Modulation of ligand binding to components of the GABAA receptor complex by ammonia: implications for the pathogenesis of hyperammonemic syndromes

The effects of 5-2500 microM concentrations of neutral ammonium salts on the binding of ligands to components of the GABAA receptor complex were investigated. [3H]Flunitrazepam binding to the benzodiazepine receptor was enhanced by ammonium (10-500 microM), but not sodium tartrate with EC50 = 98 microM and Emax = 31%. Further increasing ammonium tartrate concentrations (500-2500 microM) decreased [3H]flunitrazepam binding to control levels. The ammonium tartrate-induced increase in [3H]flunitrazepam binding was manifested as a 50% decrease in Kd. Furthermore, GABA increased the potency of ammonium tartrate in enhancing [3H]flunitrazepam binding by 63%. [3H]Ro 15-1788 and [3H]Ro 15-4513 binding to the benzodiazepine receptor was not significantly enhanced by ammonium tartrate (Emax approximately 13%). Ammonium tartrate also increased, then decreased the binding of 500 nM [3H]muscimol to the GABAA receptor (EC50 = 52 microM, Emax = 30%) in a concentration-dependent manner, but had no effect on [3H]SR 95-531 binding (Emax < 16%). The ammonium tartrate-induced alterations in [3H]muscimol binding were demonstrated in saturation assays as the loss of the high affinity binding site and a 27% increase in the Bmax of the low affinity binding site. These results indicate that ammonia biphasically enhances, then returns ligand binding to both the GABA and benzodiazepine receptor components of the GABAA receptor complex to control levels in a barbiturate-like fashion. This suggests that ammonia may enhance GABAergic neurotransmission at concentrations commonly encountered in hepatic failure, an event preceding the suppression of inhibitory neuronal function observed at higher (> 1 mM) ammonia concentrations. This increase in GABAergic neurotransmission is consistent with the clinical picture of lethargy, ataxia and cognitive deficits associated with liver failure and congenital hyperammonemia.

Kinetics of nucleotide-induced changes in the tryptophan fluorescence of the molecular chaperone Hsc70 and its subfragments suggest the ATP-induced conformational change follows initial ATP binding

The kinetics of nucleotide-induced changes of tryptophan fluorescence have been measured for recombinant bovine 70 kDa heat shock cognate protein (Hsc70), a 60 kDa subfragment (amino acid residues 1-554) which has ATPase and peptide binding activities, and a 44 kDa subfragment (residues 1-386) which has only ATPase activity. The fluorescence changes resulting from ATP binding to Hsc70 and the 60 kDa fragment are biphasic, and can be interpreted as arising from a two-step process in which ATP initially binds in a bimolecular reaction, followed by a conformational change of the protein-MgATP complex. Fluorescence changes resulting from ADP binding indicate a single-step, bimolecular process. Under single-cycle conditions of the ATPase reaction, a fluorescence change is observed whose rate constant correlates with product release in Hsc70, and with product release/ATP hydrolysis (which are kinetically indistinguishable under single-cycle conditions) in the 60 kDa fragment. These data support a scheme for Hsc70 in which a conformational transition is induced after initial ATP binding but prior to hydrolysis, and the reverse transition is induced by product release. The 60 kDa fragment shows behavior that is quantitatively similar to that of Hsc70. The 44 kDa ATPase fragment does not show biphasic kinetics for ATP binding, and does not show fluorescence changes that suggest conformational changes of the type seen in Hsc70 and the 60 kDa fragment.

Replacing a surface loop endows ribonuclease A with angiogenic activity

Angiogenin (ANG) promotes the formation of blood vessels in animals. This hormone is a small, monomeric protein that is homologous to bovine pancreatic ribonuclease A (RNase). ANG is a poor ribonuclease but its ribonucleolytic activity is essential for its angiogenic activity. RNase is not angiogenic. A hybrid protein was produced in which 13 residues of a divergent surface loop of ANG were substituted for the analogous 15 residues of RNase. The value of kcat/Km for the cleavage of uridylyl(3'-->5')adenosine by this hybrid protein was 20-fold less than that of RNase but 10(5)-fold greater than that of ANG. The thermal stability of the hybrid protein was also less than that of RNase. Nevertheless, the RNase/ANG hybrid protein promotes angiogenesis in mice at least as extensively as does authentic ANG. Thus we present a protein endowed with a noncognate biological activity simply by replacing a single element of secondary structure. In addition, a 13-residue peptide corresponding to the surface loop of ANG inhibits endogenous angiogenesis in mice. These results support a model in which both a surface loop and a catalytic site are necessary for the promotion of blood vessel formation by ANG or RNase. The dissection of structure/function elements in ANG reveals a unique opportunity to develop new molecules that modulate neovascularization.

