Oxygen diffusion-concentration in erythrocyte plasma membranes studied by the fluorescence quenching of anionic and cationic pyrene derivatives

Shape transformations of red blood cells in the capillary and their possible connections to oxygen transportation

In this work, a series of numerical simulations have been performed to obtain the steady shapes of red blood cells under a shear force field in the capillary. Two possible classes of steady shapes, the axisymmetric parachute and the non-axisymmetric parachute, are found. If we assume that oxygen diffusion across the red cell membrane is mediated by membrane curvature, it is found that the non-axisymmetric parachute will be more favorable due to its special shape which enables it to have a larger portion of membrane patch capable of releasing oxygen to tissues.

Diffusion of nitric oxide and oxygen in lipoproteins and membranes studied by pyrene fluorescence quenching

Oxygen and nitric oxide are small hydrophobic molecules that usually need to diffuse a considerable distance to accomplish their biological functions and necessarily need to traverse several lipid membranes. Different methods have been used to study the diffusion of these molecules in membranes and herein we focus in the quenching of fluorescence of pyrenes inserted in the membrane. The pyrene derivatives have long fluorescence lifetimes (around 200 ns) that make them very sensitive to fluorescence quenching by nitric oxide, oxygen and other paramagnetic species. Results show that the apparent diffusion coefficients in membranes are similar to those in water, indicating that diffusion of these molecules in membranes is not considerably limited by the lipids. This high apparent diffusion in membranes is a consequence of both a favorable partition of these molecules in the hydrophobic interior of membranes and a high diffusion coefficient. Altering the composition of the membrane results in slight changes in diffusion, indicating that in most cases the lipid membranes will not hinder the passage of oxygen or nitric oxide. The diffusion of nitric oxide in the lipid core of low density lipoprotein is also very high, supporting its role as an antioxidant. In contrast to the high permeability of membranes to nitric oxide and oxygen, the permeability to other reactive species such as hydrogen peroxide and peroxynitrous acid is nearly five orders of magnitude lower.

Exploring O2 diffusion in A-type cytochrome c oxidases: molecular dynamics simulations uncover two alternative channels towards the binuclear site

Cytochrome c oxidases (Ccoxs) are the terminal enzymes of the respiratory chain in mitochondria and most bacteria. These enzymes couple dioxygen (O₂) reduction to the generation of a transmembrane electrochemical proton gradient. Despite decades of research and the availability of a large amount of structural and biochemical data available for the A-type Ccox family, little is known about the channel(s) used by O₂ to travel from the solvent/membrane to the heme a₃-Cu_B binuclear center (BNC). Moreover, the identification of all possible O₂ channels as well as the atomic details of O₂ diffusion is essential for the understanding of the working mechanisms of the A-type Ccox. In this work, we determined the O₂ distribution within Ccox from Rhodobacter sphaeroides, in the fully reduced state, in order to identify and characterize all the putative O₂ channels leading towards the BNC. For that, we use an integrated strategy combining atomistic molecular dynamics (MD) simulations (with and without explicit O₂ molecules) and implicit ligand sampling (ILS) calculations. Based on the 3D free energy map for O₂ inside Ccox, three channels were identified, all starting in the membrane hydrophobic region and connecting the surface of the protein to the BNC. One of these channels corresponds to the pathway inferred from the X-ray data available, whereas the other two are alternative routes for O₂ to reach the BNC. Both alternative O₂ channels start in the membrane spanning region and terminate close to Y288_I. These channels are a combination of multiple transiently interconnected hydrophobic cavities, whose opening and closure is regulated by the thermal fluctuations of the lining residues. Furthermore, our results show that, in this Ccox, the most likely (energetically preferred) routes for O₂ to reach the BNC are the alternative channels, rather than the X-ray inferred pathway.

