Influence of the redox state of ubiquinones and plastoquinones on the order of lipid bilayers studied by fluorescence anisotropy of diphenylhexatriene and trimethylammonium diphenylhexatriene

Plastoquinone In and Beyond Photosynthesis

Plastoquinone-9 (PQ-9) is an essential component of photosynthesis that carries electrons in the linear and alternative electron transport chains, and is also a redox sensor that regulates state transitions and gene expression. However, a large fraction of the PQ pool is located outside the thylakoid membranes, in the plastoglobules and the chloroplast envelopes, reflecting a wider range of functions beyond electron transport. This review describes new functions of PQ in photoprotection, as a potent antioxidant, and in chloroplast metabolism as a cofactor in the biosynthesis of chloroplast metabolites. It also focuses on the essential need for tight environmental control of PQ biosynthesis and for active exchange of this compound between the thylakoid membranes and the plastoglobules. Through its multiple functions, PQ connects photosynthesis with metabolism, light acclimation, and stress tolerance.

Cultivation at high osmotic pressure confers ubiquinone 8-independent protection of respiration on Escherichia coli

Ubiquinone 8 (coenzyme Q8 or Q8) mediates electron transfer within the aerobic respiratory chain, mitigates oxidative stress, and contributes to gene expression in Escherichia coli In addition, Q8 was proposed to confer bacterial osmotolerance by accumulating during growth at high osmotic pressure and altering membrane stability. The osmolyte trehalose and membrane lipid cardiolipin accumulate in E. coli cells cultivated at high osmotic pressure. Here, Q8 deficiency impaired E. coli growth at low osmotic pressure and rendered growth osmotically sensitive. The Q8 deficiency impeded cellular O₂ uptake and also inhibited the activities of two proton symporters, the osmosensing transporter ProP and the lactose transporter LacY. Q8 supplementation decreased membrane fluidity in liposomes, but did not affect ProP activity in proteoliposomes, which is respiration-independent. Liposomes and proteoliposomes prepared with E. coli lipids were used for these experiments. Similar oxygen uptake rates were observed for bacteria cultivated at low and high osmotic pressures. In contrast, respiration was dramatically inhibited when bacteria grown at the same low osmotic pressure were shifted to high osmotic pressure. Thus, respiration was restored during prolonged growth of E. coli at high osmotic pressure. Of note, bacteria cultivated at low and high osmotic pressures had similar Q8 concentrations. The protection of respiration was neither diminished by cardiolipin deficiency nor conferred by trehalose overproduction during growth at low osmotic pressure, but rather might be achieved by Q8-independent respiratory chain remodeling. We conclude that osmotolerance is conferred through Q8-independent protection of respiration, not by altering physical properties of the membrane.

CoQ10 selective miscibility and penetration into lipid monolayers with lower lateral packing density

CoQ₁₀ is ubiquitously present in eukaryotic cells. It acts as electron carrier in the electron transport chain of the inner membrane of the mitochondria to facilitate aerobic cellular respiration. A highly stable lipid nanodispersion formulation containing CoQ₁₀ (BPM31510) is currently in clinical investigation for treatment of cancer. This study was designed to determine whether biophysical interactions between CoQ₁₀ and lipid, in part, explain the observed stability and cellular accumulation of CoQ₁₀ in cells and tissues. A lipid monolayer at the air-water interface was used as an experimental membrane model to measure CoQ₁₀ penetration and solubility. Lipid monolayers with varying proportions of CoQ₁₀ were laterally compressed to measure CoQ₁₀ miscibility and lateral organization. Additionally, lipid monolayers with varying lateral packing densities were spread at the air-water interface and CoQ₁₀ was injected in proximity to measure its rate of penetration. Our results demonstrate that CoQ₁₀ selectively penetrates into lipid monolayers with a lower lateral packing density, and is excluded by monolayers of higher packing densities. Data also indicates that CoQ₁₀-lipid mixing is non-ideal. CoQ₁₀ presence in lipid monolayers is biphasic, with one phase occupying the interstitial space between the DMPC lipids, and the other phase is present as pure CoQ₁₀ domains. This work provides further insight into mechanism of action of CoQ₁₀ based formulations that can significantly increase intracellular CoQ₁₀ concentration to show pleotropic effects on cellular functions.

