Entropy-driven interactions of anesthetics with membrane proteins

Propofol Causes Sustained Ca2+ Elevation in Endothelial Cells by Stimulating Ryanodine Receptor and Suppressing Plasmalemmal Ca2+ Pump

ABSTRACT

Propofol, a general anesthetic administered intravenously, may cause pain at the injection site. The pain is in part due to irritation of vascular endothelial cells. We here investigated the effects of propofol on Ca2+ transport and pain mediator release in human umbilical vein endothelial cells (EA.hy926). Propofol mobilized Ca2+ from cyclopiazonic acid (CPA)-dischargeable pool but did not cause Ca2+ release from the lysosomal Ca2+ stores. Propofol-elicited Ca2+ release was suppressed by 100 μM ryanodine, suggesting the participation of ryanodine receptor channels. Propofol did not affect ATP-triggered Ca2+ release but abolished the Ca2+ influx triggered by ATP; in addition, propofol also suppressed store-operated Ca2+ entry elicited by CPA. Ca2+ clearance during CPA-induced Ca2+ discharge was unaffected by a low Na+ (50 mM) extracellular solution, but strongly suppressed by 5 mM La3+ (an inhibitor of plasmalemmal Ca2+ pump), suggesting Ca2+ extrusion was predominantly through the plasmalemmal Ca2+ pump. Propofol mimicked the effect of La3+ in suppressing Ca2+ clearance. Propofol also stimulated release of pain mediators, namely, reactive oxygen species and bradykinin. Our data suggest propofol elicited Ca2+ release and repressed Ca2+ clearance, causing a sustained cytosolic [Ca2+]i elevation. The latter may cause reactive oxygen species and bradykinin release, resulting in pain.

Propofol-based sedation does not negatively influence oxygenator running time compared to midazolam in patients with extracorporeal membrane oxygenation

OBJECTIVE

Patients on extracorporeal membrane oxygenation are frequently in need for sedation. Use of propofol has been associated with impaired oxygenator function due to adsorption to the membrane as well as lipid load. The aim of our retrospective analysis was to compare two different sedation regimens containing either propofol or midazolam with respect to oxygenator running time.

METHODS

Midazolam was used in 73 patients whereas propofol was used in 49 patients, respectively. In the propofol group, veno-arterial-extracorporeal membrane oxygenation was used predominantly (84%), while veno-venous-extracorporeal membrane oxygenation was used more often in the midazolam group (64%).

RESULTS

Oxygenator running time until first exchange was 7 days in both groups ( p = 0.759). No statistically significant differences could be observed between the subgroup of patients receiving lipid-free (n = 24) and lipid-containing (n = 31) parenteral nutrition, respectively. Laboratory parameters like triglycerides, free hemoglobin, fibrinogen, platelets, and activated partial thromboplastin time were not significantly different between both sedation regimens ( p = 0.462, p = 0.489, p = 0.960, p = 0.134, and p = 0.843) and were not associated with oxygenator running time.

CONCLUSION

The use of propofol as sedative seems suitable in patients undergoing extracorporeal membrane oxygenation therapy.

The effect of Diprivan (propofol) on phosphorylcholine surfaces during cardiopulmonary bypass — an in vitro investigation

It is well known that extracorporeal surfaces have the ability to bind such drugs as fentanyl, nitroglycerine and propofol. Adsorption of the injectable anesthetic agent Diprivan® (propofol) onto uncoated and heparin-coated extracorporeal surfaces during cardiopulmonary bypass (CPB) has been previously investigated; however, propofol adsorption onto synthetic-coated extracorporeal surfaces has not been published previously. The focus of this investigation was on the interaction of propofol and the synthetic biomimetic coating from the Sorin Group called Mimesys® (phosphorylcholine (PC)). A randomized series of six in vitro experiments were done with propofol using both PC-coated circuits without arterial filters and those with arterial filters. The circuits were identical in all experiments and priming remained the same, with 750 mls of normal saline and 1250 mls of fresh bovine blood (hematocrit 41.0 ± 1.0%). After circulation and collection of baseline samples, the first (low) dose of 4 mg (4000 mcg) of Diprivan 1% was added to the perfusate, followed by the second (high) dose of 40 mg (40,000 mcg) and the final challenge (extreme dose) of 356 mg (356,000 mcg) of Diprivan.

