Voltage dependence of theChara proton pump revealed by current-voltage measurement during rapid metabolic blockade with cyanide

Summary It is generally agreed that solute transport across theChara plasma membrane is energized by a proton electrochemical gradient maintained by an H⁺-extruding ATPase. Nonetheless, as deduced from steady-state current-voltage (I-V) measurements, the kinetic and thermodynamic constraints on H⁺-ATPase function remain in dispute. Uncertainties necessarily surround long-term effects of the relatively nonspecific antagonists used in the past; but a second, and potentially more serious problem has sprung from the custom of subtracting, across the voltage spectrum, currents recorded following pump inhibition from currents measured in the control. This practice must fail to yield the trueI-V profile for the pump when treatments alter the thermodynamic pressure on transport.We have reviewed these issues, using rapid metabolic blockade with cyanide and fitting the resultant whole-cellI-V and difference-current-voltage (dI-V) relations to a reaction kinetic model for the pump and parallel, ensemble leak. Measurements were carried out after blocking excitation with LaCl₃, so that steady-state currents could be recorded under voltage clamp between –400 and +100 mV. Exposures to 1mm NaCN (CN) and 0.4mm salicylhydroxamic acid (SHAM) depolarized (positive-going)Chara membrane potentials by 44–112 mV with a mean half time of 5.4±0.8 sec (n=13). ATP contents, which were followed in parallel experiments, decayed coincidently with a mean half time of 5.3±0.9 sec ([ATP] _t=0, 0.74±0.3mm; [ATP] _t=x , 0.23±0.02mm). Current-voltage response to metabolic blockade was described quantitatively in context of these changes in ATP content and the consequent reduction in pump turnover rate accompanied by variable declines in ensemble leak conductance. Analyses ofdI-V curves (±CN+SHAM) as well as of families ofI-V curves taken at times during CN+SHAM exposures indicated a stoichiometry for the pump of one charge (H⁺) transported per ATP hydrolyzed and an equilibrium potential near –420 mV at neutral external pH; under these conditions, the pump accounted for approximately 60–75% of the total membrane conductance nearV _m. Complementary results were obtained also in fitting previously publishedI-V data gathered over the external pH range 4.5–7.5 Kinetic features deduced for the pump were dominated by a slow step preceding H⁺ unloading outside, and by recycling and loading steps on the inside which were in rapid equilibrium. These characteristics predict, in marked contrast to the situation forNeurospora, that cytoplasmic acid loads inChara should shift the pumpI-V curve negative-going along the voltage axis with little change in maximum current output at positive voltages.     

Comparison of theα-globin gene cluster structure in Perissodactyla

Summary To investigate molecular evolution in a mammalian order with a comprehensive fossil record, we have constructed-globin-like gene cluster maps for members of the order Perissodactyla. Although the arrangement of genes is the same in the five Equidae examined, the tapir and rhinoceros differ from each other and the horse in the position and number of their genes, but not in the arrangement of their and genes. In contrast to morphological work, a dendrogram derived from restriction site maps associates the tapir with the horse rather than with the rhinoceros; however, this phylogeny is not statistically significant. Among the Equidae,Equus caballus emerges as an outgroup, in agreement with data from other disciplines.     

Biexponential Kinetics of (R)-α-[3H]Methylhistamine Binding to the Rat Brain H3 Histamine Receptor

The H3 histamine receptor is a high-affinity receptor reported to mediate inhibition of CNS histidine decarboxylase activity and depolarization-induced histamine release. We have used (R)-alpha-[3H]methylhistamine, a specific, high-affinity agonist, to characterize ligand binding to this receptor. Saturation binding studies with rat brain membranes disclosed a single class of sites (KD = 0.68 nM; Bmax = 78 fmol/mg of protein). Competition binding assays also yielded an apparently single class of sites with a rank order of potency for ligands characteristic of an H3 histamine receptor: N alpha-methylhistamine, (R)-alpha-methylhistamine greater than histamine, thioperamide greater than impromidine greater than burimamide greater than dimaprit. In contrast, kinetic studies disclosed two classes of sites, one with fast, the other with slow on-and-off rates. Density of (R)-alpha-[3H]methylhistamine binding followed the order: caudate, midbrain (thalamus and hippocampus), cortex greater than hypothalamus greater than brainstem greater than cerebellum. These data are consistent with an H3 histamine receptor, distinct from H1 and H2 receptors, that occurs in two conformations with respect to agonist association and dissociation or with multiple H3 receptor subtypes that are at present pharmacologically undifferentiated.     