Cytochrome c oxidases (Ccoxs), the terminal enzymes of the respiratory electron transport chain in eukaryotes and many prokaryotes, are key enzymes in aerobic respiration. These proteins couple the reduction of molecular dioxygen to water with the creation of a transmembrane electrochemical proton gradient. Over the last decades, most of the Ccoxs research focused on the mechanisms and energetics of reduction and/or proton pumping, and little emphasis has been given to the pathways used by dioxygen to reach the binuclear center, where dioxygen reduction takes place. In particular, the existence and the characteristics of the channel(s) used by O₂ to travel from the solvent/membrane to the binuclear site are still unclear. In this work, we combine all-atom molecular dynamics simulations and implicit ligand sampling calculations in order to identify and characterize the O₂ delivery channels in the Ccox from Rhodobacter sphaeroides. Altogether, our results suggest that, in this Ccox, O₂ can diffuse via three well-defined channels that start in membrane region (where O₂ solubility is higher than in the water). One of these channels corresponds to the pathway inferred from the X-ray data available, whereas the other two are alternative routes for O₂ to reach the binuclear center.

Multiscale analysis of hypoxemia in methemoglobin anemia

Methemoglobinemia is a disease that results from abnormally high levels of methemoglobin (MetHb) in the red blood cell (RBC), which is caused by simultaneous uptake of oxygen (O(2)) and nitric oxide (NO) in the human lungs. MetHb is produced in the RBC by irreversible NO-induced oxidation of the oxygen carrying ferrous ion (Fe(2+)) present in the heme group of the hemoglobin (Hb) molecule to its non-oxygen binding ferric state (Fe(3+)). This paper studies the role of NO in the pathophysiology of methemoglobinemia and presents a multiscale quantitative analysis of the relation between the levels of NO inhaled by the patient and the hypoxemia resulting from the disease. Reactions of NO occurring in the RBC with both Hb and oxyhemoglobin are considered in conjunction with the usual reaction between oxygen and Hb to form oxyhemoglobin. Our dynamic simulations of NO and O(2) uptake in the RBC (micro scale), alveolar capillary (meso scale) and the entire lung (macro scale) under continuous, simultaneous exposure to both gases, reveal that NO uptake competes with the reactive uptake of O(2), thus suppressing the latter and causing hypoxemia. We also find that the mass transfer resistances increase from micro through meso to macro scales, thus decreasing O(2) saturation as one goes up the scales from the cellular to the organ (lung) level. We show that NO levels of 203 ppm or higher while breathing in room air may be considered to be fatal for methemoglobinemia patients since it causes severe hypoxemia by reducing the O(2) saturation below a critical value of 88%, at which Long Term Oxygen Therapy (LTOT) becomes necessary.

Specific labelling of cell populations in blood with targeted immuno-fluorescent/magnetic glyconanoparticles

Current performance of iron oxide nanoparticle-based contrast agents in clinical use is based on the unspecific accumulation of the probes in certain organs or tissues. Specific targeted biofunctional nanoparticles would significantly increase their potential as diagnostic and therapeutic tools in vivo. In this study, multimodal fluorescent/magnetic glyco-nanoparticles were synthesized from gold-coated magnetite (glyco-ferrites) and converted into specific probes by the covalent coupling of protein G and subsequent incubation with an IgG antibody. The immuno-magnetic-fluorescent nanoparticles were applied to the specific labelling of peripheral blood mononuclear cells (PBMCs) in a complex biological medium, as human blood. We have been able to label specifically PBMCs present in blood in a percentage as low as 0.10-0.17%. Red blood cells (RBCs) were also clearly labelled, even though the inherent T(2) contrast arising from the high iron content of these cells (coming mainly from haemoglobin). The labelling was further assessed at cellular level by fluorescence microscopy. In conclusion, we have developed new contrast agents able to label specifically a cell population under adverse biological conditions (low abundance, low intrinsic T(2), high protein content). These findings open the door to the application of these probes for the labelling and tracking of endogenous cell populations like metastatic cancer cells, or progenitor stem cells that exist in very low amount in vivo.

Ruthenium polypyridyl peptide conjugates: membrane permeable probes for cellular imaging

Two novel polyarginine labelled ruthenium polypyridyl dyes are reported, one conjugated to five, (Ru-Ahx-R5), and one to eight arginine residues, (Ru-Ahx-R8); both complexes exhibit long-lived, intense, and oxygen-sensitive luminescence; (Ru-R8) is passively, efficiently and very rapidly transported across the cell membrane into the cytoplasm without requirement for its permeablisation.

