Effect of lipid hydroperoxide on Xenopus oocytes and on neurotransmitter receptors synthesized in Xenopus oocytes injected with exogenous mRNA

The effect of 13-L-hydroperoxylinoleic acid (LOOH) on both Xenopus oocytes and neurotransmitter receptors synthesized in the oocytes was studied by electrophysiological and ion flux measurement. Addition of LOOH to the incubation mixture of the oocytes raised the membrane potential and decreased the membrane resistance of the oocytes. These effects of LOOH on the oocytes were reversed within a few hours by incubation with frog Ringer solution. Addition of LOOH also caused an increase of Li+ and 45Ca2+ uptake into the oocytes. However, production of alkoxy radicals by the addition of FeCl2 to the incubation mixture containing LOOH did not accelerate the damage to the oocytes by LOOH. So essential toxicity is caused possibly by an increase in the membrane permeability resulting from disturbance of the lipid bilayer arrangement, not from production of active alkoxy radicals during decomposition of LOOH. Nicotinic acetylcholine and gamma-aminobutyric acid receptors were synthesized in Xenopus oocytes by injecting mRNA prepared from Electrophorus electricus electroplax and rat brain. LOOH noncompetitively inhibited the function of these receptors and also increased the rate of desensitization of the receptors.

Expression of the functional D-glucose transport system in Xenopus oocytes injected with mRNA of rat small intestine

mRNA prepared from rat small intestine was injected into Xenopus oocytes. The injected oocytes showed a clear electrical response to D-glucose (Glu) in the form of membrane depolarization and conductance increase, while none was shown to D-fructose. The membrane electrical response of the injected oocytes evoked by Glu followed the Michaelis-Menten type kinetics and was dependent on the membrane potential of the oocyte. Replacing the Na+ of the bathing buffer with choline+ resulted in no response to Glu. Thus, a Glu transport system coupled to a Na+ gradient was expressed in Xenopus oocytes by injecting mRNA from rat small intestine.

Minimal model to account for the membrane conductance increase and desensitization of gamma-aminobutyric acid receptors synthesized in the Xenopus oocytes injected with rat brain mRNA

gamma-Aminobutyric acid (GABA) receptors, which translocate chloride anion with binding GABA, were synthesized in Xenopus oocytes by injecting rat brain mRNA. GABA-elicited responses in the oocytes were measured electrophysiologically by the current-clamped method. Five different measurements were made to establish the relationship between GABA concentration and the electrical responses: (1) the GABA-elicited conductance increase before desensitization; (2) the rate of desensitization of GABA receptors; (3) the rate of recovery of desensitized receptors upon removal of GABA; (4) the GABA-elicited conductance increase after desensitization equilibrium; (5) the fraction of the active form of GABA receptors after desensitization equilibrium. These results were interpreted on the basis of the minimal model proposed for nicotinic acetylcholine receptor in Electrophorus electricus electroplax [Hess, G. P., Cash, D. J., & Aoshima, H. (1983) Annu. Rev. Biophys. Bioeng. 12, 443-473]. Estimated equilibrium and rate constants in the model for GABA receptors could successfully explain the results of the five above measurements.

Induction of muscarinic acetylcholine, serotonin and substance P receptors in Xenopus oocytes injected with mRNA prepared from the small intestine of rats

Serotonin and muscarinic acetylcholine (ACh) receptors were clearly induced in Xenopus oocyte injected with mRNA prepared from the small intestines of rats. Their response to ACh and serotonin was composed of 4 distinct components: fast and slow depolarization, slow hyperpolarization and large membrane potential fluctuation. About three-quarters of the injected oocytes responded to substance P. The response of the injected oocytes to substance P was transient and decayed even in the presence of substance P, indicating the presence of desensitization. However, the injected oocytes showed no response to 6 other drugs analyzed: adrenaline, noradrenaline, dopamine, gamma-aminobutyric acid, glycine and glutamate.