Inhibition of hexose transport in the human erythrocyte by 5, 5′-dithiobis(2-nitrobenzoic acid): Role of an exofacial carrier sulfhydryl group

Summary The sulfhydryl reagent 5, 5-dithiobis (2-nitrobenzoic acid) (DTNB) was used to study the functional role of an exofacial sulfhydryl group on the human erythrocyte hexose carrier. Above 1mm DTNB rapidly inhibited erythrocyte 3-O-methylglucose influx, but only to about half of control rates. Efflux was also inhibited, but to a lesser extent. Uptake inhibition was completely reversed by incubation and washing with 10mm cysteine, whereas it was only partially reduced by washing in buffer alone, suggesting both covalent and noncovalent interactions. The covalent thiol-reversible reaction of DTNB occurred on the exofacial carrier, since (i) penetration of DTNB into cells was minimal, (ii) blockade of potential uptake via the anion transporter did not affect DTNB-induced hexose transport inhibition, and (iii) DTNB protected from transport inhibition by the impermeant sulfhydryl reagent glutathione-maleimide-I. Maltose at 120mm accelerated the covalent transport inhibition induced by DTNB, whereas 6.5 m cytochalasin B had the opposite effect, indicating under the one-site carrier model that the reactive sulfhydryl is on the outward-facing carrier but not in the substrate-binding site. In contrast to glutathione-maleimide-I, however, DTNB did not restrict the ability of the carrier to reorient inwardly, since it did not affect equilibrium cytochalasin B binding. Thus, carrier conformation determines exposure of the exofacial carrier sulfydryl, but reaction of this group may not always lock the carrier in an outward-facing conformation.     

Antipeptide antibodies to the 141-1141-1141-1receptor confirm the extracellular orientation of the amino-terminus and the putative first extracellular loop

Summary We developed site-directed rabbit antisera against synthetic peptides selected from the deduced amino acid sequence of the hamster lung ₂-adrenergic receptor (amino acids 16–31 and 174–189, respectively). All antisera directed against peptide 1 (four of four rabbits) as well as two antisera directed against peptide 2 (two of four rabbits) recognized the purified ₂-adrenergic receptor in immunoblot conditions when used at a dilution of 1500. Antisera directed against peptide 1 as well as peptide 2 were able to immunoprecipitate iodinated as well as¹²⁵I-cyanopindolol tabeled ₂-adrenergic receptor. This last result implies that the recognized epitopes do not contain the¹²⁵I-cyanopindolol binding domain of the ₂-adrenergic receptor. Immunoblot experiments performed on membrane fractions from hamster lung tissue showed that immunoreactive bands at 64,000, 57,000, 47,000, 44,000 and 38,000 daltons were specifically detected. When purified ₂-adrenergic receptor was iodinated and submitted to glycolytic and/or tryptic treatments, species with similar molecular weights could be recovered. Then, the immunoreactive bands probably correspond to native ₂-adrenergic receptor and to degradative or nonglycosylated species of this molecule. The antisera were also able to detect immunoreactive molecules in murine and human cell lines, suggesting conservation of the probed sequences between these species. Enzymatic linked immunosorbent assay tests on intact cells and immunofluorescence studies confirmed that the amino-terminus and putative first extracellular loop are extracellularly located. Immunofluorescence studies on mouse brain primary cultures showed that cells expressing ₂-adrenergic receptor-like molecules exhibited a neuronal phenotype.     

A Glycine Site Associated with N-Methyl-d-Aspartic Acid Receptors: Characterization and Identification of a New Class of Antagonists

Membranes from rat telencephalon contain a single class of strychnine-insensitive glycine sites. That these sites are associated with N-methyl-D-aspartic acid (NMDA) receptors is indicated by the observations that [3H]glycine binding is selectively modulated by NMDA receptor ligands and, conversely, that several amino acids interacting with the glycine sites increase [3H]N-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) binding to the phencyclidine site of the NMDA receptor. The endogenous compound kynurenate and several related quinoline and quinoxaline derivatives inhibit glycine binding with affinities that are much higher than their affinities for glutamate binding sites. In contrast to glycine, kynurenate-type compounds inhibit [3H]TCP binding and thus are suggested to form a novel class of antagonists of the NMDA receptor acting through the glycine site. These results suggest the existence of a dual and opposite modulation of NMDA receptors by endogenous ligands.     