Gut bacteria provide precursors of benzodiazepine receptor ligands in a rat model of hepatic encephalopathy

Benzodiazepine receptor (BZR) ligands are elevated in animals and humans with fulminant hepatic failure (FHF) and contribute to the pathogenesis of the associated hepatic encephalopathy (HE). As gut factors are proposed to play a role in the pathogenesis of HE, we investigated gut flora as a source of BZR ligands. Rats received daily oral neomycin, vancomycin and metronidazole (AB +) or saline (AB -) before and concurrent with the induction of FHF with thioacetamide. BZR ligands were extracted from brain and plasma and quantified using radiometric techniques. Plasma BZR ligand concentrations in AB(+) and AB(-) rats with HE were higher than AB(+) and AB(-) control rats. Brain BZR ligand concentrations increased in AB(+) and AB(-) rats with HE. Stool cultures from antibiotic treated rats with HE indicated the presence of multidrug resistant Acinetobacter lwoffii. Although no significant concentrations of BZR ligands were detected in culture media inoculated with A. lwoffii, administering A. lwoffii to normal rats significantly elevated BZR ligand levels in brain, but not plasma. Thus, antibiotic treatment of rats is associated with the growth of a resistant strain of bacterium which produces an inactive BZR ligand precursor. BZR ligands may be synthesized from these precursors in the brain and efficiently cleared by a normal liver following brain-to-plasma transfer. Impairment of this clearance process in FHF facilitates their accumulation, enabling agonist BZR ligands to contribute to the pathogenesis of HE by further enhancing GABAergic neurotransmission.

Brain and plasma levels of opioid peptides are altered in rats with thioacetamide-induced fulminant hepatic failure: implications for the treatment of hepatic encephalopathy with opioid antagonists

Although plasma levels of Met-enkephalin and beta-endorphin are elevated in patients suffering from liver failure, it is not known whether central nervous system (CNS) opioidergic neurotransmission is altered in these patients. Such changes may contribute to the motor dysfunction, psychiatric abnormalities and CNS depression observed in hepatic encephalopathy (HE). Therefore, Met- and Leu-enkephalin, dynorphin A and beta-endorphin levels were measured in discrete brain regions and plasma from thioacetamide-treated rats in Stages II to IV of HE. Pituitary and plasma beta-endorphin, Met- and Leu-enkephalin concentrations increased with the severity of HE by 50 to 290%. Pituitary and brainstem dynorphin A levels increased whereas plasma levels decreased in rats with thioacetamide-induced fulminant hepatic failure. Both striatal Met- and Leu-enkephalin levels increased and hypothalamic concentrations decreased in HE. Concurrent with the increase in striatal Met-enkephalin levels was a 26 to 48% decrease in the density of striatal and hypothalamic delta receptors. No change in either the density or affinity of radioligand binding to mu or delta receptors was observed in the CNS. Finally, administering (+/-)-naloxone (5 and 10 mg/kg) or (+/-)-naltrexone (5-15 mg/kg), but not (+)-naloxone (10 mg/kg), significantly increased the motor activity of rats with Stage III HE. Whereas elevated plasma levels of opioid peptides may play a role in the peripheral manifestations of hepatic failure (ascites and hypotension), increased CNS levels of these peptides may be involved in the neuropsychiatric abnormalities characteristic of HE. Thus, opioid antagonists may be effective in ameliorating some of the neurological manifestations of HE.

Striatal met-enkephalin and substance P levels are decreased in mice infected with the LP-BM5 murine leukemia virus

Mice infected with the LP-BM5 murine leukemia virus mixture develop severe immunosuppression and an encephalopathy characterized by spatial learning deficits. Twelve weeks after infection of C57BL/6J mice with LP-BM5, significant (50-60%) reductions in Met-enkephalin and substance P levels were observed in the striatum, whereas somatostatin levels were unchanged. In addition, a 39% decrease in hypothalamic substance P concentrations was observed, with no alteration in Metenkephalin levels. The apparent selectivity of the decrease in neuropeptide concentrations indicates that a functional alteration of the primary striatal efferent neurons occurs in this infection, which may contribute to the impairment of spatial learning observed in these mice. Moreover, this decrease in striatal neuropeptide levels is similar to the neuropathological changes in basal ganglia observed in HIV-infected individuals and is consistent with previous studies suggesting that the LP-BM5-infected mouse may serve as a useful model of AIDS dementia.

