Transmembrane distribution of lipophilic cations in response to an electrochemical potential in reconstituted cytochrome c oxidase vesicles and in vesicles exhibiting a potassium ion diffusion potential

Molecular probe location in reverse micelles determined by NMR dipolar interactions

The location and interactions of solutes in microheterogeneous environments, such as reverse micelles, critically influence understanding of many phenomena that utilize probe molecules to characterize properties in chemical, biological, and physical systems. The information gained in such studies depends substantially on the location of the probe used. Often, intuition leads to the assumption that ionic probe molecules reside in the polar water pool of a system. In this work, the location of a charged polar transition metal coordination complex in a reverse micellar system is determined using NMR spectroscopy. Despite the expected Coulomb repulsion between the surfactant headgroups and the negatively charged complex, the complex spends significant time penetrating into the hydrophobic portion of the reverse micellar interface. These results challenge the assumption that ionic probe molecules reside solvated by water in microheterogeneous environments and suggest that probe molecule location be carefully considered before interpreting data from similar systems.

Analysis of the tangled relationships between P-glycoprotein-mediated multidrug resistance and the lipid phase of the cell membrane

P-glycoprotein (Pgp), the so-called multidrug transporter, is a plasma membrane glycoprotein often involved in the resistance of cancer cells towards multiple anticancer agents in the multidrug-resistant (MDR) phenotype. It has long been recognized that the lipid phase of the plasma membrane plays an important role with respect to multidrug resistance and Pgp because: the compounds involved in the MDR phenotype are hydrophobic and diffuse passively through the membrane; Pgp domains involved in drug binding are located within the putative transmembrane segments; Pgp activity is highly sensitive to its lipid environment; and Pgp may be involved in lipid trafficking and metabolism. Unraveling the different roles played by the membrane lipid phase in MDR is relevant, not only to the evaluation of the precise role of Pgp, but also to the understanding of the mechanism of action and function of Pgp. With this aim, I review the data from different fields (cancer research, medicinal chemistry, membrane biophysics, pharmaceutical research) concerning drug-membrane, as well as Pgp-membrane, interactions. It is emphasized that the lipid phase of the membrane cannot be overlooked while investigating the MDR phenotype. Taking into account these aspects should be useful in the search of ways to obviate MDR and could also be relevant to the study of other multidrug transporters.

Kinetics of doxorubicin handling in the LLC-PK1 kidney epithelial cell line is mediated by both vesicle formation and P-glycoprotein drug transport

The subcellular distribution of doxorubicin was evaluated in living non-fixed LLC-PK1 cells, which maintain the structural and functional characteristics of the kidney proximal tubule epithelium and also express P-glycoprotein. After 10 min incubation, doxorubicin fluorescence was detectable in the nucleus. The intensity of nuclear fluorescence progressively increased, reaching the maximum at the end of the first hour. Then, the nuclear signal started to decrease and, at 2 h, doxorubicin fluorescence disappeared almost completely from the cell nucleus. Cytoplasmic fluorescent vesicles first appeared in the perinuclear region after 10 min doxorubicin exposure and increased in number and size over a period of 2 h. From 2 to 5 h, fluorescent vesicles moved unidirectionally to the cell periphery. Disappearance of doxorubicin punctate fluorescence in LLC-PK1 cells treated with methylamine or monensin demonstrated that drug accumulation occurred inside acidic compartments. In addition, the cytoplasmic pattern of doxorubicin fluorescence was very similar to that observed upon exposure to the acidotropic tracer LysoSensor Blue. Involvement of P-glycoprotein in doxorubicin handling by LLC-PK1 cells was suggested by modified intracellular doxorubicin distribution after cell incubation with verapamil and vinblastine. Moreover, the fluorescent P-glycoprotein substrate Bodipy FL Verapamil was shown to accumulate in LLC-PK1 cells in a manner that is quite similar to that observed for doxorubicin. P-glycoprotein expression was evaluated by immunoblot using the JSB-1 and C219 monoclonal antibodies. Immunofluorescence analysis was performed using the JSB-1 monoclonal antibody. P-glycoprotein immuno-reactivity was found both on the plasma membrane and intracytoplasmically in a perinuclear position. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis revealed that MDR1 gene was expressed. This study indicates that a rapid intracellular redistribution accompanies the process of doxorubicin uptake by LLC-PK1 cells. Although these cells are non-tumour cells derived from the normal epithelium of the proximal renal tubule, they display a model of doxorubicin redistribution which is characteristic of doxorubicin-resistant tumour cells.

