin, glutaraldehyde, heat, and 5,5′-dithiobis(2-nitrobenzoic) acid. ATP could be distinguished from other nucleoside triphosphates in these comparisons, which may be related to the specificity with which ATP is utilized as a substrate. All nucleoside derivatives examined provided substantial protection against two or more of the above inactivating agents, indicating relatively nonspecific binding of nucleotides by brain hexokinase, consistent with a similar lack of specificity in the inhibition of this enzyme by nucleoside derivatives. The fluorescence of 2-p-toluidinylnaphthalene-6-sulfonate (TNS) and of tetraiodofluorescein (TIF) was enhanced by binding to brain hexokinase. TNS binding was not affected by the presence of various relevant metabolites (Glc, glucose 6-phosphate, ATP), nor did TNS inhibit the enzyme. In contrast, substantial (approximately 70%) decreases in the fluorescence of bound TIF resulted from the addition of various nucleoside derivatives, and TIF served as a competitive inhibitor of brain hexokinase. These observations are consistent with the view that TIF binds to a nucleotide binding site of the enzyme. The inability of nucleotides to totally displace TIF was taken to indicate the existence of an additional TIF binding site (or sites) discrete from the catalytic site, and probably identical to the site(s) at which TNS binds with no effect on catalytic activity. The effects of saturating levels of ATP and ADP were not additive indicating that both compounds were displacing TIF from the same site i.e., a common nucleotide binding site. Glc, mannose, and 2-deoxyglucose greatly enhanced the ability of nucleotides to displace TIF, while fructose, galactose, and N-acetylglucosamine did not, indicating the existence of interactions between hexose and nucleotide binding sites; the hexoses themselves were not effective at displacing TIF. The enhanced binding of nucleotides in the presence of the first three hexoses but not the latter three can be directly correlated with the relative ability of these hexoses to induce specific conformational changes in the enzyme. The hexoses themselves were not effective at displacing TIF. Glucose 6-phosphate and 1,5-anhydroglucitol 6-phosphate could also displace TIF, and as with the nucleotides, a maximum of approximately 70% decrease in fluorescence was observed and the effectiveness of glucose 6-phosphate was enhanced in the presence of Glc. Other hexose 6-phosphates tested were not effective at displacing TIF. The specificity with which hexose 6-phosphates displaced TIF could be correlated with their ability to induce specific conformational change in the enzyme. The results are discussed as they relate to the kinetic mechanism and allosteric regulation by nucleotides that have been proposed for this enzyme.     

Allethrin interactions with the nicotinic acetylcholine receptor channel

Interactions of the synthetic pyrethroid allethrin with the nicotinic acetylcholine (ACh) receptor/channel were studied in membranes from Torpedo electric organ. Allethrin did not inhibit binding of [3H]ACh to the receptor sites, but inhibited noncompetitively binding of [3H]perhydrohistrionicotoxin ([3H]H12-HTX) to the ionic channel sites in a dose-dependent manner. The inhibition constant (Ki) of [3H]H12-HTX binding in absence of receptor agonist was 30 micro M, while in presence of 100 micro M carbamylcholine it was 4 micro M. This inhibitory effect of allethrin had a negative temperature coefficient. The high affinity binding of allethrin to the channel sites of the nicotinic ACh-receptor may be indicative of a postsynaptic site of action for pyrethroids, in addition to their known action on the sodium channel.     

Effect of aging on neurotransmitter receptor binding in rat and human brain.

β-Adrenergic and GABA receptor binding were measured in brain areas of rats 3 to 24 months of age. While GABA receptor binding was not significantly different across age in any area, β-adrenergic receptor binding was significantly reduced in the cerebellum and brain stem, but not cerebral cortex, of 24-month-old animals. The loss in β-adrenergic receptor binding does not correlate in a temporal fashion with the reported decrease in norepinephrine-stimulated cyclic AMP accumulation in the cerebellum which occurs as early as 12 months of age. An age-related reduction in β-adrenergic binding was also noted in human cerebellar tissue obtained at autopsy, suggesting that the cerebellar dysfunction seen with aging may be related to a loss of cerebellar neurons which receive noradrenergic input.     

