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

Co-supplementation of Vitamin K2 and Selenium Synergistically Improves Metabolic Status and Reduces Cardiovascular Risk Markers in Dyslipidemic Rabbits

This work investigated the impact of vitamin K2 and selenium co-supplementation on metabolic profile and indicators of cardiovascular health in dyslipidemic rabbits. Fifty adult male rabbits were equally allocated into 5 groups: Control group, Dyslipidemic group: received 0.5% cholesterol in diet for 12 weeks, groups 3, 4 and 5 dyslipidemic rabbits daily treated with vitamin K2 (10 mg/kg bw) or/and selenium (1 mg/kg bw) for 8 weeks. Co-supplementation of vitamin K2 and selenium significantly decreased body weight gain and blood pressure elevation in dyslipidemic rabbits compared to un-treated ones. Consuming vitamin K2 plus selenium also markedly lowered serum lipids encompassing cholesterol, triglycerides and LDL and elevated HDL relative to placebo. Additionally, such co-supplementation reduced fasting glucose and insulin, enhancing insulin sensitivity with respect to placebo. Regarding cardiovascular risk markers, dyslipidemic rabbits received vitamin K2 concurrently with selenium displayed lower levels of atherogenic index (LDL/HDL), serum C-reactive protein, heart fatty acid-binding protein and asymmetric dimethylarginine as well as aortic ox-LDL, lipid peroxidation and calcium but higher levels of serum nitric oxide and aortic total antioxidants than un-treated ones. Concomitant administration of vitamin K2 and selenium improved metabolic profile, markers of cardiovascular health and atherosclerosis in dyslipidemic rabbits.

Synthesis of natural polyprenols for the production of biological prenylquinones and tocochromanols

We elaborate the chemical synthesis of polyprenols by chain lengthening, which is considerably less time-consuming than the other previously described methods. Our method eliminates critical steps requiring low temperature and toxic chemicals, which are difficult to perform in ordinary laboratories. The critical step of acetylene addition in liquid ammonia was replaced by a new approach, namely, the use of sodium acetylide in dimethoxyethane at room temperature, where the reaction is completed within one hour. This method is of general significance as it can also be applied to the synthesis of any other acetylides. Our method provides reasonable yields and can be scaled depending on the requirements. All the reactions were followed by high-performance liquid chromatography, allowing the formation of undesired isomers and other side-products to be controlled. The resulting polyprenols were further used in the synthesis of plastoquinones, although a variety of biological prenylquinones can be synthesized this way. Moreover, we found a new method for the direct formation of tocochromanols (plastochromanols, tocochromanols) from polyprenols and aromatic head groups.

Acyl-Nω-methylserotonins and Branched-chain Acylserotonins in Lemon and Other Citrus Seeds—New Lipids with Antioxidant Properties and Potential Pharmacological Applications

We have found 15 previously unknown compounds in seeds of lemon and other citrus species, such as tangerine, grapefruit and pomelo. The structure of these compounds was characterized by HR–MS spectrometry, fluorescence spectroscopy and chemical synthesis. These compounds were predominantly long-chain (C20–C25), saturated acyl-Nω-methylserotonins with the main contribution of C22 and C24 homologues, usually accounting for about 40% and 30% of all acylserotonins, respectively. The other, previously undescribed, minor compounds were branched-chain acylserotonins, as well as normal-chain acylserotonins, recently found in baobab seed oil. Within the seed, acylserotonins were found nearly exclusively in the inner seed coat, where probably their biosynthesis proceeds. On the other hand, lemon seedlings contained only trace amounts of these compounds that were not found in adult leaves. The compounds identified in the present studies were shown to have antioxidant properties in vitro, using 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. In the investigated reaction in hexane, Me-C22 and Me-C24-serotonins were less active than n-C22 and n-C24-serotonins and δ-tocopherol, while branched-chain acylserotonins (iso-C21 and -C25) showed higher antioxidant activity than all the normal-chain compounds. On the other hand, all these compounds showed a similar but considerably lower antioxidant activity in acetonitrile than in hexane.