Drug assay was done by an independent laboratory, using the standardized method of High Performance Liquid Chromatography (HPLC). Measurements of total propofol were done at baseline, 20 minutes, 40 minutes and 60 minutes after each injection. Oxygenator performance was also measured prior to the addition of propofol and repeated after exposure to 4 mg, 40mg and 356 mg propofol for 60 min, 120 min and 180 minutes of circulation. Results indicate that phosphorylcholine coating does not prevent the adsorption of propofol during extracorporeal circulation and the oxygenator’s gas exchange ability is not affected by prolonged exposure to an extreme dose of the medication during high flow extracorporeal circulation.

Four-alpha-helix bundle with designed anesthetic binding pockets. Part II: halothane effects on structure and dynamics

As a model of the protein targets for volatile anesthetics, the dimeric four-alpha-helix bundle, (Aalpha(2)-L1M/L38M)(2), was designed to contain a long hydrophobic core, enclosed by four amphipathic alpha-helices, for specific anesthetic binding. The structural and dynamical analyses of (Aalpha(2)-L1M/L38M)(2) in the absence of anesthetics (another study) showed a highly dynamic antiparallel dimer with an asymmetric arrangement of the four helices and a lateral accessing pathway from the aqueous phase to the hydrophobic core. In this study, we determined the high-resolution NMR structure of (Aalpha(2)-L1M/L38M)(2) in the presence of halothane, a clinically used volatile anesthetic. The high-solution NMR structure, with a backbone root mean-square deviation of 1.72 A (2JST), and the NMR binding measurements revealed that the primary halothane binding site is located between two side-chains of W15 from each monomer, different from the initially designed anesthetic binding sites. Hydrophobic interactions with residues A44 and L18 also contribute to stabilizing the bound halothane. Whereas halothane produces minor changes in the monomer structure, the quaternary arrangement of the dimer is shifted by about half a helical turn and twists relative to each other, which leads to the closure of the lateral access pathway to the hydrophobic core. Quantitative dynamics analyses, including Modelfree analysis of the relaxation data and the Carr-Purcell-Meiboom-Gill transverse relaxation dispersion measurements, suggest that the most profound anesthetic effect is the suppression of the conformational exchange both near and remote from the binding site. Our results revealed a novel mechanism of an induced fit between anesthetic molecule and its protein target, with the direct consequence of protein dynamics changing on a global rather than a local scale. This mechanism may be universal to anesthetic action on neuronal proteins.

Cytotoxic effect of a new 1,3,4-thiadiazolium mesoionic compound (MI-D) on cell lines of human melanoma

The structural characteristics of mesoionic compounds, which contain distinct regions of positive and negative charges associated with a poly-heteroatomic system, enable them to cross cellular membranes and interact strongly with biomolecules. Potential biological applications have been described for mesoionic compounds. 1,3,4-Thiadiazolium mesoionic compound (MI-D), a new mesoionic compound, has been demonstrated to be extremely cytotoxic to B16-F10 murine melanoma cells when compared to fotemustine and dacarbazine, drugs of reference in melanoma treatment protocols, describing inhibition of tumours grown in vitro and in vivo. We now evaluate the effects of mesoionic compound MI-D on different human melanoma cell lines. The drug decreased the viability and proliferation of MEL-85, SK-MEL, A2058 and MEWO cell lines in vitro, showing a considerable cytotoxic activity on these human cells. Adhesion of MEL-85 cells was evaluated in the presence of the drug using different extracellular matrix (ECM) constituents. MI-D decreased MEL-85 adhesion to laminin, fibronectin and matrigel. The morphology and actin cytoskeleton organisation of MEL-85 cells were also modified on MI-D treatment. These results on human melanoma cell lines indicate that MI-D is a very encouraging drug against melanoma, a tumour that is extremely resistant to chemotherapy.