ATPase kinetics of recombinant bovine 70 kDa heat shock cognate protein and its amino-terminal ATPase domain

Steady-state kinetic, pre-steady-state kinetic, and equilibrium binding measurements have been applied to determine the rate constants of individual steps of the ATPase cycle for the recombinant bovine 70 kDa heat shock cognate protein and its amino-terminal 44 kDa ATPase fragment. At 25 degrees C, pH 7.0, in the presence of 75 mM KCl and 4.5 mM Mg2+, the measured association rate constants for MgATP approximately hsc70 and MgADP approximately hsc70 are (2.7 +/- 0.5) x 10(5) and (4.1 +/- 0.5) x 10(5) M-1 s-1, respectively, while the dissociation rate constants are 0.0114 (+/- 0.0002) and 0.0288 (+/- 0.0018) s-1, respectively. MgATP (Kd = 0.042 microM) therefore binds to hsc70 more tightly than MgADP (Kd = 0.11 microM). ADP release is inhibited by inorganic phosphate (Pi), suggesting that product dissociation is ordered with Pi released first and ADP second. The rate of chemical hydrolysis of ATP is 0.0030 (+/- 0.0003) s-1 for hsc70 and 0.0135 (+/- 0.0033) s-1 for the 44 kDa fragment. The rate of Pi release is 0.0038 (+/- 0.0010) s-1 for hsc70 and 0.0051 (+/- 0.0006) s-1 for the 44 kDa fragment. For the 44 kDa fragment, Pi release is the slowest step in the ATPase cycle, while for hsc70, Pi release and chemical hydrolysis of MgATP have similar rates; in both cases, ADP release is a relatively rapid step in the ATPase cycle.

The protective effects of stress control may be mediated by increased brain levels of benzodiazepine receptor agonists

Control over stress protects against many of the deleterious effects of stress exposure, but the endogenous mediators responsible for these prophylactic effects have remained elusive. Using behavioral pharmacology, in vitro radioligand binding and neurochemical analyses, we demonstrate that exposure to escapable stress results in brain and behavior changes reminiscent of benzodiazepine administration. The stress control group shows significant protection against picrotoxinin-induced seizures, reductions in [35S]t-butylbicyclophosphorothionate (TBPS) binding and a 3-fold increase of benzodiazepine-like substances in brain in comparison to both yoked-inescapable shock and non-shock controls. These observations suggest that coping behavior leads to the release of endogenous benzodiazepine-like compounds in brain which protect the organism from stress pathology.

The action of diazepam in the isolated rat detrusor muscle

Diazepam is one of the benzodiazepines, a group of drugs that depresses the central nervous system. It also inhibits the contractility of smooth muscles in the periphery, but the mechanism of this inhibitory action has not been clarified. Our study was undertaken to investigate the effect of diazepam on the contractility of the detrusor muscle. Detrusor muscle strips isolated from rat urinary bladder were examined by isometric myography. Diazepam, as well as baclofen, a gamma-aminobutyric acid (GABA)B receptor agonist, reduced the electric field stimulation-induced contractions; delta-aminovaleric acid, a GABAB receptor antagonist, completely antagonized the inhibitory effect of baclofen, but not the inhibitory action of diazepam. Diazepam reduced the basal tone of detrusor muscle concentration dependently, and this inhibitory action was not affected by tetrodotoxin. Diazepam suppressed the contractile responses to bethanechol, adenosine triphosphate and potassium chloride. Diazepam diminished the calcium-induced recovery of tension in calcium-free PSS. A23187, a calcium ionophore, partially recovered the basal tone which had been reduced by diazepam in normal physiologic salt solution (PSS). The loss of tension in calcium free PSS containing diazepam could not be recovered by addition of A23187. On the other hand, the loss of tension in calcium-free PSS containing 3,4,5-trimethoxybenzoic acid 8(diethylamino)octyl ester (TMB-8), an inhibitor of intracellular calcium release, was considerably recovered by addition of A23187. Based on these results, it is suggested that diazepam inhibits the contractility of detrusor muscle acting directly on the smooth muscle cell, which is unrelated to the activation of GABA receptors. Its inhibitory action appears to be mediated through interference with the influx of extracellular calcium.