Acylserotonins - a new class of plant lipids with antioxidant activity and potential pharmacological applications

During analysis of components of baobab (Adansonia digitata) seed oil, several new fluorescent compounds were detected in HPLC chromatograms that were not found previously in any seed oils investigated so far. After preparative isolation of these compounds, structural analysis by NMR spectroscopy, UHPLC-HR-MS, GC-FID and spectroscopic methods were applied and allowed identification of these substances as series of N-acylserotonins containing saturated C22 to C26 fatty acids with minor contribution of C27 to C30 homologues. The main component was N-lignocerylserotonin and the content of odd carbon-atom-number fatty acids was unusually high among the homologues. The suggested structure of the investigated compounds was additionally confirmed by their chemical synthesis. Synthetic N-acylserotonins showed pronounced inhibition of membrane lipid peroxidation of liposomes prepared from chloroplast lipids, especially when the peroxidation was initiated by a water-soluble azo-initiator, AIPH. Comparative studies of the reaction rate constants of the N-acylserotonins and tocopherols with a stable radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) in solvents of different polarity revealed that N-acylserotonins showed similar activity to δ-tocopherol in this respect. The described compounds have been not reported before either in plants or in animals. This indicates that we have identified a new class of plant lipids with antioxidant properties that could have promising pharmacological activities.

Tuning the reduction potentials of benzoquinone through the coordination to Lewis acids

Electron transfer promoted by the coordination of a substrate molecule to a Lewis acid or hydrogen bonding group is a critical step in many biological and catalytic transformations. This computational study investigates the nature of the interaction between benzoquinone and one and two Lewis acids by examining the influence of Lewis acid strength on the ability to alter the two reduction potentials of the coordinated benzoquinone molecule. To investigate this interaction, the coordination of the neutral (Q), singly reduced ([Q]˙-), and doubly reduced benzoquinone ([Q]2-) molecule to eight Lewis acids was analyzed. Coordination of benzoquinone to a Lewis acid became more favorable by 25 kcal mol-1 with each reduction of the benzoquinone fragment. Coordination of benzoquinone to a Lewis acid also shifted each of the reduction potentials of the coordinated benzoquinone anodically by 0.50 to 1.5 V, depending on the strength of the Lewis acid, with stronger Lewis acids exhibiting a larger effect on the reduction potential. Coordination of a second Lewis acid further altered each of the reduction potentials by an additional 0.70 to 1.6 V. Replacing one of the Lewis acids with a proton resulted in the ability to modify the pKa of the protonated Lewis acid-Q/[Q]˙-/[Q]2- adducts by about 10 pKa units, in addition to being able to alter the ability to transfer a hydrogen atom by 10 kcal mol-1, and the capacity to transfer a hydride by about 30 kcal mol-1.

Singlet oxygen oxidation products of carotenoids, fatty acids and phenolic prenyllipids

Singlet oxygen (¹O₂) is the major reactive oxygen species ROS causing photooxidative stress in plants which is formed predominantly in the reaction center of photosystem II during photosynthesis. To avoid deleterious effects of ¹O₂ oxygen on photosynthetic membrane components, plant synthesize a variety of ¹O₂ quenchers of lipophilic character, such as carotenoids or phenolic prenyllipids (tocopherols, plastochromanol-8, plastoquinol). In the process of chemical quenching of ¹O₂ by the antioxidants, both short-lived products, such as oxidized carotenoids, or relative long-lived compounds, such as oxidized phenolic prenyllipids are formed. The other target of ¹O₂ are unsaturated fatty acids of membrane lipids that undergo peroxidation as a result of the reaction. Some of the ¹O₂ oxidation products, like β-cyclocitral can be components of ¹O₂-signallingsignaling pathway leading to acclimatory responses of plants, while some others further fulfill antioxidant functions, like hydroxy-plastochromanol or hydroxy-plastoquinol. As most of the ¹O₂ oxidation products are specific compounds formed only as a results of ¹O₂ action, they can be very useful, specific molecular markers of ¹O₂-dependent oxidative stress in vivo.