Water-soluble acetylcholine receptor from Torpedo californica. Solubilization,purification and characterization

The nicotinic acetylcholine receptor from electrogenic tissue of Torpedo californica was solubilized by tryptic digestion of membrane fragments obtained from autolysed tissue, without use of detergent. The water-soluble acetylcholine receptor was purified by affinity chromatography on a cobra-toxin-Sepharose resin. The purified receptor bound 4000–6000 pmol per mg protein of α-[¹²⁵]bungarotoxin, and toxin-binding was specifically inhibited by cholinergic ligands. Gel filtration revealed a single molecular species of Stokes radius $\text{125 ± 10}$ Å and on sucrose gradient centrifugation one major peak was observed of 20–22 S. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and β-mercaptoethanol revealed two major polypeptides of mol. wt. 30 000 and 48 000.     

Release of ascorbate from a synaptosomal fraction of rat brain

We have studied factors controlling the release of endogenous ascorbate from synaptosomes prepared from various regions of the rat brain. Ascorbate was spontaneously released from synaptosomes, and this efflux could be enhanced by incubation at 37°C. A further additional ascorbate release could be induced by potassium depolarization or, in striatal, hippocampal and cortical synaptosomes, by incubation with the amino acid glutamate. Spontaneous, depolarization and glutamate-evoked ascorbate release were shown to occur by separate mechanisms. Glutamate-evoked ascorbate release occurred by a heteroexchange mechanism. In cerebellar synaptosomes there was no evidence for such heteroexchange; however, in synaptosomes of this brain region kainic acid induced ascorbate release, probably by acting on excitatory amino acid receptors. The results are discussed in relation to the changes in extracellular brain ascorbate occurring in vivo.     

Change in the intrinsic fluorescence of acetylcholine receptor purified from Narke japonica upon binding with cholinergic ligands

Effects of various cholinergic ligands on the intrinsic fluorescence of acetylcholine receptor purified from the electric organ of Narke japonica were investigated. Binding with acetylcholine decreased the fluorescence by 7–8%, and that with carbamylcholine by 4–5% at 20 °C. Decamethonium and d-tubocurarine did not affect significantly the fluorescence intensity, while hexamethonium enhanced it. These changes were completely inhibited by preincubation of the receptor with α-bungarotoxin, which indicated that the observed intrinsic fluorescence change was due to the specific binding of each ligand. Data of the quenching experiment using iodide ion as an extrinsic quencher suggested the occurrence of the conformational change in the receptor upon binding with various cholinergic ligands. Considering these results together with those on intrinsic fluorescence change, conformational change provoked by binding with acetylcholine or carbamylcholine seems to differ from that provoked by binding with other cholinergic ligands examined.     

Simultaneously entry of palmitic acid into proteolipid protein in different myelin subfractions of rat brain

Rats of 20-days of age were injected intracranially with radioactive palmitic acid to study its incorporation into proteolipid protein (PLP) of myelin and myelin subfractions. At short times (120 min), the radioactivity present in PLP was shown to be due to palmitic acid bound to the protein by ester linkages. The specific radioactivity of palmitic acid labeled PLP was identical in all the myelin subfractions except the myelin-like fraction, in which it was lower, suggesting that the entry of the fatty acid into PLP of the different subfractions occurs simultaneously.Experiments using time staggered injections of ¹⁴C- and ³H-labeled palmitic acid also showed that entry of the fatty acid into PLP of the various subfractions was simultaneous. These results seem to indicate that the acylation of PLP occurs in the myelin membrane and that synthesis and transport of this protein are events unrelated to the acylation process.     

