Fluorescence properties of plastoquinol, ubiquinol and alpha-tocopherol quinol in solution and liposome membranes

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.

Fluorescent signals associated with respiratory Complex I revealed conformational changes in the catalytic site

Fluorescent signals associated with Complex I (NADH:ubiquinone oxidoreductase type I) upon its reduction by NADH without added acceptors and upon NADH:ubiquinone oxidoreduction were studied. Two Complex I-associated redox-dependent signals were observed: with maximum emission at 400 nm (λex = 320 nm) and 526 nm (λex = 450 nm). The 400 nm signal derived from ubiquinol accumulated in Complex I/DDM (n-dodecyl β-D-maltopyranoside) micelles. The 526 nm redox signal unexpectedly derives mainly from FMN (flavin mononucleotide), whose fluorescence in oxidized protein is fully quenched, but arises transiently upon reduction of Complex I by NADH. The paradoxical flare-up of FMN fluorescence is discussed in terms of conformational changes in the catalytic site upon NADH binding. The difficulties in revealing semiquinone fluorescent signal are considered.

Transmembrane NADH Oxidation with Tetracyanoquinodimethane

The design of efficient schemes for nicotinamide adenine dinucleotide (NAD) regeneration is essential for the development of enzymatic biotechnological processes in order to sustain continuous production. In line with our motivation for the encapsulation of redox cascades in liposomes to serve as microbioreactors, we developed a straightforward strategy for the interfacial oxidation of entrapped NADH by ferricyanide as an external electron acceptor. Instead of the commonly applied enzymatic regeneration methods, we employed a hydrophobic redox shuttle embedded in the liposome bilayer. Tetracyanoquinodimethane (TCNQ) mediated electron transfer across the membrane and thus allowed us to shortcut and emulate part of the electron transfer chain functionality without the involvement of membrane proteins. To describe the experimental system, we developed a mathematical model which allowed for the determination of rate constants and exhibited handy predictive utility.

Tocopherol Cyclases-Substrate Specificity and Phylogenetic Relations

In the present studies, we focused on substrate specificity of tocopherol cyclase, the key enzyme in the biosynthesis of the tocopherols and plastochromanol-8, the main plant lipid antioxidants, with special emphasis on the preference for tocopherols and plastochromanol-8 precursors, taking advantage of the recombinant enzyme originating from Arabidopsis thaliana and isolated plastoglobules, thylakoids and various model systems like micelles and thylakoids. Plastoglobules and triacylglycerol micelles were the most efficient reaction environment for the cyclase. In various investigated systems, synthesis of γ-tocopherol proceeded considerably faster than that of plastochromanol-8, probably mainly due to different localization of the corresponding substrates in the analyzed lipid structures. Moreover, our study was complemented by bioinformatics analysis of the phylogenetic relations of the cyclases and sequence motifs, crucial for the enzyme activity, were proposed. The analysis revealed also a group of tocopherol cyclase-like proteins in a number of heterotrophic bacterial species, with a conserved region common with photosynthetic organisms, that might be engaged in the catalytic activity of both groups of organisms.

Experimental evidence suggesting that H2O2 is produced within the thylakoid membrane in a reaction between plastoquinol and singlet oxygen

Plastoquinol (PQH2-9) and plastoquinone (PQ-9) mediate photosynthetic electron transfer. We isolated PQH2-9 from thylakoid membranes, purified it with HPLC, subjected the purified PQH2-9 to singlet oxygen ((1)O2) and analyzed the products. The main reaction of (1)O2 with PQH2-9 in methanol was found to result in formation of PQ-9 and H2O2, and the amount of H2O2 produced was essentially the same as the amount of oxidized PQH2-9. Formation of H2O2 in the reaction between (1)O2 and PQH2-9 may be an important source of H2O2 within the lipophilic thylakoid membrane.