Acetylcholine receptor-controlled ion translocation caused by phenyltrimethylammonium and nereistoxin: simple estimation of equilibrium constants of the minimal model

The rate of slow Li+ influx and the fraction of active form of acetylcholine receptor (AChR) of Electrophorus electricus membrane vesicles at equilibrium between the active and desensitized forms of the receptor were measured in the presence of various concentrations of phenyltrimethylammonium (PTA) and nereistoxin (NTX), by a simple filtration assay and flame emission spectroscopy. The equilibrium constants of these ligands in the minimal model, which accounts for the AChR-mediated ion flux, were estimated simply from these two measurements, since the equilibrium constants for acetylcholine (ACh) and carbamylcholine (Carb) estimated from two kinetic measurements agreed well with those estimated from five sophisticated kinetic measurements of AChR-mediated ion fluxes. PTA showed high potency but not high efficacy, and showed inhibition when large doses were applied. NTX showed both low potency and low efficacy and acted as an inhibitor when it was added with Carb. The apparent dissociation constants of these three agonists evaluated from the minimal model and the equilibrium constants agreed with those obtained by assay of inhibition of radiolabeled ligand binding.

Li+ uptake into Xenopus and Cynops oocytes injected with exogenous mRNA, observed by flame emission spectroscopy

Li+ uptake into Xenopus oocytes was measured by flame emission spectroscopy. Li+ uptake into the oocytes increased proportionally with incubation time and was dependent on either pH or temperature. Maximum uptake of Li+ was observed around pH 7. Li+ uptake into Xenopus oocytes increased by a factor of roughly 7 over the range 4-30 degrees C. When mRNA prepared from electroplax of Electrophorus electricus was injected into Xenopus or Cynops oocytes, Li+ uptake into the injected oocytes increased by the addition of carbamylcholine (Carb), an agonist of the acetylcholine receptor (AChR). This increase of Li+ uptake by Carb was inhibited by d-tubocurarine, an antagonist of nicotinic AChR. Thus, a new method was established for detection of the activity of nicotinic AChR synthesized in oocytes injected with exogenous mRNA.

New translation system of mRNA coding for neurotransmitter receptors using oocytes of the newt, Cynops pyrrhogaster

Eel electroplax mRNA was injected into oocytes of newts (Cynops pyrrhogaster) and the injection induced synthesis of a nicotinic acetylcholine receptor in the oocyte membrane. The time course of the induction and dose-response relationship of the receptor were recorded electrophysiologically. The receptor responses and their developmental changes were similar to those of Xenopus oocytes injected with eel mRNA. Newt oocytes lived much longer (7-10 days) than Xenopus oocytes (3-4 days) under our experimental conditions. In non-injected newt oocytes, native transmitter receptors were not observed. In addition, newts have large oocytes (1.6-1.9 mm in diameter), into which a large amount of mRNA could be easily injected. Thus, newt oocytes may be a more useful system to translate exogenous mRNAs coding for neurotransmitter receptors than Xenopus oocytes.

Time course of the induction of acetylcholine receptors in Xenopus oocytes injected with mRNA from Electrophorus electricus electroplax

mRNA from the electroplax of adult Electrophorus electricus was injected into Xenopus oocytes. At various times after injection, the induction of the nicotinic acetylcholine (ACh) receptor in the oocyte membrane was studied electrophysiologically using a two-electrode current clamp. When the ACh sensitivity was induced, membrane potential and conductance rapidly rose from the resting value to their peaks and slowly fell (desensitization) in response to bath-applied 0.1 mM ACh. It took a latency period of 8 +/- 2.6 h (n = 10) from mRNA injection to the first appearance of the ACh sensitivity. During about 10 h incubation after the onset, the increasing speed in the peak of ACh-induced conductance change with incubation time was slow at first and accelerated later. The speed was then decelerated until the induction stages of 60 h. Apparent desensitization properties of the induced receptors changed with an increase of incubation time. During the early induction stages, the conductance decline after its peak followed two exponentials in a minute time region of ACh application: an early slow and a late fast one. The rate of decline in the late fast component slowed down markedly with incubation time. Finally the desensitization followed a single exponential.