Entropic Trapping Binding Mechanism: Its Likely Role in Receptor−Ligand and Other Biochemical Systems

Entropy-driven binding continues to be discovered in receptor-ligand systems as well as in other important biochemical systems. In receptor-ligand systems the "thermodynamic agonist-antagonist discrimination" is often found: in some such receptor types the binding of antagonists is entropy-driven, in others this is a characteristic of the agonist binding. The interpretation of the entropy-driven binding mechanism in the systems in question is still rather elusive. Experimental findings clearly indicate that the entropic binding mechanisms which are usually considered cannot provide a consistent interpretation in these cases. Entropic trapping was therefore proposed in our earlier papers as a possible binding model which does not contradict the experimental facts. It is pointed out here that through the work which has appeared subsequently the existence of such a mechanism has been firmly established and that its role is strongly corroborated by the more recent data from several biochemical systems.

Multiple effects of trichloroethanol on calcium handling in rat submandibular acinar cells

The effect of trichloroethanol (TCEt), the active metabolite of chloral hydrate, on the intracellular concentration of calcium ([Ca(2+)](i)) was investigated in rat submandibular glands (RSMG) acini loaded with fura-2. TCEt (1 - 10 mM) increased the [Ca(2+)](i) independently of the presence of calcium in the extracellular medium. Dichloroethanol (DCEt) and monochloroethanol (MCEt) reproduced the stimulatory effect of TCEt but at much higher concentrations (about 6 fold higher for DCEt and 20 fold higher for MCEt). TCEt mobilized an intracellular pool of calcium, which was depleted by a pretreatment with thapsigargin, an inhibitor of the sarcoplasmic and endoplasmic reticulum calcium-dependent ATPases, but not with FCCP, an uncoupler of mitochondria. TCEt 10 mM inhibited by 50% the thapsigargin-sensitive microsomal Ca(2+)-ATPase. DCEt 10 mM and MCEt 10 mM inhibited the ATPase by 20 and 10%, respectively. TCEt inhibited the increase of the [Ca(2+)](i) and the production of inositol phosphates in response to carbachol, epinephrine and substance P. TCEt inhibited the uptake of calcium mediated by the store-operated calcium channel (SOCC). ATP and Bz-ATP increased the [Ca(2+)](i) in RSMG acini and this effect was blocked by extracellular magnesium, by Coomassie blue and by oxydized ATP (oATP). TCEt potentiated the increase of the [Ca(2+)](i) and of the uptake of extracellular calcium in response to ATP and Bz-ATP. TCEt had no effect on the uptake of barium and of ethidium bromide in response to purinergic agonists. These results suggest that TCEt, at sedative concentrations, exerts various effects on the calcium regulation: (1) it mobilizes a thapsigargin-sensitive intracellular pool of calcium in RSMG acini; (2) it inhibits the uptake of calcium via the SOCC; (3) it inhibits the activation by G protein-coupled receptors of a polyphosphoinositide-specific phospholipase C. It does not interfere with the activation of the ionotropic P2X receptors. The use of chloral hydrate should be avoided in studies exploring the in vivo responses to sialagogues.

Effects of volatile anesthetic on the Ca2+-ATPase activation by dimerization. Distance-dependent quenching analysis and fluorescence energy transfer studies

The phenomenological distance-dependent quenching (DDQ) model was employed to investigate the character of the interaction between volatile anesthetics (VAs) and the plasma membrane Ca2+-ATPase (PMCA). The simultaneous analysis of the frequency-domain and steady-state data of tryptophan (Trp) fluorescence quenching by a VA points to a specific character of the apparent quenching effect of the VA, possibly arising from a significant contribution of static quenching. The apparent contributions of both static and dynamic quenching may be due to VA binding in the PMCA, which results in the modification of the conformational substates of the enzyme. To characterize further the molecular consequences of VA binding, we investigated its effects on the process of PMCA activation by self-association. VA shifted the equilibrium from enzyme dimers to monomers, as monitored by the loss of fluorescence energy transfer. The shift was apparently due to the VA-induced decrease in the affinity of PMCA molecules for self-association. Addition of a large molecular mass dextran to increase the proximity between enzyme monomers induced re-association of the VA-impaired PMCA, while the Ca2+-ATPase activity was not recovered. The results are congruent with a dual VA effect on PMCA, a shift in the monomer/dimer equilibrium, and an inactivation of both monomers and dimers.