Simultaneous analyses of oxidized and reduced forms of photosynthetic quinones by high-performance liquid chromatography

Plastoquinones and phylloquinones are the major plant quinones localized in chloroplasts, and they act as photosynthetic electron redox mediators in thylakoid membranes. These quinones are analyzed by two processes: extraction with organic solvents and quinone assay by high-performance liquid chromatography (HPLC) analysis. Solvent choice is very important from the viewpoint of stability of the redox state in the extraction processes and during storage of plant quinones. We introduce procedures and solvents to avoid changes in the redox state of quinones, in addition to achieving high extraction efficiency. Traditional methods have problems of low sensitivity and require preparation steps to remove interfering substances, such as plant pigments. HPLC systems have been developed utilizing the fluorescent properties of quinols (reduced forms) to measure quinones. Plastoquinones were detected by reversed-phase HPLC with dual detectors (ultra-violet and fluorescence detection). However, the peak of phylloquinone and plastoquinone isomers with shorter side chains often overlaps with a large peak of fast-eluting pigments. To address these issues, HPLC with fluorescence detection after post-column reduction to convert quinones to fluorescent quinol was applied for measurement of fast-eluting quinones (low hydrophobicity quinones and quinols) such as phylloquinone. Using post-column reduction methods with sodium borohydride or platinum black, not only the reduced forms (fluorescent) but also the oxidized forms (non-fluorescent) could be clearly measured by HPLC with a fluorescence detector.

Fluorescence of bioaerosols: mathematical model including primary fluorescing and absorbing molecules in bacteria

This paper describes a mathematical model of fluorescent biological particles composed of bacteria, viruses, or proteins. The fluorescent and/or light absorbing molecules included in the model are amino acids (tryptophan, etc.); nucleic acids (DNA, RNA, etc.); coenzymes (nicotinamide adenine dinucleotides, flavins, and vitamins B₆ and K and variants of these); and dipicolinates. The concentrations, absorptivities, and fluorescence quantum yields are estimated from the literature, often with large uncertainties. The bioparticles in the model are spherical and homogeneous. Calculated fluorescence cross sections for particles excited at 266, 280, and 355 nm are compared with measured values from the literature for several bacteria, bacterial spores and albumins. The calculated 266- and 280-nm excited fluorescence is within a factor of 3.2 of the measurements for the vegetative cells and proteins, but overestimates the fluorescence of spores by a factor of 10 or more. This is the first reported modeling of the fluorescence of bioaerosols in which the primary fluorophores and absorbing molecules are included.

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.

Ferredoxin:NADP+ oxidoreductase bound to cytochrome b₆f complex is active in plastoquinone reduction: implications for cyclic electron transport

In this study, we have compared three isolation methods of cytochrome b₆f complex, obtained from spinach (Spinacia oleracea), differing in the preservation of the cytochrome b₆f-associated ferredoxin:NADP+ oxidoreductase (FNR). Although the complexes isolated by all the methods showed the presence of the FNR peptide(s), when incorporated into liposome membranes, the NADPH-PQ (plastoquinone) oxidoreductase activity was not detected for the cytochrome b₆f complex isolated with the original method including a NaBr wash. Some activity was found for the complex isolated with the omission of the wash, but the highest activity was detected for the complex isolated with the use of digitonin. The reaction rate of PQ reduction of the investigated complexes in liposomes was not significantly influenced by the addition of free FNR or ferredoxin. The reaction was inhibited by about 60% in the presence of 2 µM 2-n-nonyl-4-hydroxyquinoline N-oxide, an inhibitor of the cytochrome b₆ f complex at the Q(i) site, while it was not affected by triphenyltin or isobutyl cyanide that interacts with the recently identified heme c(i) . The obtained data indicate that FNR associated with the cytochrome b₆ f complex can participate in the cyclic electron transport as PSI-PQ or NADPH-PQ oxidoreductase. Moreover, we have shown that PQ can be non-enzymatically reduced by ascorbate in liposomes and this reaction might be involved in non-photochemical reduction pathways of the PQ-pool in chloroplasts.