Transport of the anti-cancer drug doxorubicin across cytoplasmic membranes and membranes composed of phospholipids derived from Escherichia coli occurs via a similar mechanism

An assay was developed to measure and directly compare transport of doxorubicin across right-side-out cytoplasmic membrane vesicles (ROV) and across model membranes (LUVET) composed of pure phospholipids, isolated from the corresponding cells. Escherichia coli was used as a model organism, since mutants are available which differ in phospholipid composition. Both in LUVET and ROV only passive diffusion across the bilayer is involved, because effects of drug concentration, pH, divalent cations, the phospholipid composition, and the active transport inhibitor verapamil were comparable. Permeability coefficients were about 2-3-times higher in ROV compared to LUVET. Furthermore, in LUVET an average activation energy of 87 kJ/mol and in ROV of 50 kJ/mol was observed. These differences are suggested to result from differences in membrane order between LUVET and ROV and differences in the temperature dependence of membrane order in LUVET and ROV, respectively. Because no background carrier-facilitated doxorubicin transport seems to be present, ROV are an excellent model system to study the effect of phospholipid composition on drug transport after expression of a multidrug resistance-conferring protein. Furthermore, data of passive diffusion of doxorubicin obtained with LUVET are representative for more complex, biologically relevant membrane systems.

Possible proton relay pathways in cytochrome c oxidase

As the final electron acceptor in the respiratory chain of eukaryotic and many prokaryotic organisms, cytochrome c oxidase (EC 1.9.3.1) catalyzes the reduction of oxygen to water and generates a proton gradient. To test for proton pathways through the oxidase, site-directed mutagenesis was applied to subunit I of the Rhodobacter sphaeroides enzyme. Mutants were characterized in three highly conserved regions of the peptide, comprising possible proton loading, unloading, and transfer sites: an interior loop between helices II and III (Asp132Asn/Ala), an exterior loop between helices IX and X (His411Ala, Asp412Asn, Thr413Asn, Tyr414Phe), and the predicted transmembrane helix VIII (Thr352Ala, Pro358Ala, Thr359Ala, Lys362Met). Most of the mutants had lower activity than wild type, but only mutants at residue 132 lost proton pumping while retaining electron transfer activity. Although electron transfer was substantially inhibited, no major structural alteration appears to have occurred in D132 mutants, since resonance Raman and visible absorbance spectra were normal. However, lower CO binding (70-85% of wild type) suggests some minor change to the binuclear center. In addition, the activity of the reconstituted Asp132 mutants was inhibited rather than stimulated by ionophores or uncoupler. The inhibition was not observed with the purified enzyme and a direct pH effect was ruled out, suggesting an altered response to the electrical or pH gradient. The results support an important role for the conserved II-III loop in the proton pumping process and are consistent with the possibility of involvement of residues in helix VIII and the IX-X loop.

Generation of proton-motive force by an archaeal terminal quinol oxidase from Sulfolobus acidocaldarius

The terminal quinol oxidase of the cytochrome aa3 type was isolated from the extreme thermoacidophilic archaeon Sulfolobus acidocaldarius. In micellar solution, the enzyme oxidized various quinols and exerted the highest activity with the physiological substrate caldariella quinol. The enzyme was functionally reconstituted into monolayer liposomes composed of archaeal tetraether lipids also derived from S. acidocaldarius. With the electron donor system ascorbate and N,N,N',N'-tetramethyl-p-phenylenediamine, the reconstituted enzyme was more active in the archaeal lipids as compared to lipids derived from Escherichia coli at temperatures above 50 degrees C. Due to the low proton permeability of the tetraether lipids, it was possible to generate a steady-state transmembrane electrical potential (delta psi, interior negative), and transmembrane pH gradient (delta pH, interior alkaline) at temperatures up to 70 degrees C. The successful functional reconstitution of the cytochrome aa3-type quinol oxidase from Sulfolobus identifies it as the key energy converter in the respiratory system of this hyperthermophilic archaeon.