Antioxidation of 2-phenylbenzofuran derivatives: structural-electronic effects and mechanisms

Stilbenoid-type 2-phenylbenzofuran derivatives, which are widely distributed in nature, are now promising antioxidant agents. In the present study, a quantum computational approach principally based on the DFT/B3LYP method with the 6-311++G(d,p) basis set was used to shed light on free radical scavenging for the isolated compounds stemofurans A-K and S-W. On the basis of the findings and from a thermodynamic perspective, the antioxidant activity of all studied compounds in the gaseous phase was mostly controlled by the O-H bond dissociation enthalpy (BDE), consistent with the hydrogen atom transfer (HAT) mechanism. The solvent effect was investigated, and the hydroxyl radicals of these studied compounds possessed the lowest proton affinity (PA) enthalpy and the sequential proton loss electron transfer (SPLET) pathway occurred in water, methanol and acetone. The studied compounds interacted with DPPH radicals, which is kinetic evidence of the involvement of two intermediates and one transition state. From both thermodynamics and kinetics perspectives, it can be proposed that stemofuran U is likely to be a leader compound in antioxidant drug development due to the presence of a 4'-OH moiety. Regarding the structure-bioactivity relationship, methylation can lead to a decrease in BDE.

Role of the NAD(P)H quinone oxidoreductase NQR and the cytochrome b AIR12 in controlling superoxide generation at the plasma membrane

MAIN CONCLUSION

The quinone reductase NQR and the b-type cytochrome AIR12 of the plasma membrane are important for the control of reactive oxygen species in the apoplast. AIR12 and NQR are two proteins attached to the plant plasma membrane which may be important for generating and controlling levels of reactive oxygen species in the apoplast. AIR12 (Auxin Induced in Root culture) is a single gene of Arabidopsis that codes for a mono-heme cytochrome b. The NADPH quinone oxidoreductase NQR is a two-electron-transferring flavoenzyme that contributes to the generation of O 2•- in isolated plasma membranes. A. thaliana double knockout plants of both NQR and AIR12 generated more O 2•- and germinated faster than the single mutant affected in AIR12. To test whether NQR and AIR12 are able to interact functionally, recombinant purified proteins were added to plasma membranes isolated from soybean hypocotyls. In vitro NADH-dependent O 2•- production at the plasma membrane in the presence of NQR was reduced upon addition of AIR12. Electron donation from semi-reduced menadione to AIR12 was shown to take place. Biochemical analysis showed that purified plasma membrane from soybean hypocotyls or roots contained phylloquinone and menaquinone-4 as redox carriers. This is the first report on the occurrence of menaquinone-4 in eukaryotic photosynthetic organisms. We propose that NQR and AIR12 interact via the quinone, allowing an electron transfer from cytosolic NAD(P)H to apoplastic monodehydroascorbate and control thereby the level of reactive oxygen production and the redox state of the apoplast.

Natural variation in tocochromanols content in Arabidopsis thaliana accessions - the effect of temperature and light intensity

In this study, 25 accessions of Arabidopsis thaliana originating from a variety of climate conditions were grown under controlled circumstances of different light intensity and temperature. The accessions were analyzed for prenyllipids content and composition, as well as expression of the genes involved in tocochromanol biosynthesis (vte1-5). It was found that the applied conditions did not strongly affect total tocochromanols content and there was no apparent correlation of the tocochromanol content with the origin of the accessions. However, the presented results indicate that the temperature, more than the light intensity, affects the expression of the vte1-5 genes and the content of some prenyllipids. An interesting observation was that under low growth temperature, the hydroxy-plastochromanol (PC-OH) to plastochromanol (PC) ratio was considerably increased regardless of the light intensity in most of the accessions. PC-OH is known to be formed as a result of singlet oxygen stress, therefore this observation indicates that the singlet oxygen production is enhanced under low temperature. Unexpectedly, the highest increase in the PC-OH/PC ratio was found for accessions originating from cold climate (Shigu, Krazo-1 and Lov-5), even though such plants could be expected to be more resistant to low temperature stress.