Simultaneous determination of in vivo plastoquinone and ubiquinone redox states by HPLC-based analysis

For a better understanding of the metabolic interaction between chloroplasts and mitochondria, it is important to analyze the in vivo redox states of the electron transport chains in both organelles at the same time. For this purpose, we devised an HPLC-based measurement system simultaneously analyzing plastoquinone (PQ) and ubiquinone (UQ) contents and redox states. Using this system, we discovered that, in addition to PQ, the reduction levels of UQ were elevated under high-light conditions. We also provide direct evidence that mitochondrial alternative oxidase contributes to alleviate UQ over-reduction under such conditions.

Tocochromanols, plastoquinol, and other biological prenyllipids as singlet oxygen quenchers-determination of singlet oxygen quenching rate constants and oxidation products

Singlet oxygen quenching rate constants for tocopherol and tocotrienol homologues have been determined in organic solvents of different polarities, as well as for other biological prenyllipids such as plastoquinol, ubiquinol, and alpha-tocopherolquinol. The obtained results showed that the quenching activity of tocochromanols was mainly due to the chromanol ring of the molecule and the activity increased with the number of the methyl groups in the ring and solvent polarity. Among prenylquinols, alpha-tocopherolquinol was the most active scavenger of singlet oxygen followed by ubiquinol and plastoquinol. The oxidation products of tocopherols were identified as 8a-hydroperoxy-tocopherones which are converted to the corresponding tocopherolquinones under acidic conditions. The primary oxidation products of prenylquinols, containing unsaturated side chains, were the corresponding prenylquinones that were further oxidized to hydroxyl side-chain derivatives. In the case of plastochromanol, the gamma-tocotrienol homologue found in some seed oils, mainly the hydroxyl derivatives were formed, although 8a-hydroperoxy-gamma-tocopherones were also formed to a minor extent, both from plastochromanol and from its hydroxyl, side-chain derivatives. The obtained results were discussed in terms of the activity of different prenyllipids as singlet oxygen scavengers in vivo.

Tocopherol quinone content of green algae and higher plants revised by a new high-sensitive fluorescence detection method using HPLC--effects of high light stress and senescence

A rapid, sensitive fluorescence method was applied here for detection of oxidized tocopherol quinones in total plant tissue extracts using HPLC, employing a post-column reduction of these compounds by a Zn column. Using this method, we were able to detect both alpha- and gamma-tocopherol quinones in Chlamydomonas reinhardii with a very high degree of sensitivity. The levels of both compounds increased under high light stress in the presence of pyrazolate in parallel to a decrease in the content of the corresponding tocopherols. The formation of tocopherol quinones from tocopherols was apparently due to their oxidation by singlet oxygen, which is formed in photosystem II under high light stress. alpha-Tocopherol quinone was also detected in a variety of higher plants of different age, and its level was found to increase during senescence in leaves grown under natural conditions. In contrast to alpha-tocopherol quinone, gamma-tocopherol quinone was not found in the higher plant species investigated with the exception of young runner bean leaves, where the levels of both compounds increased dramatically during cold and light stress. Taking advantage of native fluorescence of the reduced alpha-tocopherol quinone (alpha-tocopherol quinol), it can be detected in plant tissue extracts with a high sensitivity. In young runner bean leaves, alpha-tocopherol quinol was found at a level similar to alpha-tocopherol.

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.