Function of plastochromanol and other biological prenyllipids in the inhibition of lipid peroxidation-A comparative study in model systems

Lipid peroxidation is an oxidation reaction leading to the generation of lipid hydroperoxides. Here we present comparative data on the inhibition of lipid peroxidation by a variety of biological prenyllipids in liposomes prepared from natural lipid membranes. Lipid peroxidation was initiated by hydrophilic and hydrophobic azo initiators, as well as by singlet oxygen generated via photosensitized reaction of hydrophobic zinc tetraphenylporphine. When lipid peroxidation was initiated in the water phase, tocopherols and plastochromanol-8 were more effective than prenylquinols, such as plastoquinol-9, ubiquinol-10 or α-tocopherolquinol. However, if the peroxidation was initiated within the hydrophobic interior of liposome membranes, long-chain prenyllipids, such as plastoquinol-9 and plastochromanol-8, were considerably more active than tocopherols in the inhibition of the reaction. In the latter system, tocopherols showed even prooxidant activity. The prooxidant activity of α-tocopherol was prevented by plastoquinol, suggesting the reduction of α-tocopheroxyl radical by the quinol. All the investigated prenyllipids were able to inhibit singlet oxygen-mediated lipid peroxidation but the most active were prenylquinols in this respect. Among all the prenyllipids investigated, plastochromanol-8 was the most versatile antioxidant in the inhibition of lipid peroxidation initiated by the three different methods.

Configuration of polyisoprenoids affects the permeability and thermotropic properties of phospholipid/polyisoprenoid model membranes

The influence of α-cis- and α-trans-polyprenols on the structure and properties of model membranes was analyzed. The interaction of Ficaprenol-12 (α-cis-Prenol-12, α-Z-Prenol-12) and Alloprenol-12 (α-trans-Prenol-12, α-E-Prenol-12) with model membranes was compared using high performance liquid chromatography (HPLC), differential scanning calorimetry (DSC) and fluorescent methods. l-α-Phosphatidylcholine from egg yolk (EYPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) as the main lipid components of unilamellar (SUVs) and multilamellar (MLVs) vesicles were used. The two-step extraction procedure (n-pentane and hexane, respectively) allowed to separately analyze the fractions of polyprenol as non-incorporated (Prenol(NonInc)) and incorporated (Prenol(Inc)) into liposomes. Consequently, distribution coefficients, P', describing the equilibrium of prenol content between phospholipid (EYPC) membrane and the aqueous phase gave different logP' for α-cis- and α-trans-Prenol-12, indicating that the configuration of the α-terminal residue significantly alters the hydrophobicity of the polyisoprenoid molecule and consequently the affinity of polyprenols for EYPC membrane. In fluorescence experiments α-trans-Pren-12 increased up to 1.7-fold the permeability of EYPC bilayer for glucose while the effect of α-cis-Pren-12 was almost negligible. Considerable changes of thermotropic behavior of DPPC membranes in the presence of both prenol isomers were observed. α-trans-Pren-12 completely abolished the pretransition while in the case of α-cis-Pren-12 it was noticeably reduced. Furthermore, for both prenol isomers, the temperature of the main phase transition (T(m)) was shifted by about 1°C to lower values and the height of the peak was significantly reduced. The DSC analysis profiles also showed a new peak at 38.7°C, which may suggest the concomitant presence of more that one phase within the membrane. Results of these experiments and the concomitant occurrence of alloprenols and ficaprenols in plant tissues suggest that cis/trans isomerization of the α-residue of polyisoprenoid molecule might comprise a putative mechanism responsible for modulation of the permeability of cellular membranes.