A second, slower inactivation process in acetylcholine receptor-rich membrane vesicles prepared from Electrophorus electricus

An agonist such as carbamylcholine or phenyltrimethylammonium induced a second, slower complete inactivation of acetylcholine receptor prepared from Electrophorus electricus. The rate of this inactivation of the receptor followed first-order kinetics. The rate constant of the inactivation increased with the agonist concentration until it reached a plateau, the value of which was 0.19 h-1 at 4.5 degrees C. The reaction was also temperature dependent, and the activation energy of the inactivation caused by 1 mM carbamylcholine was estimated to be 7.6 kcal/mol. The inactive receptor was reconverted to the active form with a rate constant of about 0.015 h-1 at 4.5 degrees C when the carbamylcholine concentration (0.1 mM) was reduced by 15-fold dilution in eel Ringer's solution. These results can be interpreted by adding, to the minimal reaction scheme proposed by the Hess group, a second, slower, reversible inactivation process either through the intact form or through the first desensitized form of the receptor binding two agonist molecules.

Inhibition schemes for acetylcholine receptor-mediated ion translocation in the presence of various kinds of inhibitors

Some basic inhibition schemes for acetylcholine receptor (AChR)-mediated ion translocation in the presence of agonist, cholinergic ligand, and inhibitors are proposed on the basis of the minimum reaction scheme (Hess, G.P., Cash, D.J., & Aoshima, H. (1983) Annu. Rev. Biophys. Bioeng. 12, 443-473). Equations for the rate coefficients of ion flux before and after desensitization, JA and JD, and for desensitization, a, were derived from each scheme, assuming that binding of inhibitors to AChR does not affect the values of rate and equilibrium constants of cholinergic ligand to the receptor. In the presence of inhibitors, AChR-mediated transmembrane Li+ influx caused by carbamylcholine (Carb) was measured by using Electrophorus electricus membrane vesicles, a simple filtration assay and flame emission spectroscopy. The dependence of the ratios of rate coefficients on the concentration of ligand and inhibitors was examined in detail. The inhibition constants of d-tubocurarine and caffeine were estimated to be about 31 nM and 0.84 mM, respectively, on the basis of a simple competitive inhibition scheme, and that of procaine was estimated to be 0.11 mM on the basis of a simple noncompetitive one.

Cocaine and phencyclidine inhibition of the acetylcholine receptor: analysis of the mechanisms of action based on measurements of ion flux in the millisecond-to-minute time region

The effects of cocaine and of phencyclidine and procaine on acetylcholine receptor-controlled ion flux were measured in the millisecond-to-minute time region. Chemical kinetic measurements of ion flux were made in membrane vesicles prepared from the electric organ of Electrophorus electricus and in PC-12 cells, a sympathetic neuronal cell line. A quench-flow technique was used to measure ion flux in the millisecond-to-second range in membrane vesicles. Cocaine and phencyclidine both inhibit acetylcholine receptor-controlled ion flux, but by different mechanisms. Both compounds decrease the initial rate of ion flux, an effect observed with the local anesthetic procaine. This inhibition cannot be prevented by saturating concentrations of acetylcholine (1 mM). These results from chemical kinetic experiments are consistent with electrophysiological measurements which indicate that local anesthetics act by interfering with the movement of ions through receptor-formed channels. The chemical kinetic experiments, however, give additional information about the action of phencyclidine. They indicate that phencyclidine also increases the rate of receptor inactivation (desensitization) and changes the equilibrium between active and inactive receptor conformations, effects not observed in the presence of cocaine or procaine.

Mechanism of inactivation (desensitization) of acetylcholine receptor. Investigations by fast reaction techniques with membrane vesicles

Exposure of the acetylcholine receptor to acetylcholine, or its stable analogue carbamylcholine, inactivates (desensitizes) the receptor. Inactivation of receptor-controlled ion (86Rb+) flux in the presence of different concentrations of carbamylcholine (12.5 microM to 28 mM) was measured in the millisecond to minute time region, using a quench flow technique and membrane vesicles prepared from the electric organ of Electrophorus electricus. Three different kinetic measurements were made to establish the relationship between carbamylcholine concentration and the ion translocation process: (i) the rate of inactivation of the ion translocation process; (ii) the rate of recovery of the inactivated receptor upon removal of carbamylcholine; and (iii) the rate of the ion flux mediated by equilibrium mixtures of active and inactive receptor forms. The kinetics of these three processes follow single-exponential rate laws, and simple analytical expressions for their ligand concentration dependence could be used. Therefore, it was possible to determine the value of the rate constants in a scheme relating the ligand binding steps to ion translocation, and to predict the dependence of these rate constants on carbamylcholine concentration over the 200-fold range investigated.