Effect of adriamycin treatment on the lifetime of pyrene butyric acid in single living cells

We investigated the fluorescence lifetime of pyrene butyric acid (PBA) using various O2 concentrations in cells. Both in living and freshly fixed cells, PBA lifetime decreased with oxygen concentration. We recorded decay curves in single cells and measured PBA lifetime and NAD(P)H intensity values. Under nitrogen atmosphere, the probe lifetime differences (199 and 209 ns in living and freshly fixed cells, respectively) suggest a supplemental pathway for the deactivation of the probe when the cell functions are not stopped. We propose reactive oxygen species (ROS) to be the additional quenchers that cause this decrease. We further studied the effect of drugs generating ROS the anthracycline doxorubicin (adriamycin). For living cells, PBA lifetime decreased after adriamycin (ADR) treatment (200 and 1000 ng/ml). This supports our hypothesis that under nitrogen atmosphere and for freshly fixed cells, PBA lifetimes increase to an unchanging value due to absence of quenchers.

Direct Measurement of Nitric Oxide and Oxygen Partitioning into Liposomes and Low Density Lipoprotein

Nitric oxide (*NO) has been proposed to play a relevant role in modulating oxidative reactions in lipophilic media like biomembranes and lipoproteins. Two factors that will regulate *NO reactivity in the lipid milieu are its diffusion and solubility, but there is no data concerning the actual diffusion (D) and partition coefficients (KP) of *NO in biologically relevant hydrophobic phases. Herein, a "equilibrium-shift" method was designed to directly determine the *NO and O2 partition coefficients in liposomes and low density lipoprotein (LDL) relative to water. It was found that *NO partitions 4.4- and 3.4-fold in liposomes and LDL, respectively, whereas O2 behaves similarly with values of 3.9 and 2.9, respectively. In addition, actual diffusion coefficients in these hydrophobic phases were determined using fluorescence quenching and found that *NO diffuses approximately 2 times slower than O2 in the core of LDL and 12 times slower than in buffer (DNOLDL=3.9 x 10(-6) cm2 s(-1),DO2LDL=7.0 x 10(-6) cm2 s(-1),DNObuffer=DO2buffer=4.5 x 10(-5) cm2 s(-1)). The influence of *NO and O2 partitioning and diffusion in membranes and lipoproteins on *NO reaction with lipid radicals and auto-oxidation is discussed. Particularly, the 3-4-fold increase in O2 and *NO concentration within biological hydrophobic phases provides quantitative support for the idea of an accelerated auto-oxidation of *NO in lipid-containing structures, turning them into sites of enhanced local production of oxidant and nitrosating species.

Microfluorometric study of oxygen dependence of (1"-pyrene butyl)-2-rhodamine ester probe in mitochondria of living cells

The access to oxygen concentration is of importance in various organelles of living cells, especially in mitochondria. A new probe, (1"-pyrene butyl)-2-rhodamine ester, was designed to target this organelle. We present here the properties of the probe in such an environment. Microspectrofluorometry confirms the localization of the probe in the mitochondrial environment at low doses (1 microM). At these doses, the cell toxicity experiments show no effect on the cell growth. The emission spectrum measured at an excitation wavelength of 340 nm (pyrene centered) indicates energy transfer from the pyrene to the rhodamine chromophore, as also observed in an ethanol solution. With excitation at 337 nm, the excited state decays biexponentially with lifetime decays of 6-9 ns and 90 ns. The first corresponds to the intrinsic fluorescence of the cell and the latter corresponds to the pyrene chromophore. In degassed conditions the pyrene lifetime decay increases up to 130 ns. Under an oxygen atmosphere the lifetime decays decrease to 62 ns. The lifetime changes with the oxygen concentration were compared with the range obtained during our previous study in ethanol solution (5-220 ns). The observed differences were interpreted by studying the lifetime of the probe in simplified environments, liposome suspensions and protein solutions. In this paper we show that the new probe can be used to measure the fluctuation of oxygen concentration in the surroundings of mitochondria.