Measurement of a folate binding protein from rat liver cytosol by radioimmunoassay

A radioimmunoassay has been developed for the folate binding protein from rat liver cytosol with a molecular weight of 150,000 which was recently purified to homogeneity (Suzuki, N., and Wagner, C., 1980, Arch. Biochem. Biophys.199, 236–248). This method has indicated that the binding protein (FBP-CII) is found primarily in the liver. A significant amount of FBP-CII was also found in the kidney and much reduced levels in spleen, serum, brain, lung, and heart. No FBP-CII could be detected in small intestine, skeletal muscle, or testes. Small amounts of cross-reacting material were found in the livers of mouse, dog, chick, and humans. Levels of FBP-CII were not decreased in the livers of folate-deficient rats. Assays of rat fetal liver and kidney 2 days prior to birth showed much lower levels which increased rapidly at birth. These data are consistent with the FBP-CII fulfilling a role as a folate storage protein in rat liver.     

Effect of adrenocorticotrophic hormone on the activity of sterol ester hydrolase from rat brain

Adrenocorticotrophic hormone (ACTH) stimulates in vitro the hydrolysis of oleoylcholesterol by a sterol ester hydrolase from rat brain synaptosomes. The stimulatory effect involves an alkaline shift of the pH optimum of catalysis and culminates at pH 6 with a 15 to 20-fold increase in lipolytic rates. The effect requires trace amount of organic solvent in the substrate emulsion and is dependent on the NH₂-terminal sequence extending through the basic amino acid residues at positions 15–18. The hormonal stimulation is decreased when a lipid-depleted preparation is used as enzyme source, and fully restored upon addition of lecithin. The results raise the possibility that ACTH may have a neuro-hormonal role in brain via modulation of local lipolytic processes.     

Purification and characterization of a protein inhibitor of calcium-dependent proteases from rat liver

Soluble extracts of rat liver contain a protein inhibitor of calcium-dependent proteases. The inhibitor has an apparent M_r = 250,000 and is separated from the calcium-dependent proteases by gel-filtration chromatography in the presence of EGTA. The inhibitor has been purified by affinity chromatography using a calcium-dependent protease covalently linked to Affi-Gel 15. The inhibitor specifically binds to this affinity resin in a calcium-dependent manner and elutes in the presence of EDTA or EGTA. The purified inhibitor appears as a single protein with M_r = 125,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Presumably it is a dimer under nondenaturing conditions. The inhibitor inhibits each of two calcium-dependent proteases from rat liver and from other tissues and species. However, it has no effect on any other protease tested.     

Muscarinic receptor sites in human foetal brain

The specific binding of the muscarinic cholinergic ligand N-methylscopolamine to human foetal brain has been measured. A level of binding of 64 pmol/g protein was found with a dissociation constant, K_d of 0.27 nM. Values of 0.17 nM min^?1 and 0.048 min^?1 for the association rate constant, K_on, and the dissociation rate constant K_off respectively, were obtained. The pharmacological properties of the binding site were found to be very similar to those reported for muscarinic receptors from adult mammalian brain except that the binding of pirenzepine and the nicotinic antagonists d-tubocurarine and decamethonium shows differences from that seen in adult brain.     

Characterization of bovine brain calmodulin-dependent protein phosphatase

Calmodulin-dependent protein phosphatase of bovine brain exhibited a pH optimum of 7 and appeared to require sulfhydryl groups for activity. Phosphatase activity was inhibited by both NaF and ZnCl2, but was stimulated approximately 2-fold by MnCl2. The enzyme exhibited broad substrate specificity, dephosphorylating casein, troponin I, protamine, histone, and phosvitin, and was not phosphorylated by cAMP-dependent protein kinase. With 32P-labeled casein as a substrate, phosphatase was activated 15-fold by calmodulin; the dissociation constant of phosphatase for calmodulin was 30 nM. Activation of the enzyme by calmodulin as a function of Ca2+ was highly cooperative; the Hill coefficient was 4.9. At a saturating concentration of calmodulin, half-maximal activation of phosphatase was obtained at 0.3 microM Ca2+. Calmodulin increased the Vmax from 1.7 to 41 nmol mg protein-1 min-1 with no significant change in its Km. Formation of a Ca2+-dependent complex between calmodulin and the phosphatase was demonstrated by a calmodulin-Sepharose affinity column, gel-filtration chromatography, and sedimentation on a sucrose density gradient. The rate of formation and dissociation of the calmodulin X phosphatase complex was rapid and readily reversible in response to changes in Ca2+ concentration. The calmodulin X phosphatase complex consists of 1 mol of calmodulin and 1 mol of phosphatase.     