Acetylcholine-induced cation translocation across cell membranes and inactivation of the acetylcholine receptor: chemical kinetic measurements in the millisecond time region

Acetylcholine-induced flux of inorganic ions across membranes and inactivation of the acetylcholine receptor were measured at pH 7.0, 1 degrees C, over a 5000-fold concentration range of acetylcholine. Receptor-containing electroplax membrane vesicles prepared from Electrophorus electricus and a quench-flow technique were used, allowing flux to be measured in the 2-msec to 1-min time region. Five different measurements were made: (i) rate of ion translocation with the active state of the receptor, (ii) rate of the slower ion translocation after equilibration of active and inactive receptor states, (iii) rate of inactivation, (iv) equilibrium between active and inactive forms of the receptor, and (v) reactivation of inactivated receptor. The kinetics of the steps in the receptor-controlled ion flux follow single-exponential rate laws, and simple analytical expressions for their ligand concentration dependence can be used. Thus, the rate and equilibrium constants in a scheme that relates the ligand binding steps to ion translocation could be evaluated. It was found that the dependence of the receptor-controlled ion translocation over the concentration range investigated obeys the integrated rate equation based on the proposed mechanism. The flux rate before inactivation was approximately 10(7) ions sec-1 per receptor, which is comparable with that measured electrophysiologically in muscle cells. The half-time of inactivation is approximately 100 msec when the receptor is saturated with acetylcholine. The specific reaction rate of the ion translocation (J) is 3 X 10(7) M-1 sec-1. The results support a minimum reaction mechanism previously proposed on the basis of experiments in which carbamylcholine was used.

Specific reaction rate of acetylcholine receptor-controlled ion translocation: a comparison of measurements with membrane vesicles and with muscle cells

The specific reaction rate (J) of the acetylcholine receptor-controlled ion translocation has been determined. In eel Ringer's solution (pH 7.0) at 1 degrees C, J = 3 X 10(7) M-1 sec-1. J is an intrinsic constant that is characteristic of the receptor and independent of other properties of a receptor-containing cell that also determine the rates of ion translocation. Membrane vesicles (prepared from the electric organ of Electrophorus electricus) and a flow-quench technique that has a millisecond time resolution were used to measure the receptor-controlled ion translocation. Using the value of J and the molar concentrations of receptor sites and inorganic ions, we calculated that 6 X 10(3) ions are translocated per msec per receptor. Analysis of electrical noise in frog muscle cells at temperatures above 8 degrees C [Nether, E. & Stevens, C. F. (1977) Annu. Rev. Biophys. Bioeng. 6, 345-381] gave a value of about 1 X 10(4) ions msec-1 per channel. Thus, each technique gives essentially the same result. It is now possible, therefore, to correlate the results obtained when receptor function is measured in two different ways in membrane vesicles and in muscle cells: (i) chemical kinetic measurements, using membrane vesicles, which relate the ligand binding and ion translocation processes and (ii) analysis of acetylcholine noise in muscle cells [Katz, B. & Miledi, R. (1972) J. Physiol. (London) 224, 665-699], which allows one to measure elementary steps in the formation of ion channels through the cell membrane.

Electron spin resonance studies on the lipoxygenase reaction by spin trapping and spin labelling methods

The rate of oxygenation and that of trapping linoleic acid free radicals in the lipoxygenase [EC 1.13.11.12] reaction were measured in the presence of linoleic acid, oxygen, and nitrosobenzene at various concentrations, with a Clark oxygen electrode and ESR spectroscopy. The results were interpreted under the assumption that the free radical of linoleic acid, an intermediate of the lipoxygenase reaction, reacts competitively with oxygen or nitrosobenzene. The oxidation of the iron in the active site of lipoxygenase caused by the spin label reagent, 2-(10-carboxydecyl)-2-hexyl-4,4-dimethyl-3-oxazolidinyloxyl, was also observed by ESR- and fluorescence-spectroscopy.

