Interaction of drugs with a model membrane protein. Effect of dibucaine on cytochrome oxidase proteoliposomes

Kinetics of inhibition of purified and mitochondrial cytochrome c oxidase by psychosine (beta-galactosylsphingosine)

1. Psychosine (beta-galactosylsphingosine) is the toxic agent in Krabbe's disease (globoid cells leukodystrophy). It inhibits purified bovine heart mitochondrial cytochrome c oxidase; there is a rapid phase of inhibition (complete within 10-15 s) and a slower phase (complete within 10-15 min). Both phases are also seen in rat liver mitochondria. IC50 is about 200 microM psychosine in the purified enzyme and less than 20 microM in mitochondria. Psychosine inhibition is due to binding to cytochrome oxidase, not cytochrome c. 2. Bovine heart submitochondrial particles show inhibition similar to rat liver mitochondria. However, although proteoliposomes containing bovine heart cytochrome oxidase show an identical fast phase, they have no noticeable slow phase of inhibition. Addition of phospholipid liposomes to submitochondrial particles relieved the majority of psychosine inhibition, consistent with the removal of those molecules binding in the slow phase. Psychosine can inhibit cytochrome oxidase molecules facing in either direction in proteoliposomes and submitochondrial particles, suggesting that it can rapidly interact with both sides of a membrane when added externally. 3. At high ionic strength, the presence of psychosine decreases the Vmax. of cytochrome oxidase with little effect on the Km for cytochrome c. This non-competitive inhibition suggests that the psychosine-enzyme complex is kinetically inactive and not labile over the time course of the assay. Psychosine does not inhibit the reduction of haem a or haem a3 by artificial electron donors, but does inhibit the reduction of haem a by cytochrome c.

Inhibition of cytochrome oxidase by dibucaine

Dibucaine-HCl inhibited mitochondrial cytochrome c oxidase activity in intact mitochondria with 50% inhibition occurring at 1.1 mM dibucaine-HCl. Dibucaine-HCl did not prevent the reduction of cytochrome oxidase by ascorbate plus N,N,N',N'-tetramethyl-p-phenylenediamine dihydrochloride (TMPD) when measured at 604 nm but prevented 50% of the absorbance change at 445 nm; dithionite reduced the oxidase completely. Dibucaine prevented binding of CO to oxidase reduced with ascorbate plus TMPD by preventing the reduction of cytochrome a3. The midpotenials of cytochrome c and cytochrome oxidase, the visible absorbance wavelength maxima, and the position and intensity of the signals of the EPR spectrum of the oxidase were not affected. Dibucaine-HCl prevented ascorbate plus TMPD-driven reduction of the near infra-red detectable copper center associated with cytochrome a: dithionite subsequently reduced this center. Dibucaine-HCl inhibited cytochrome oxidase activity by interacting between cytochrome a and its associated copper. Since respiration was 8-fold less sensitive in submitochondrial particles, this site of inhibition is on the cytoplasmic side of the membrane.

The inhibition of a membrane-bound enzyme as a model for anaesthetic action and drug toxicity

The inhibition of the membrane-bound enzyme cytochrome c oxidase by aliphatic n-alcohols and other neutral organic compounds was studied as a model for anaesthetic action and drug toxicity. The n-alcohols (C1 to C14) displayed a variation in inhibition constant of over 500,000-fold. The inhibition constants correlated well with the number of carbon atoms in the n-alcohols and also their n-octanol/water partition coefficients. General anaesthetic potency is known to be similarly well correlated with octanol/water partition coefficients. The free-energy change for transferring a methylene group of the n-alcohol to the more hydrophobic environment bound to the enzyme is similar to that for transferring a methylene group from water to pure alcohol. These results are consistent with the n-alcohols inhibiting by binding to an octanol-like environment on the enzyme or the protein/phospholipid interface. Neither negatively charged carboxylates nor positively charged amine analogues were observed to cause any inhibition, indicating that this postulated binding site may be uncharged. Inhibition of cytochrome c oxidase by n-alcohols was also demonstrated in both bovine heart and rat liver sonicated submitochondrial fragments.