Function of isoprenoid quinones and chromanols during oxidative stress in plants

Isoprenoid quinones and chromanols in plants fulfill both signaling and antioxidant functions under oxidative stress. The redox state of the plastoquinol pool (PQ-pool), which is modulated by interaction with reactive oxygen species (ROS) during oxidative stress, has a major regulatory function in both short- and long-term acclimatory responses. By contrast, the scavenging of ROS by prenyllipids affects signaling pathways where ROS play a role as signaling molecules. As the primary antioxidants, isoprenoid quinones and chromanols are synthesized under high-light stress in response to any increased production of ROS. During photo-oxidative stress, these prenyllipids are continuously synthesized and oxidized to other compounds. In turn, their oxidation products (hydroxy-plastochromanol, plastoquinol-C, plastoquinone-B) can still have an antioxidant function. The oxidation products of isoprenoid quinones and chromanols formed specifically in the face of singlet oxygen, can be indicators of singlet oxygen stress.

Roles of MPBQ-MT in Promoting α/γ-Tocopherol Production and Photosynthesis under High Light in Lettuce

2-methyl-6-phytyl-1, 4-benzoquinol methyltransferase (MPBQ-MT) is a vital enzyme catalyzing a key methylation step in both α/γ-tocopherol and plastoquinone biosynthetic pathway. In this study, the gene encoding MPBQ-MT was isolated from lettuce (Lactuca sativa) by rapid amplification of cDNA ends (RACE), named LsMT. Overexpression of LsMT in lettuce brought about a significant increase of α- and γ-tocopherol contents with a reduction of phylloquinone (vitamin K1) content, suggesting a competition for a common substrate phytyl diphosphate (PDP) between the two biosynthetic pathways. Besides, overexpression of LsMT significantly increased plastoquinone (PQ) level. The increase of tocopherol and plastoquinone levels by LsMT overexpression conduced to the improvement of plants' tolerance and photosynthesis under high light stress, by directing excessive light energy toward photosynthetic production rather than toward generation of more photooxidative damage. These findings suggest that the role and function of MPBQ-MT can be further explored for enhancing vitamin E value, strengthening photosynthesis and phototolerance under high light in plants.

Physiological and antioxidant responses of two accessions of Arabidopsis thaliana in different light and temperature conditions

During their lifetime, plants need to adapt to a changing environment, including light and temperature. To understand how these factors influence plant growth, we investigated the physiological and antioxidant responses of two Arabidopsis accessions, Shahdara (Sha) from the Shahdara valley (Tajikistan, Central Asia) in a mountainous area and Lovvik-5 (Lov-5) from northern Sweden to different light and temperature conditions. These accessions originate from different latitudes and have different life strategies, both of which are known to be influenced by light and temperature. We showed that both accessions grew better in high-light and at a lower temperature (16°C) than in low light and at 23°C. Interestingly, Sha had a lower chlorophyll content but more efficient non-photochemical quenching than Lov-5. Sha, also showed a higher expression of vitamin E biosynthetic genes. We did not observe any difference in the antioxidant prenyllipid level under these conditions. Our results suggest that the mechanisms that keep the plastoquinone (PQ)-pool in more oxidized state could play a role in the adaptation of these accessions to their local climatic conditions.

Isochorismate synthase 1 is required for thylakoid organization, optimal plastoquinone redox status, and state transitions in Arabidopsis thaliana