Oxygen-mediated cold-acclimation in cucumber (Cucumis sativus) seedlings

Cold acclimation of etiolated cucumber seedlings, consisting of cooling at 12 degrees C for 48 h followed by a warming period at 25 degrees C, led to tolerance to subsequent chilling at 2 degrees C. Tolerance, as evidenced by freedom from chilling injury and continued growth, developed during the warming period in a time-course manner for 12 h but decreased with prolonged warming. A similar increase and subsequent decrease was also observed in the content of palmitic, linoleic and linolenic acids in total lipid fraction from cucumber hypocotyl tissue. During the warming period supra-ambient oxygen stimulated, whereas subambient oxygen inhibited, the increase in fatty acid content as well as development of chilling tolerance. A strong correlation between oxygen-mediated changes in fatty acid content and associated development of cold tolerance suggests that both these processes are interrelated. Cold acclimation, but not cold stress, led to an increase followed by a decrease in CO2 evolution suggesting that a respiratory upsurge is yet another feature of cold acclimation in cucumbers.

Diffusion of nitric oxide into low density lipoprotein

A key early event in the development of atherosclerosis is the oxidation of low density lipoprotein (LDL) via different mechanisms including free radical reactions with both protein and lipid components. Nitric oxide (( small middle dot)NO) is capable of inhibiting LDL oxidation by scavenging radical species involved in oxidative chain propagation reactions. Herein, the diffusion of ( small middle dot)NO into LDL is studied by fluorescence quenching of pyrene derivatives. Selected probes 1-(pyrenyl)methyltrimethylammonium (PMTMA) and 1-(pyrenyl)-methyl-3-(9-octadecenoyloxy)-22,23-bisnor-5-cholenate (PMChO) were chosen so that they could be incorporated at different depths of the LDL particle. Indeed, PMTMA and PMChO were located in the surface and core of LDL, respectively, as indicated by changes in fluorescence spectra, fluorescence quenching studies with water-soluble quenchers and the lifetime values (tau(o)) of the excited probes. The apparent second order rate quenching constants of ( small middle dot)NO (k(NO)) for both probes were 2.6-3.8 x 10(10) m(-1) s(-1) and 1.2 x 10(10) m(-1) s(-1) in solution and native LDL, respectively, indicating that there is no significant barrier to the diffusion of ( small middle dot)NO to the surface and core of LDL. Nitric oxide was also capable of diffusing through oxidized LDL. Considering the preferential partitioning of ( small middle dot)NO in apolar milieu (6-8 for n-octanol:water) and therefore a larger ( small middle dot)NO concentration in LDL with respect to the aqueous phase, a corrected k(NO) value of approximately 0.2 x 10(10) m(-1) s(-1) can be determined, which still is sufficiently large and consistent with a facile diffusion of ( small middle dot)NO through LDL. Applying the Einstein-Smoluchowsky treatment, the apparent diffusion coefficient (D(')NO) of ( small middle dot)NO in native LDL is on average 2 x 10(-5) cm(2) s(-1), six times larger than that previously reported for erythrocyte plasma membrane. Thus, our observations support that ( small middle dot)NO readily traverses the LDL surface accessing the hydrophobic lipid core of the particle and affirm a role for ( small middle dot)NO as a major lipophilic antioxidant in LDL.

Disruption effects of carbon tetrachloride on rat liver microsomes

Lipid fluidity in rat liver microsomes was assessed by the steady state fluorescence polarization of stearic acid labelled at positions 2, 7 or 12 with 9-anthroyl groups. Oxygen solubility and/or mobility was determined by measuring the quenching rate of different pyrene derivatives containing an aromatic group separated from the microsome surface by 1, 4 or 11 methylene groups. The fluorescence of these compounds, as well as that from methylpyrene and benzo(alpha)pyrene, was quenched by carbon tetrachloride. The effect of the latter compound and of n-heptanol on the fluorescence polarization of the anthracene derivatives and the quenching rate by oxygen was also determined. The results obtained indicate that the lipid order decreases and the solubility and/or mobility of oxygen increases towards the bilayer core. The addition of carbon tetrachloride decreases the order of the membrane and increases the rate of fluorescence quenching by oxygen. The largest effect of the additive on the fluorescence quenching rate by oxygen is observed at intermediate positions in the bilayer. n-Heptanol addition decreases the membrane order and increases the rate of fluorescence quenching by oxygen or carbon tetrachloride, the maximum effect being observed at the microsomal surface. The differences between the effects of the two additives are discussed in terms of their different localizations. Fluorescence quenching by oxygen is considerably more affected by carbon tetrachloride than fluorescence depolarization. In addition, the maximum effect takes place at different positions in the membrane.(ABSTRACT TRUNCATED AT 250 WORDS)