Spin label study of the perturbation effect of the local anaesthetics tetracaine and dibucaine on synaptosomes at pharmacological concentrations

The method of electron spin resonance spectroscopy of spin probes was used to determine the lowest concentrations of the local anaesthetics dibucaine and tetracaine exerting perturbations on synaptosome membranes. The perturbation depends on the temperature and the membrane depth, as well as on the concentration and the structure of the anaesthetics. Using spin labelled stearic acid at the 5th carbon position a negligible effect of the anaesthetics on the order parameter was found in the membrane both at 1 degree and 22 degrees, within the buffer concentration 0.01-10 mmol/l, but at 37 degrees and concentrations higher than 0.1 mmol/l the disordering effect was significant and of comparable efficiency for dibucaine and tetracaine. Employing stearic acid labelled at the 16th carbon position, disordering of the hydrocarbon core of the membrane caused by tetracaine or dibucaine was detected at 1 degree and 22 degrees, as well as at 37 degrees. The disordering effect occurred at buffer concentrations higher than 0.01 mmol/l for dibucaine, and higher than 0.1 mmol/l for tetracaine. At equal anaesthetic membrane concentrations and at the 16th carbon membrane depth, dibucaine was approximately twice as effective as tetracaine in perturbing synaptosomes. Tetracaine induced nonlamellar phases in the rat brain lipid membrane as detected by 31P NMR spectroscopy. The dynamic and structural perturbation effects of the local anaesthetics was found in that concentration range at which the anaesthetics influence various activities of biological membranes.

Effect of positively charged local anesthetics on a membrane-bound phosphatase in Acholeplasma laidlawii

The plasma membrane p-nitrophenylphosphatase activity of Acholeplasma laidlawii was stimulated by the spin-labeled local anesthetic 2-[N-methyl-N-(2,2,6,6-tetramethylpiperidinooxyl)]ethyl p-hexyloxybenzoate, abbreviated as C6SL, and its methylated quaternary analog, C6SLMeI. The tertiary amine C6SL (at a concentration of 5 X 10(-5) M) was more potent at pH 6.5 than at pH 7.7. In contrast, the permanently-charged C6SLMeI was equally potent, independently of pH. These results suggest that cationic forms of the anesthetics are responsible for stimulating the enzyme. Electron spin resonance studies of C6SL- and C6SLMeI-labeled membranes showed that these anesthetics in their cationic forms interacted electrostatically with components of the Acholeplasma membrane. For C6SL, this interaction was pH dependent and correlated with the pH dependency of the anesthetic-induced enzyme stimulation in the Acholeplasma membranes. Further, studies using 5-doxylstearic acid labels and non-spin-labeled anesthetics at various pH values showed that the membrane-fluidizing effect of anesthetics was not correlated with anesthetic-induced pNPPase stimulation. Our observations are consistent with the hypothesis that electrostatic interactions between cationic local anesthetics and anionic membrane components may lead to functional changes mediated by membrane proteins.

Interaction of local anaesthetics with cytochrome oxidase studied with fluorescence quenching

The interaction of a series of eight local anaesthetics with cytochrome oxidase chosen as a membrane model protein has been studied with fluorescence technique using quinacrine as a fluorescent probe. The existence of hydrophobic interactions with a non polar region of cytochrome oxidase complex has been shown. The ability of the drug molecules to displace quinacrine bound to cytochrome oxidase correlate as closely with their anaesthetic potency as with their octanol-water partition coefficient. Our results are in good agreement with a recent model of local anaesthetic action on nerve membranes presenting a site of anaesthesia including both lipid binding and protein binding environments.

Interaction of drugs with a model membrane protein. Effects of local anesthetics on electron transfer and hydrogen ion uptake in ionophore stimulated cytochrome oxidase proteoliposomes

Cytochrome oxidase extracted from beef heart was incorporated into vesicles composed of soy bean phospholipids (asolectin). The oxidation of externally added cytochrome c by such vesicles is associated with proton uptake from the external medium. The rates of both cytochrome c oxidation and proton uptake were stimulated by addition of ionophores such as trifluoromethoxy carbonyl cyanide phenylhydrazone (FCCP), nigericin and valinomycin. These agents probably dissipate pH and/or electrical potential gradients which develop as a result of enzyme activity and which have a "restraining" influence on the turnover of the oxidase. Local anesthetics inhibited oxidase activity but had a much greater effect on the stimulated (ionophore-treated) than the unstimulated enzyme. In addition, pretreating proteoliposomes with local anesthetics completely prevented the stimulating effects of these ionophores. Based on this and previous studies, a model was developed in which local anesthetics interacted with the phospholipid component of the oxidase complex resulting in reduced internal electron transfer and dissociation of the oxidase from the regulatory role of the proton gradient.