Isochorismate synthase 1 (ICS1) is a crucial enzyme in the salicylic acid (SA) synthesis pathway, and thus it is important for immune defences. The ics1 mutant is used in experiments on plant-pathogen interactions, and ICS1 is required for the appropriate hypersensitive disease defence response. However, ICS1 also takes part in the synthesis of phylloquinone, which is incorporated into photosystem I and is an important component of photosynthetic electron transport in plants. Therefore, photosynthetic and molecular analysis of the ics1 mutant in comparison with wild-type and SA-degrading transgenic NahG Arabidopsis thaliana plants was performed. Photosynthetic parameters in the ics1 mutant, when compared with the wild type, were changed in a manner observed previously for state transition-impaired plants (STN7 kinase recessive mutant, stn7). In contrast to stn7, deregulation of the redox status of the plastoquinone pool (measured as 1-q p) in ics1 showed significant variation depending on the leaf age. SA-degrading transgenic NahG plants targeted to the cytoplasm or chloroplasts displayed normal (wild-type-like) state transition. However, ics1 plants treated with a phylloquinone precursor displayed symptoms of phenotypic reversion towards the wild type. ics1 also showed altered thylakoid structure with an increased number of stacked thylakoids per granum which indicates the role of ICS1 in regulation of state transition. The results presented here suggest the role of ICS1 in integration of the chloroplast ultrastructure, the redox status of the plastoquinone pool, and organization of the photosystems, which all are important for optimal immune defence and light acclimatory responses.

Functional characterization of long-chain prenyl diphosphate synthases from tomato

The electron transfer molecules plastoquinone and ubiquinone are formed by the condensation of aromatic head groups with long-chain prenyl diphosphates. In the present paper we report the cloning and characterization of two genes from tomato (Solanum lycopersicum) responsible for the production of solanesyl and decaprenyl diphosphates. SlSPS (S. lycopersicum solanesyl diphosphate synthase) is targeted to the plastid and both solanesol and plastoquinone are associated with thylakoid membranes. A second gene [SlDPS (S. lycopersicum solanesyl decaprenyl diphosphate synthase)], encodes a long-chain prenyl diphosphate synthase with a different subcellular localization from SlSPS and can utilize geranyl, farnesyl or geranylgeranyl diphosphates in the synthesis of C45 and C50 prenyl diphosphates. When expressed in Escherichia coli, SlSPS and SlDPS extend the prenyl chain length of the endogenous ubiquinone to nine and ten isoprene units respectively. In planta, constitutive overexpression of SlSPS elevated the plastoquinone content of immature tobacco leaves. Virus-induced gene silencing showed that SlSPS is necessary for normal chloroplast structure and function. Plants silenced for SlSPS were photobleached and accumulated phytoene, whereas silencing SlDPS did not affect leaf appearance, but impacted on primary metabolism. The two genes were not able to complement silencing of each other. These findings indicate a requirement for two long-chain prenyl diphosphate synthases in the tomato.

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.

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.

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.

Plastoquinol is the main prenyllipid synthesized during acclimation to high light conditions in Arabidopsis and is converted to plastochromanol by tocopherol cyclase

Plants have evolved various strategies to acclimate to high light conditions at different levels of organization. High light stress stimulates synthesis of different antioxidant enzymes and low molecular weight antioxidants, mainly in chloroplasts. In the present studies we showed that plastoquinol, in addition to alpha-tocopherol, is the main lipid-soluble antioxidant synthesized during acclimation of Arabidopsis plants to high light conditions. The level of plastoquinol increased >10-fold and independently of tocopherols, as revealed using tocopherol biosynthetic mutants. The high light-induced increase in plastoquinol level was mainly attributable to the photochemically non-active fraction of this compound localized in plastoglobuli, which are the storage site of prenyllipids for their antioxidant action. Our data also revealed that tocopherol cyclase is required for plastochromanol biosynthesis from plastoquinol in vivo. Plastochromanol accumulated in increasing amounts in leaves during growth and it was also identified in seeds. The obtained data suggest that plastochromanol may, similarly to other prenyllipids, fulfill antioxidant function in leaves and seeds, especially during aging.