Occurrence, biosynthesis and function of isoprenoid quinones

Isoprenoid quinones are one of the most important groups of compounds occurring in membranes of living organisms. These compounds are composed of a hydrophilic head group and an apolar isoprenoid side chain, giving the molecules a lipid-soluble character. Isoprenoid quinones function mainly as electron and proton carriers in photosynthetic and respiratory electron transport chains and these compounds show also additional functions, such as antioxidant function. Most of naturally occurring isoprenoid quinones belong to naphthoquinones or evolutionary younger benzoquinones. Among benzoquinones, the most widespread and important are ubiquinones and plastoquinones. Menaquinones, belonging to naphthoquinones, function in respiratory and photosynthetic electron transport chains of bacteria. Phylloquinone K(1), a phytyl naphthoquinone, functions in the photosynthetic electron transport in photosystem I. Ubiquinones participate in respiratory chains of eukaryotic mitochondria and some bacteria. Plastoquinones are components of photosynthetic electron transport chains of cyanobacteria and plant chloroplasts. Biosynthetic pathway of isoprenoid quinones has been described, as well as their additional, recently recognized, diverse functions in bacterial, plant and animal metabolism.

The cold but not hard fats in ectotherms: consequences of lipid restructuring on susceptibility of biological membranes to peroxidation, a review

The production of reactive oxygen species is a regular feature of life in the presence of oxygen. Some reactive oxygen species possess sufficient energy to initiate lipid peroxidation in biological membranes, self-propagating reactions with the potential to damage membranes by altering their physical properties and ultimately their function. Two of the most prominent patterns of lipid restructuring in membranes of ectotherms involve contents of polyunsaturated fatty acids and ratios of the abundant phospholipids, phosphatidylcholine and phosphatidylethanolamine. Since polyunsaturated fatty acids and phosphatidylethanolamine are particularly vulnerable to oxidation, it is likely that higher contents of these lipids at low body temperature elevate the inherent susceptibility of membranes to lipid peroxidation. Although membranes from animals living at low body temperatures may be more prone to oxidation, the generation of reactive oxygen species and lipid peroxidation are sensitive to temperature. These scenarios raise the possibility that membrane susceptibility to lipid peroxidation is conserved at physiological temperatures. Reduced levels of polyunsaturated fatty acids and phosphatidylethanolamine may protect membranes at warm temperatures from deleterious oxidations when rates of reactive oxygen species production and lipid peroxidation are relatively high. At low temperatures, enhanced susceptibility may ensure sufficient lipid peroxidation for cellular processes that require lipid oxidation products.

Fluorescence studies of the interactions of ubiquinol-10 with liposomes

Ubiquinone-10 plays a central role in energy production and its reduced form, ubiquinol-10 is also capable of acting as a potent radical scavenging antioxidant against membrane lipid peroxidation. Efficiency of this protection depends mostly on its localization in lipid bilayer. The intrinsic fluorescence of ubiquinol-10 and of the exogenous probe, Laurdan, has been used to determine the location of ubiquinol-10 in unilamellar liposomes of egg phosphatidylcholine (EggPC) and dimyristoyl phosphatidylcholine. Laurdan fluorescence moiety is positioned at the hydrophilic-hydrophobic interface of the phospholipid bilayer and its parameters reflect the membrane polarity and microheterogeneity, which we have used to explore the coexistence of microdomains with distinct physical properties. In liquid-crystalline bilayers ubiquinol has a short fluorescence lifetime (0.4 ns) and a high steady-state anisotropy. In a concentration-dependent manner, ubiquinol-10 influences the Laurdan excitation, emission and generalized polarization measurements. In EggPC liposomes ubiquinol-10 induces a decrease in membrane water mobility near the probe, while in dimyristoyl liposomes a decrease in the membrane water content was found. Moreover the presence of ubiquinol results in the formation of coexisting phospholipid domains of gel and liquid-crystalline phases. The results indicate that ubiquinol-10 molecules are mainly located at the polar-lipid interface, inducing changes in the physico-chemical properties of the bilayer microenvironment.