Plastoquinol as a singlet oxygen scavenger in photosystem II

It has been found that in Chlamydomonas reinhardtii cells, under high-light stress, the level of reduced plastoquinone considerably increases while in the presence of pyrazolate, an inhibitor of plastoquinone and tocopherol biosynthesis, the content of reduced plastoquinone quickly decreases, similarly to alpha-tocopherol. In relation to chlorophyll, after 18 h of growth under low light with the inhibitor, the content of alpha-tocopherol was 22.2 mol/1000 mol chlorophyll and that of total plastoquinone (oxidized and reduced) was 19 mol/1000 mol chlorophyll, while after 2 h of high-light stress the corresponding amounts dropped to 6.4 and 6.2 mol/1000 mol chlorophyll for alpha-tocopherol and total plastoquinone, respectively. The degradation of both prenyllipids was partially reversed by diphenylamine, a singlet oxygen scavenger. It was concluded that plastoquinol, as well as alpha-tocopherol is decomposed under high-light stress as a result of a scavenging reaction of singlet oxygen generated in photosystem II. The levels of both alpha-tocopherol and of the reduced plastoquinone are not affected significantly in the absence of the inhibitor due to a high turnover rate of both prenyllipids, i.e., their degradation is compensated by fast biosynthesis. The calculated turnover rates under high-light conditions were twofold higher for total plastoquinone (0.23 nmol/h/ml of cell culture) than for alpha-tocopherol (0.11 nmol/h/ml). We have also found that the level of alpha-tocopherolquinone, an oxidation product of alpha-tocopherol, increases as the alpha-tocopherol is consumed. The same correlation was also observed for gamma-tocopherol and its quinone form. Moreover, in the presence of pyrazolate under low-light growth conditions, the synthesis of plastoquinone-C, a hydroxylated plastoquinone derivative, was stimulated in contrast to plastoquinone, indicating for the first time a functional role for plastoquinone-C. The presented data also suggest that the two plastoquinones may have different biosynthetic pathways in C. reinhardtii.

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.

Plastoquinones are effectively reduced by ferredoxin:NADP+ oxidoreductase in the presence of sodium cholate micelles. Significance for cyclic electron transport and chlororespiration

The effect of sodium cholate and other detergents (Triton X-100, sodium dodecyl sulphate, octyl glucoside, myristyltrimethylammonium bromide) on the reduction of plastoquinones (PQ) with a different length of the side-chain by spinach ferredoxin:NADP(+) oxidoreductase (FNR) in the presence of NADPH has been studied. Both NADPH oxidation and oxygen uptake due to plastosemiquinone autoxidation were highly stimulated only in the presence of sodium cholate among the used detergents. Sodium cholate at the concentration of 20 mM was found to be the most effective on both PQ-4 and PQ-9-mediated oxygen uptake. The FNR-dependent reduction of plastoquinones incorporated into sodium cholate micelles was stimulated by spinach ferredoxin but inhibited by Mg(2+) ions. It was concluded that the structure of sodium cholate micelles facilitates contact of plastoquinone molecules with the enzyme and creates favourable conditions for the reaction similar to those found in thylakoid membranes for PQ-9 reduction. The obtained results were discussed in terms of the function of FNR as a ferredoxin:plastoquinone reductase both in cyclic electron transport and chlororespiration.

Cytochrome c is reduced mainly by plastoquinol and not by superoxide in thylakoid membranes at low and medium light intensities: its specific interaction with thylakoid membrane lipids

We have found that, at low light intensity (5-10 micromol photons x m(-2) x s(-1)), photoreduction of cyt (cytochrome) c by isolated thylakoids was not inhibited by dinitrophenylether of iodonitrothymol, an inhibitor of the cyt b6- f complex, and the inhibition was only partial at medium light intensity (50-200 micromol photons x m(-2) x s(-1)). The photoreduction was not significantly influenced by superoxide dismutase. The conclusion that cyt c could be reduced directly by the plastoquinone pool was confirmed by the observation that plastoquinol-9 reduced cyt c efficiently when it was incorporated into liposome membranes prepared from thylakoid membrane lipids. It was shown that the cyt is specifically bound to thylakoid lipid liposomes owing to the presence of negatively charged lipids, phosphatidylglycerol and sulphoquinovosyldiacylglycerol, and the reduction was stimulated by the presence of monogalactosyldiacylglycerol, an inverted micelles-forming lipid, in the membranes, where the cyt c reduction by plastoquinol probably takes place. The results obtained are also discussed in terms of reliability of the method of cyt c photoreduction for determining superoxide production by illuminated thylakoids.