Studies of corticotropin receptors on rat adipocytes

Synthetic [¹²⁵I]-Tyr²³, Phe², Nle⁴-adrenocorticotropin (ACTH)-(1–38) ([¹²⁵I]-ACTH analog) with full biological potency and near theoretical specific radioactivity (1800 ± 75 Ci/mmol) was used to investigate ACTH receptors on isolated rat adipocytes derived from 42-day-old rats. Binding to adipocytes was studied in the presence of 1% bovine serum albumin (BSA) as well as 4% BSA. The interaction of the [¹²⁵I]-ACTH analog with adipocytes was highly specific, rapid, saturable, and reversible. Scatchard analysis of the binding data obtained in medium containing 1% BSA revealed a single class of binding sites with an apparent K_D = 170 ± 11.9 pM. Competition experiments with unlabeled ACTH also yielded a comparable value for the apparent K_D (143 ± 16.5 pm). The number of receptors per adipocyte was quite low (521–841/cell). The stimulation of lipolysis by ACTH was closely correlated with the binding, the apparent K_m being 145–177 pm. At a concentration of 4% BSA in the incubation medium, the binding curve was shifted significantly to the right (apparent K_D = 446 ± 77 pM) and the binding capacity was also significantly enhanced (1663 ± 208/cell) without any change in the apparent K_m for glycerol release (187 ± 7.1 pm).     

Kinin-forming activity in rat brain

The present study shows that rat brain contains a kinin-forming activity which is distinguishable from plasma kallikrein. Kinin-forming activity was found in an acetone powder of frozen brain tissue (between 27 and 175.5 ng generated bradykinin/g fresh brain tissue/h). Analysis by high pressure liquid chromatography (HPLC) indicated that the kinin formed chromatographed like true bradykinin (BK). After subcellular fractionation using differential centrifugation of homogenized fresh brain tissue the kinin-forming activity was found mainly in a microsomal (P-3) fraction after preincubation with 2 μM melittin. Further fractionation of P-3 fraction using discontinuous sucrose gradient centrifugation identified activity in both the 1 M sucrose layer (5.8 ± 3.1 ng kinin/mg protein/h) and at the interface between the 0.8 and 0.3 sucrose layers (9.4 ± 4 ng kinin/mg protein/h). Melittin pretreatment did not change these values. The distribution pattern of the kallikrein-like activity was different from that of cathepsin d-like acid protease. The two kinin-forming activities were equally sensitive to treatment with various trypsin inhibitors but were clearly distinguishable from plasma kallikrein: brain activity was inhibited completely by Trasylol but not by soybean trypsin inhibitor (SBTI) or ovomucoid while plasma kallikrein was completely inhibited by SBTI and partially by ovomucoid and Trasilol. Our results clearly distinguish between plasma kallikrein, brain cathepsin d-like acid protease activity and an apparent brain kinin-forming activity, but do not by themselves establish a central biosynthetic pathway for kinin generation.     

In vivo autoregulation of rat adenohypophyseal thyrotropin-releasing hormone receptor

The possible mechanism of attenuation of thyrotropin response to exogenous thyrotropin-releasing hormone $\text{in vivo}$ after repeated administrations of the releasing hormone has been studied. To this end, the effect of prolonged hormone treatment on the binding of hormone to its receptor in the anterior pituitary gland has been evaluated. The data show that prolonged hormonal treatment resulted in a reduction in the number (B_max) but not the binding affinity (K_D) of the receptor. The effect was reversible and depended on the duration of treatment. This phenomenon of down regulation or the decrease in the receptor number was found not to be due to either the metabolism of releasing hormone or its ability to activate pituitary-thyroid-axis.     