Effect of Coenzyme Q10 Incorporation on the Characteristics of Nanoliposomes

Coenzyme Q(10) (CoQ(10)) is incorporated in nanoliposomes composed of egg yolk phospholipid, cholesterol, and Tween 80. Atomic force microscopy, performed to characterize vesicle surface topology, shows some visible influence of CoQ(10) on the nanoliposomal structure. CoQ(10) incorporation can suppress the increase of the z-average diameter of nanoliposomes during storage for 8 months at 4 degrees C. The liposomal lipid peroxidation caused by Fe(III)/ascorbate is also significantly inhibited. Perturbation of acyl chain motion of lipids due to the presence of CoQ(10) in the bilayer is examined by fluorescence probe diphenyl-hexatriene and Raman spectroscopy. Fluorescence probe studies indicate that CoQ(10) incorporation results in the microviscosity increase of nanoliposomes. The steric structure of nanoliposomes reflected by Raman spectroscopy changes obviously and shows CoQ(10) content dependency. The order parameters for the lateral interaction between chains increase. The trans conformation decrease and the gauche conformation increase as the weight contents of CoQ(10) incorporation are at 1%, 5%, 10%, and 32.5%. However, the order parameters for the longitudinal interaction in chains was higher than that of pure nanoliposomes as the weight content of CoQ(10) is at 25%. Results suggest that CoQ(10)might intercalate between lipid molecules and perturb the bilayer structure.

Evidence that cytochrome b559 is involved in superoxide production in photosystem II: effect of synthetic short-chain plastoquinones in a cytochrome b559 tobacco mutant

Light-induced production of superoxide (O2*-) in spinach PSII (photosystem II) membrane particles was studied using EPR spin-trapping spectroscopy. The presence of exogenous PQs (plastoquinones) with a different side-chain length (PQ-n, n isoprenoid units in the side-chain) enhanced O2*- production in the following order: PQ-1>PQ-2>>PQ-9. In PSII membrane particles isolated from the tobacco cyt (cytochrome) b559 mutant which carries a single-point mutation in the beta-subunit and also has a decreased amount of the alpha-subunit, the effect of PQ-1 was less than in the wild-type. The increase in LP (low-potential) cyt b559 content, induced by the incubation of spinach PSII membrane particles at low pH, resulted in a significant increase in O2*- formation in the presence of PQ-1, whereas it had little effect on O2*- production in the absence of PQ-1. The enhancement of O2*- formation induced by PQ-1 was not abolished by DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea]. Under anaerobic conditions, dark oxidation of LP cyt b559 increased, as pH was decreased. The presence of molecular oxygen significantly enhanced dark oxidation of LP cyt b559. Based on these findings it is suggested that short-chain PQs stimulate O2*- production via a mechanism that involves electron transfer from Pheo- (pheophytin) to LP cyt b559 and subsequent auto-oxidation of LP cyt b559.

Fluorescence Lifetimes Study of α-Tocopherol and Biological Prenylquinols in Organic Solvents and Model Membranes

We have found that for biological prenyllipids, such as plastoquinol-9, alpha-tocopherol quinol, and alpha-tocopherol, the shortest fluorescence lifetimes were found in aprotic solvents (hexane, ethyl acetate) whereas the longest lifetimes were those of ubiquinonol-10 in these solvents. For all the investigated prenyllipids, fluorescence lifetime in alcohols increased along with an increase in solvent viscosity. In a concentrated hexane solution, the lifetimes of prenylquinols considerably decreased. This contrasts with methanol solutions, which is probably due to the self-association of these compounds in aprotic solvents. We have also found a correlation of the Stokes shift of prenyllipids fluorescence with the orientation polarizability of the solvents. Based on data obtained in organic solvents, measurements of the fluorescence lifetimes of prenyllipids in liposomes allowed an estimation of the relative distance of their fluorescent rings from the liposome membrane surface, and was found to be the shortest for alpha-tocopherol quinol in egg yolk phosphatidylcholine liposomes, and increased in the following order: alpha-tocopherol in dipalmitoyl phosphatidylcholine liposomes < alpha-tocopherol < plastoquinol-9 < ubiquinol-10 in egg-yolk phosphatidylcholine liposomes.