The lipid/protein interface as a target site for general anesthetics: a multiple-site kinetic analysis of synaptosomal Ca2+-ATPase

There is a long-standing controversy on whether membrane lipids or proteins are the target for general anesthetics. The plasma membrane-associated Ca2+-ATPase of synaptosomes has recently been established as a model system for general anesthesia, the protein interior being the proposed target site (M.M. Lopez, D. Kosk-Kosicka, J. Biol. Chem. 270 (1995) 28239-28245). Multiple-site kinetics is now applied as a mechanistic tool to analyze inhibition by organic solvents and general anesthetics. A close fit to the experimental data points was achieved using the complex equations for a competitive displacement of lipid activators from multiple sites on the protein surface. Inhibitor dissociation constants were about 1. 6x105-fold higher than the microscopic lipid dissociation binding constants that are derived here for the first time. Binding of lipid therefore is by -7.1 kcal/mole favored over that of the tested inhibitors. The latter are nevertheless effective because in the model used displacement of only few of the lipid solvation molecules cause complete inhibition. The lipid/protein interface rather than protein or lipid alone appeared to be the anesthetic target site.

Transient kinetics and thermodynamics of anthroylouabain binding to Na/K-ATPase

The Na/K-ATPase is an integral membrane protein enzyme which uses energy derived from hydrolysis of ATP to pump Na+ out of and K+ into the cell. Ouabain belongs to a class of drugs known as cardiac glycosides, which are useful for treating congestive heart failure. Therapeutic value is achieved when these drugs bind to and inhibit the Na/K-ATPase of cardiac muscle. We gain insight into this important interaction by measuring the thermodynamics of the interaction of anthroylouabain (AO), a fluorescent derivative of ouabain, with the Na/K-ATPase. AO has the useful property that its fluorescence intensity is greatly enhanced (approximately 10x) when it binds to the enzyme. Using this enhancement, we measure temperature dependence of transient kinetics for the association and dissociation of AO interacting with membrane fragments of Na/K-ATPase purified from dog kidney. Using a standard Eyring analysis, we find that the overall association of AO with the enzyme is driven by substantial contributions from both enthalpy and entropy, and that in an energy diagram for the association pathway, the free energy change is quite similar to that of ouabain deduced from previously published results [E. Erdmann, W. Schoner, BBA 307 (1973) 386]. However, in the transition state, there are substantial differences for the enthalpy and entropy, presumably due to the presence of the anthracene moiety.

Spectroscopic analysis of halothane binding to the plasma membrane Ca2+-ATPase

The intrinsic tryptophan (Trp) fluorescence of the plasma membrane Ca2+-ATPase (PMCA) is significantly quenched by halothane, a volatile anesthetic common in clinical practice. It has been proposed that halothane inhibition of the Ca2+-ATPase activity results from conformational changes following anesthetic binding in the enzyme. We have investigated whether the observed quenching reflects halothane binding to PMCA. We have shown that the quenching is dose dependent and saturable and can be fitted to a binding curve with an equilibrium constant K(Hal) = 2.1 mM, a concentration at which the anesthetic approximately half-maximally inhibits the Ca2+-ATPase activity. The relatively low sensitivity of halothane quenching of Trp fluorescence to the concentration of phosphatidylcholine and detergent in the PMCA preparation concurs with the quenching resulting from anesthetic binding in the PMCA molecule. Analysis of the Trp fluorescence quenching by acrylamide indicates that the Trp residues are not considerably exposed to the solvent (Stern-Volmer quenching constant of 2.9 M(-1)) and do not differ significantly in their accessibility to halothane. Other volatile anesthetics, diethyl ether and diisopropyl ether, reduce the quenching caused by halothane in a dose-dependent manner, suggesting halothane displacement from its binding site(s). These observations indicate that halothane quenching of intrinsic Trp fluorescence of PMCA results from anesthetic binding to the protein. The analysis, used as a complementary approach, provides new information to the still rudimentary understanding of the process of anesthetic interaction with membrane proteins.