Induction of acetylcholine receptor synthesis and aggregation: partial purification of low-molecular-weight activity

We have studied the effect of saline and acid extracts of chick brain on the total number of acetylcholine (ACh) receptors and the number of receptor clusters in cultured chick muscle cells. Myotubes in 7-day cultures responded more rapidly to brain extract than did myotubes in 4-day cultures, so the older cells were used in subsequent bioassays. A large percentage of the receptor inducing activity was soluble in 2% trifluoroacetic acid (TFA), and this material appeared by Sephadex G-25 chromatography to be about 1000 daltons in size. Activity was retained on octadecasilyl silica and was further purified by reverse-phase high-pressure liquid chromatography using a TFA-acetonitrile gradient system. Material that eluted between 35 and 40% acetonitrile, termed C4018, was 500- to 1000-fold more potent than saline extract. The receptor accumulation induced by C4018 was associated with an increased rate of receptor incorporation, presumably receptor synthesis, rather than to a decrease in receptor degradation. An increase in incorporation was detected as early as 3 hr after C4018 was added to 7-day cultures and the effect was maximal after 10 hr. C4018 also promoted the aggregation of receptors that were already incorporated in the surface membrane at the time to addition. It is not yet known if aggregation of "old" receptors and increased receptor synthesis are related or if the two phenomena are mediated by the same molecule.     

Kinetic properties of arachidonoyl-coenzyme A synthetase in rat brain microsomes

Free arachidonic acid is released rapidly in the brain at the onset of ischemia and during convulsions. The transient nature of this phenomenon indicates the existence of an active reacylation system for this fatty acid, likely an arachidonoyl-CoA synthetase-arachidonoyl transferase. The first of these enzymatic activities in brain microsomes was studied and it was found that [1-14C]arachidonic acid is rapidly activated and shows an absolute requirement for ATP and CoA. MgCl2 enhances this activity 10-fold. The optimum pH is 8.5, and the apparent Km values for the radiolabeled substrate, ATP, CoA, and MgCl2 are 36, 154, 8, and 182 microM, respectively. The apparent Vmax is 32.4 nmol/min/mg protein for arachidonic acid. The presence of Triton X-100 (0.1%) in the assay medium caused a significant reduction in apparent Km (9.4 microM) and Vmax (25.7 nmol/min/mg protein) values. The enzymatic activity is thermolabile with a T1/2 of less than 1 min at 45 degrees C and a maximal activity at 40 degrees C. The breaking point or transition temperature is 25 degrees C in an Arrhenius plot. The activation energies were 95 kJ/mol from 0 to 25 degrees C and 30 kJ/mol from 25 to 40 degrees C. Fatty acid competition studies showed inhibition by unlabeled docosahexaenoic and arachidonic acids with a Ki of 31 and 37 microM, respectively, in the absence and 18 and 7.7 microM in the presence of Triton X-100. Palmitic acid and oleic acid slightly inhibited the reaction whereas linoleic acid inhibited it to a moderate extent. It is concluded that this very active enzyme can activate arachidonic acid as well as docosahexaenoic acid in brain microsomes. In addition, this reaction may be involved in regulating the pool size of these free fatty acids in brain by rapid removal through activation, thus limiting eicosanoid formation. Moreover, the rapid formation of polyenoic acyl-coenzyme A may participate in the retention of essential fatty acids in the central nervous system.     

Immunochemical cross reactivity of the antibody elicited against L-histidine decarboxylase purified from the whole bodies of fetal rats with the enzyme from rat brain

Rat cytosolic glutathione S-transferases catalyzed the conjugation of phenethyl chloride and phenethyl bromide with glutathione. The reaction proceeded with a high degree of stereoselectivity. The glutathione conjugate possessing the (R,S,S)- absolute configuration was formed in major quantities from the racemic substrates. The use of the enantiomers of the phenethyl chloride substrates indicated that the (S)-phenethyl chloride was conjugated in preference to the (R)-enantiomer. The conjugation proceeded with inversion of configuration at the benzylic carbon consistent with an SN₂-type mechanism. The stereoselectivity was greater for phenethyl chloride than for phenethyl bromide. Varying the substrate or enzyme concentration had no effect upon the observed stereoselectivity. The diastereomeric glutathione conjugates were separated by high performance liquid chromatography. These findings represent the first demonstration of the substrate stereoselectivity of the glutathione S-transferases.