High performance liquid chromatography of the hydroperoxides produced by lipoxygenases

Separation of 13-hydroperoxylinoleic acid or 13-hydroperoxylinolenic acid from linoleic acid or linolenic acid, respectively, was carried out easily and quickly by high performance liquid chromatography on porous polymer gel (TSK-Gel LS-140) using n-hexane/ethanol as an eluent. An eluent containing a large amount of n-hexane (96%) made possible the separation of 9- and 13-hydroperoxylinoleic acids. These methods were applicable for analyses of the products obtained by the incubation of soybean lipoxygenase-1 [linoleate: oxygen oxidoreductase, EC 1.13.11.12] with linoleic acid or 13-hydroperoxylinoleic acid.

Inactivation of Streptomyces subtilisin inhibitory by chemical modifications

1. The inhibitory activity of an alkaline protease inhibitor, (Streptomyces subtilisin inhibitor) towards subtilisin is found to decrease by photooxidation sensitized by methylene blue with a clear pH dependence, the midpoint of which is about 6.0. 2. Amino acid analyses of photooxidized Streptomyces subtilisin inhibitor indicate that one of the two histidyl residues and the three methionyl residues are destroyed, concomittant with the loss of inhibitory activity. 3. In accordance with this observation, one of the clearly resolved nuclear magnetic resonances from C2-protons of the two histidyl residues is selectively diminished. This histidyl residue, sensitive to photooxidation and giving a proton magnetic resonance peak at lower field, is assigned to His-106 from peptide analyses. 4. Independent modification of methionyl residues by a reaction with H2O2 or Cl2 also decreases the inhibitory activity of Streptomyces subtilisin inhibitor. 5. Modification of lysyl, tyrosyl and tryptophanyl residues by diazonium-1-H-tetrazole does not lead to the loss of the inhibitory activity. 6. The above results indicate that one or more methionyl residue(s) are essential to the inhibitory activity of Streptomyces subtilisin inhibitor, whereas lysyl, tyrosyl and tryptophanyl residues are not essential to the inhibitory activity. Modification of His-106 is also strongly related to the loss of activity, although its distinct participation in the inactivation mechanism has not been demonstrated.

Kinetic study of lipoxygenase-hydroperoxylinoleic acid interaction

Interaction of lipoxygenase with hydroperoxylinoleic acid, which is the product of this enzyme reaction and acts as an activator, was studied kinetically by the fluorescence stopped-flow method. The kinetic features are consistent with a two-step mechanism involving a fast bimolecular association process followed by a slow unimolecular process. The dissociation constant of the bimolecular process was 3 (+/-2) - 10(-5) M, which was appreciably dependent on temperature and pH, in contrast to the rate constant of the latter process. The enthalpy and the entropy of activation for the unimolecular process were estimated to be 21 kcal/mol and 20 e.u., respectively. The pH dependence of the rate constant indicated that an ionizable group with pK of about 8.6 is involved in the interaction. Linoleic acid, the substrate of lipoxygenase, and oleic acid inhibited the interaction between the lipoxygenase and the hydroperoxylinoleic acid by reducing the rate. A series of saturated monohydric alcohols also reduced the rate of the interaction as the chain length of the alcohols increases, though methanol and ethanol increased the rate of the interaction.

Peptide hydrogen exchange rates in Streptomyces subtilisin inhibitor

The exchange reaction of the peptide NH protons of a microbial protease inhibitor (Streptomyces subtilisin inhibitor) with deuterium atoms in 2H2O (p2H 6.8) has been studied by proton magnetic resonance in the temperature range 56-71 degrees C. Both slowly and rapidly exchanging processes have been observed. The number of slowly exchanging protons is estimated to be 25 +/- 2 per subunit of the protein molecule. The decay of the slowly exchanging proton signals follows a single time-exponential function at each temperature. The observed first-order rate constants have been analyzed to give the denaturated fraction of the protein as a function of temperature with a consequent enthalpy (56 kcal/mol) and an entropy (137 cal/degree per mol) of denaturation. The results indicate the high conformational stability of this protein against heat denaturation.