Studies on the Formation of 6-Hydroxydopamine in Mouse Brain After Administration of 2,4,5-Trihydroxyphenylalanine (6-HydroxyDOPA)

2,4,5-Trihydroxyphenylalanine (6-OH-DOPA) destroys central and peripheral noradrenergic neurons, while sparing dopaminergic neurons. Previous studies indicate that 6-OH-DOPA toxicity is mediated by the formation of 6-hydroxydopamine. However, levels of 6-hydroxydopamine in brain following peripheral administration of 6-OH-DOPA have not been documented. In the current study, 6-OH-DOPA and 6-hydroxydopamine were measured in brain by HPLC with electrochemical detection after intraperitoneal injection of 6-OH-DOPA. When mice were injected with 100 mg 6-OH-DOPA/kg, 6-hydroxydopamine levels in the striatum were highest (1.9 microgram/g) at 15 min and fell slowly to 24% of the peak value at 4 h. Experiments with reserpine indicated that the relatively stability of 6-hydroxydopamine was largely dependent upon storage in synaptic vesicles. Reserpine (10 mg/kg) lowered striatal 6-hydroxydopamine levels to 21.6% of control (non-reserpine-treated) values at 1 h, and to 8.9% of control values at 4 h. Levels of 6-hydroxydopamine in the striatum at 1 h were increased 113% by pargyline (100 mg/kg), 145% by alpha-methyldopahydrazine (carbidopa; 25 mg/kg), and 261% by pargyline and carbidopa together. Levels of dopamine in the striatum were unchanged at 2.5 h after 200 mg 6-OH-DOPA/kg (with pargyline and 50 mg carbidopa/kg), whereas levels of norepinephrine in the frontal cortex fell by 77%. At the same time, 6-hydroxydopamine levels were 8.8-fold higher in the striatum (5.54 micrograms/g) than in the cortex (0.63 micrograms/g).(ABSTRACT TRUNCATED AT 250 WORDS)     

Entropy as a factor in the binding of γ-aminobutyric acid and nipecotic acid to the γ-aminobutyric acid transport system

Nipecotic acid is one of the most potent competitive inhibitors and alternative substrates for the high-affinity -aminobutyric acid transport system in neurons, but the structural basis of this potency is unclear. Because -aminobutyrate is a highly flexible molecule in solution, it would be expected to lose rotational entropy upon binding to the transport system, a change which does not favor binding. Nipecotic acid, in contrast, is a much less flexible molecule, and one would expect the loss of conformational entropy upon binding to be smaller thus favoring the binding of nipecotic acid over -aminobutyric acid. To investigate this possibility, the thermodynamic parameters, G°, H°, and S°, were determined for the binding of -aminobutyrate and nipecotic acid to the high affinity GABA transport system in synaptosomes. In keeping with expectations, the apparent entropy change for nipecotic acid binding (112±13 J·K^–1) was more favorable than the apparent entropy change for -aminobutyric acid binding (61.3±6.6 J·K^–1). The results suggest that restricted conformation per se is an important contributory factor to the affinity of nipecotic acid for the high-affinity transport system for -aminobutyric acid.This work was conducted when both authors were at the Department of Chemistry, University of Maryland, College Park.Special issue dedicated to Dr. Elling Kvamme.     

Study of amino acid formation during palmitate oxidation in rat brain mitochondria

The interrelation of palmitate oxidation with amino acid formation in rat brain mitochondria has been investigated in purified mitochondria of nonsynaptic origin by measuring the formation of aspartate, -ketoglutarate, and glutamate during palmitate oxidation, and also by assaying¹⁴C-products of [1-¹⁴C]palmitate oxidation. Oxidation of palmitate (or [1-¹⁴C]palmitate) resulted in the formation of aspartate (or¹⁴C-aspartate), and the oxidation was inhibited by aminooxyacetate (an inhibitor of transaminase), Palmitate oxidation also resulted in -ketoglutarate formation, which was sensitive to the effect of aminooxyacetate. Addition of NH₄Cl was found to increase¹⁴C-products and formation of -ketoglutarate, whereas glutamate formation was not increased unless the rate of palmitate oxidation was reduced by 50% by aminooxyacetate or -ketoglutarate was added exogenously. Exogenous -ketoglutarate was found to decrease¹⁴C-products, but not aspartate formation. These results indicated that palmitate oxidation was closely related to aspartate formation via aspartate aminotransferase. During palmitate oxidation without aminooxyacetate or added -ketoglutarate, however, -ketoglutarate was not available for glutamate formation via glutamate dehydrogenase. We discuss the possibility that this was because (a) oxidative decarboxylation of -ketoglutarate to form succinyl-CoA was favored over glutamate formation for the competition for -ketoglutarate in the same pool, and (b) the pool of -ketoglutarate produced in the aspartate aminotransferase reaction did not serve as substrate for glutamate formation.