Actions of potassium channel openers in rat detrusor urinae

This study was performed to investigate the action of potassium channel openers on the mechanical activity of detrusor muscle isolated from rats. Detrusor muscle strips, 15 mm in length, were myographied isometrically in an isolated organ bath. P 1060, RP 49356 and BRL 38277, potassium channel activators, reduced the basal tone and diminished the phasic activity of detrusor concentration-dependently. P 1060, RP 49356 and BRL 38227 suppressed the maximal responses to bethanechol and shifted the concentration-response curves of bethanechol-induced contraction to the right. RP 49356 and BRL 38227 reduced the contraction by low (20 mM) concentration of potassium. P 1060, however, diminished the high (80 mM) and low (20 mM) concentration of potassium-induced contraction. Glibenclamide, an inhibitor of ATP-dependent potassium channel, antagonized the suppressive action of P 1060, RP 49356 and BRL 38227 on the basal tone. Apamin or procaine did not antagonize it significantly. Based on these results, it is suggested that the relaxation of detrusor muscle strip caused by P 1060, RP 49356 and BRL 38227 may predominantly involve opening of the same potassium channel, i.e., the ATP-dependent potassium channel.

Thermodynamic stoichiometries of participation of water, cations and anions in specific and non-specific binding of lac repressor to DNA. Possible thermodynamic origins of the "glutamate effect" on protein-DNA interactions

The objective of this study is to quantify the contributions of cations, anions and water to stability and specificity of the interaction of lac repressor (lac R) protein with the strong-binding symmetric lac operator (Osym) DNA site. To this end, binding constants Kobs and their power dependences on univalent salt (MX) concentration (SKobs = d log Kobs/d log[MX]) have been determined for the interactions of lac R with Osym operator and with non-operator DNA using filter binding and DNA cellulose chromatography, respectively. For both specific and non-specific binding of lac R, Kobs at fixed salt concentration [KX] increases when chloride (Cl-) is replaced by the physiological anion glutamate (Glu-). At 0.25 M-KX, the increase in Kobs for Osym is observed to be approximately 40-fold, whereas for non-operator DNA the increase in Kobs is estimated by extrapolation to be approximately 300-fold. For non-operator DNA, SKobsRD is independent of salt concentration within experimental uncertainty, and is similar in KCl (SKobs,RDKCl = -9.8(+/- 1.0) between 0.13 M and 0.18 M-KCl) and KGlu (SKobs,RDKGlu = -9.3(+/- 0.7) between 0.23 M and 0.36 M-KGlu). For Osym DNA, SKobsRO varies significantly with the nature of the anion, and, at least in KGlu appears to decrease in magnitude with increasing [KGlu]. Average magnitudes of SKobsRO are less than SKobsRD, and, for specific binding decrease in the order [SKobsRO,KCl[>[SKobsRO,KAc[>[SKobsRO,KGlu[ . Neither KobsRO nor SKobsRO is affected by the choice of univalent cation M+ (Na+, K+, NH4+, or mixtures thereof, all as the chloride salt), and SKobsRO is independent of [MCl] in the range examined (0.125 to 0.3 M). This behavior of SKobsRO is consistent with that expected for a binding process with a large contribution from the polyelectrolyte effect. However, the lack of an effect of the nature of the cation on the magnitude of KobsRO at a fixed [MX] is somewhat unexpected, in view of the order of preference of cations for the immediate vicinity of DNA (NH4+ > K+ > Na+) observed by 23Na nuclear magnetic resonance. For both specific and non-specific binding, the large stoichiometry of cation release from the DNA polyelectrolyte is the dominant contribution to SKobs. To interpret these data, we propose that Glu- is an inert anion, whereas Ac- and Cl- compete with DNA phosphate groups in binding to lac repressor. A thermodynamic estimate of the minimum stoichiometry of water release from lac repressor and Osym operator (210(+/- 30) H2O) is determined from analysis of the apparently significant reduction in [SKobsRO,KGlu[ with increasing [KGlu] in the range 0.25 to 0.9 M. According to this analysis, SKobs values of specific and non-specific binding in KGlu differ primarily because of the release of water in specific binding. In KAc and KCl, we deduce that anion competition affects Kobs and SKobs to an extent which differs for different anions and for the different binding modes.