Scavenging of superoxide generated in photosystem I by plastoquinol and other prenyllipids in thylakoid membranes

We have examined scavenging of a superoxide by various prenyllipids occurring in thylakoid membranes, such as plastoquinone-9, alpha-tocopherolquinone, their reduced forms, and alpha-tocopherol, measuring oxygen uptake in hexane-extracted and untreated spinach thylakoids with a fast oxygen electrode under flash-light illumination. The obtained results demonstrated that all the investigated prenyllipids showed the superoxide scavenging properties, and plastoquinol-9 was the most active in this respect. Plastoquinol-9 formed in thylakoids as a result of enzymatic reduction of plastoquinone-9 by ferredoxin-plastoquinone reductase was even more active than the externally added plastoquinol-9 in the investigated reaction. Scavenging of superoxide by plastoquinol-9 and other prenyllipids could be important for protecting membrane components against the toxic action of superoxide. Moreover, our results indicate that vitamin K(1) is probably the most active redox component of photosystem I in the generation of superoxide within thylakoid membranes.

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.

Location of ubiquinone homologues in liposome membranes studied by fluorescence anisotropy of diphenyl-hexatriene and trimethylammonium-diphenyl-hexatriene

The measurements of diphenyl-hexatriene (DPH) and trimethylammonium-diphenyl-hexatriene (TMA-DPH) fluorescence anisotropy in dipalmitoylphosphatidylcholine (DPPC) and egg yolk lecithin (EYL) liposomes containing different concentrations of various ubiquinone (UQ) homologues have been performed. UQ-4 induced the highest DPH anisotropy increase in DPPC liposomes, whereas for higher UQ homologues the anisotropy was lowered with the increase of UQ side-chain length. These differences were less pronounced in EYL liposomes. It was concluded that at a higher content in the membranes (3-4 mol%), the short-chain ubiquinones are arranged parallel to lipid fatty acid chains, whereas long-chain homologues are progressively removed from the lipid acyl chains into the midplane region of the membrane. At the lower (1-2 mol%) concentrations, long-chain quinones seem to be evenly distributed within the membrane, especially in EYL membranes. UQ-10 in EYL liposomes perturbed TMA-DPH to a similar extend as the short-chain ubiquinones indicating that UQ-10 penetrates the interface regions of the membrane where its redox reactions occur. The localization and physical state of UQ-10 in native membranes is discussed.

Antioxidant properties of plastoquinol and other biological prenylquinols in liposomes and solution

Oxidation of biological prenylquinols, like plastoquinol-9 (PQH2-9), ubiquinol-10 (UQH2-10), reduced vitamins K1 (VK1H2) and K2 (VK2H2), alpha-tocopherol quinol (alpha-TQH2) and alpha-tocopherol (alpha-T) was followed by their fluorescence during sonication of egg yolk lecithin/prenylquinol liposomes. The order of magnitude of oxidation of the prenylquinols by free radicals generated during sonication was UQH2-10 > VK2H2 > VK1H2 > alpha-TQH2 > PQH2-9 > alpha-T. It was shown that egg yolk lecithin undergoes degradation even when sonicated briefly under atmosphere of nitrogen and at 0 degree C. A kinetic study of free radical scavenging action of the prenylquinols in solvents of different polarity was performed. The pseudo-first-order rate constants, k, for the reaction of the prenylquinols with 1,1-diphenyl-2-picrylhydrazyl (DPPH) in hexane showed that their scavenging activity changes in the order VK2H2 > VK1H2 > alpha-TQH2 > PQH2-9 > alpha-T > UQH2-10, being the highest in hexane and methanol, whereas in acetone and ethyl acetate the scavenging activity appeared much lower. The reaction rate constants, k, were apparently not dependent on the solvent polarity. The antioxidant activity of the prenylquinols in natural membranes is discussed.