Anisotropy measurements of intrinsic fluorescence of prenyllipids reveal much higher mobility of plastoquinol than alpha-tocopherol in model membranes

As an alternative to a fluorescent probe approach, the intrinsic fluorescence of reduced forms of prenylquinones has been exploited, which offers a convenient means of determining directly motional properties of these molecules. The steady-state fluorescence anisotropy measurements of plastoquinols (PQH(2)) and alpha-tocopherol (alpha-Toc) incorporated into phospholipid liposomes have been performed. The effect of prenyllipid concentration, PQH(2) side chain length and the composition of the membranes has been studied. For the data interpretation, the fundamental anisotropy of alpha-Toc, PQH(2), ubiquinol-10 and alpha-tocopherolquinol, as well as the angles between the absorption and emission transition moments have been also determined. It was concluded that alpha-Toc shows very low mobility in the lipid bilayer, whereas PQH(2)-9 displays significant motional freedom in dipalmitoylphosphatidylcholine vesicles and even higher in egg yolk lecithin membranes.

The influence of structure and redox state of prenylquinones on thermotropic phase behaviour of phospholipids in model membranes

Our study was aimed to investigate the significance of the isoprenoid side chain size as well as redox state of the quinone ring for interaction of two main classes of prenylquinones: plastoquinones (PQ) and ubiquinones (UQ) with lipid bilayers. By use of differential scanning calorimetry (DSC) we have followed the thermotropic behaviour of multilamellar vesicles prepared from dipalmitoylphosphatidylcholine (DPPC) upon incorporation of increasing amount (1.3-12 mol%) of quinone (quinol) molecules. Our studies reveal that as the side chain is shorter (from 9 to 2 isoprenoid units) the height of the calorimetric profiles is reduced and the temperature of the main transition of DPPC (T(m)) decreases (T(m)=39.4 degrees C for a sample with 12 mol% of PQ-2), and then increases up to 39.8 degrees C for PQ-1. For the samples containing quinols the effect is more pronounced even at lower concentration. The greater influence of the added prenylquinones on the pretransition demonstrates a stronger distortion of the DPPC packing in the gel state. It seems that this is the isoprenoid side chain length rather than the redox state of prenylquinones that determines their effectiveness in perturbation of thermotropic properties of lipid bilayer.

Incorporation of plastoquinone and ubiquinone into liposome membranes studied by HPLC analysis. The effect of side chain length and redox state of quinone

The efficiency of incorporation of plastoquinones and ubiquinones into phospholipid liposomes has been studied. The representatives of short (PQ1 and UQ1) middle (PQ4 and UQ4) and long (PQ9, UQ9 and UQ10) prenylquinones have been used to investigate the effect of quinone side chain length. The properties of hydroquinones have been also thoroughly examined in relation to the quinone forms. The extraction procedure was modified and further developed which enables removing of nonincorporated quinone by pentane washing and then determination of quinone content inside the lipid bilayer. The quantitatively evaluation of the amount of prenylquinone was assayed by means of HPLC analysis which offers much greater sensitivity and could be easily applied in case of hydroquinones. It has been found that PQ1 and UQ1 as well as their reduced forms were present mainly (about 80%) in the aqueous phase, when attempting to introduce them into phospholipid bilayer. In case of quinones having four and more isoprenyl units in side chain, a high level of quinone incorporation, ranging about 95%, was observed. The results pointed out that when comparing the effects of different exogenous quinones on membrane related processes, one has to consider the effectiveness of their incorporation within lipid bilayer.