Analysis of equilibrium and kinetic measurements to determine thermodynamic origins of stability and specificity and mechanism of formation of site-specific complexes between proteins and helical DNA

The concentration and nature of the electrolyte are key factors determining (1) the equilibrium extent of binding of oligocations or proteins to DNA, (2) the distribution of bound protein between specific and nonspecific sites, and (3) the kinetics of association and dissociation of both specific and nonspecific complexes. Salt concentration may therefore be used to great advantage to probe the thermodynamic basis of stability and specificity of protein-DNA complexes, and the mechanisms of association and dissociation. Cation concentration serves as a thermodynamic probe of the contributions to stability and specificity from neutralization of DNA phosphate charges and/or reduction in phosphate charge density. Cation concentration also serves as a mechanistic probe of the kinetically significant steps in association and dissociation that involve cation uptake. In general, effects of electrolyte concentration on equilibrium constants (quantified by SKobs) and rate constants (quantified by Skobs) are primarily cation effects that result from the cation-exchange character of the interactions of proteins and oligocations with polyanionic DNA. The competitive effects of Mg2+ or polyamines on the equilibria and kinetics of protein-DNA interactions are interpretable in the context of the cation-exchange model. The nature of the anion often has a major effect on the magnitude of the equilibrium constant (Kobs) and rate constant (kobs) of protein-DNA interactions, but a minor effect on SKobs and Skobs, which are dominated by the cation stoichiometry. The order of effects of different anions generally follows the Hofmeister series and presumably reflects the relative extent of preferential accumulation or exclusion of these anions from the relevant surface regions of DNA-binding proteins. The question of which anion is most inert (i.e., neither accumulated nor excluded from the relevant regions of these proteins) remains unanswered. The characteristic effects of temperature on equilibrium constants and rate constants for protein-DNA interactions also serve as diagnostic probes of the thermodynamic origins of stability and specificity and of the mechanism of the interaction, since large changes in thermodynamic and activation heat capacities accompany processes with large changes in the amount of water-accessible nonpolar surface area.(ABSTRACT TRUNCATED AT 400 WORDS)

Hydrophobic effect in protein folding and other noncovalent processes involving proteins

Large negative standard heat capacity changes (delta CP degree much less than 0) are the hallmark of processes that remove nonpolar surface from water, including the transfer of nonpolar solutes from water to a nonaqueous phase and the folding, aggregation/association, and ligand-binding reactions of proteins [Sturtevant, J. M. (1977) Proc. Natl. Acad. Sci. USA 74, 2236-2240]. More recently, Baldwin [Baldwin, R. L. (1986) Proc. Natl. Acad. Sci. USA 83, 8069-8072] proposed that the delta CP degree of protein folding could be used to quantify the contribution of the burial of nonpolar surface (the hydrophobic effect) to the stability of a globular protein. We demonstrate that identical correlations between the delta CP degree and the change in water-accessible nonpolar surface area (delta Anp) are obtained for both the transfer of nonpolar solutes from water to the pure liquid phase and the folding of small globular proteins: delta CP degree/delta Anp = -(0.28 +/- 0.05) (where delta Anp is expressed in A2 and delta CP degree is expressed in cal.mol-1.K-1; 1 cal = 4.184 J). The fact that these correlations are identical validates the proposals by both Sturtevant and Baldwin that the hydrophobic effect is in general the dominant contributor to delta CP degree and provides a straightforward means of estimating the contribution of the hydrophobic driving force (delta Ghyd degree) to the standard free energy change of a noncovalent process characterized by a large negative delta CP degree in the physiological temperature range: delta Ghyd degree congruent to (80 +/- 10)delta CP degree.

Role of the hydrophobic effect in stability of site-specific protein-DNA complexes

The site-specific binding interaction of lac repressor with a symmetric operator sequence and of EcoRI endonuclease with its specific recognition site both exhibit a characteristic dependence of equilibrium binding constant (Kobs) on temperature, in which Kobs attains a relative maximum in the physiologically relevant temperature range. This behavior, which appears to be quite general for site-specific protein-DNA interactions, is indicative of a large negative standard heat capacity change (delta C0P,obs) in the association process. By analogy with model compound transfer studies and protein folding data, we propose that this delta C0P,obs results primarily from the removal of non-polar surface from water in the association process. From delta C0P,obs we obtain semiquantitative information regarding the change in water-exposed non-polar surface area (delta Anp) and the corresponding hydrophobic driving force for association (delta G0hyd): delta G0hyd approximately equal to 8(+/- 1) x 10(1) delta C0P,obs approximately equal to -22(+/- 5) delta Anp. We propose that removal of non-polar surface from water (the hydrophobic effect) and release of cations (the polyelectrolyte effect) drive the thermodynamically unfavorable process (e.g. conformational distortions) necessary to achieve mutually complementary recognition surfaces (at a steric and functional-group level) in the specific complex.