Evaluation of the Antioxidant Properties of Propofol and its Nitrosoderivative. Comparison with Homologue Substituted Phenols

Propofol (2,6-diisopropylphenol), some substituted phenols (2,6-dimethylphenol and 2,6-ditertbutylphenol) and their 4-nitrosoderivatives have been compared for their scavenging ability towards 1,1-diphenyl-2-picrylhydrazyl and for their inhibitory action on lipid peroxidation. These products were also compared to the classical antioxidants butylated hydroxytoluene and butylated hydroxyanisole. When measuring the reactivity of the various phenolic derivatives with 1,1-diphenyl-2-picrylhydrazyl the following order of effectiveness was observed: butylated hydroxyanisole > propofol > 2,6-dimethylphenol > 2,6-di-tertbutylphenol > butylated hydroxytoluene. In cumene hydroperoxide-dependent microsomal lipid peroxidation, propofol acts as the most effective antioxidant, while butylated hydroxyanisole, 2,6-di-tertbutylphenol and butylated hydroxytoluene exhibit a rather similar effect, although lower than propofol. In the iron/ascorbate-dependent lipid peroxidation propofol, at concentrations higher than 10 microM, exhibits antioxidant properties comparable to those of butylated hydroxytoluene and butylated hydroxyanisole, 2,6-Dimethylphenol is scarcely effective in both lipoperoxidative systems. The antioxidant properties of the various molecules depend on their hydrophobic characteristics and on the steric and electronic effects of their substituents. However, the introduction of the nitroso group in the 4-position almost completely removes the antioxidant properties of the examined compounds. The nitrosation of the aromatic ring of antioxidant molecules and the consequent loss of antioxidant capacity can be considered a condition potentially occurring in vivo since nitric oxide and its derivatives are continuously formed in biological systems.

Singlet oxygen production in photosystem II and related protection mechanism

High-light illumination of photosynthetic organisms stimulates the production of singlet oxygen by photosystem II (PSII) and causes photo-oxidative stress. In the PSII reaction centre, singlet oxygen is generated by the interaction of molecular oxygen with the excited triplet state of chlorophyll (Chl). The triplet Chl is formed via charge recombination of the light-induced charge pair. Changes in the midpoint potential of the primary electron donor P(680) of the primary acceptor pheophytin or of the quinone acceptor Q(A), modulate the pathway of charge recombination in PSII and influence the yield of singlet oxygen formation. The involvement of singlet oxygen in the process of photoinhibition is discussed. Singlet oxygen is efficiently quenched by beta-carotene, tocopherol or plastoquinone. If not quenched, it can trigger the up-regulation of genes, which are involved in the molecular defence response of photosynthetic organisms against photo-oxidative stress.

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.

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.

The effect of vitamin K2 on bone metabolism in aged female rats

Reactive oxygen species (ROS) may contribute to aging and osteoporosis resulting from marked decreases in plasma antioxidants in aged osteoporotic women. On the other hand, high-dose vitamin K2 (menaquinone-4: menatrenone, MK-4) supplementation has been reported to reduce ovariectomy-induced bone loss in rats and to decrease osteoporotic fracture in postmenopausal women. However, the mechanism by which vitamin K2 prevents osteoporosis is unclear. Recently, vitamin K2 has been suggested to preserve antioxidant activity as a novel function. Therefore, we investigated the effect of vitamin K2 on the osteoporosis of aged rats by evaluating the relationships between serum antioxidant levels and bone metabolism. Aged female rats exhibited significantly lower serum alkaline phosphatase activity and osteocalcin level, together with lower serum levels of antioxidants such as 17beta-estradiol, macrophage migration inhibitory factor (MIF) and glutathione peroxidase (GPx) activity, as compared with young female rats. On the other hand, vitamin K2 supplementation (500 mg/kg, food intake) for 98 days led to a significantly increased serum vitamin K2 level (3,045+/-915 ng/ml in the vitamin K2 supplemented group vs. 4.6+/-3.4 ng/ml in the control diet group; P<0.0001) with increased serum alkaline phosphatase activity and MIF level (P<0.05). Unexpectedly, however, it failed to increase the serum level of antioxidants such as GPx. Nor did it affect bone metabolism markers such as osteocalcin and osteopontin, which were significantly lower than in the young female rats (P<0.05). Finally, the histomorphometric properties of the proximal tibia in the femur were not altered by vitamin K2. These results suggest that high-dose vitamin K2 supplementation neither improves lowered antioxidant levels nor stimulates bone formation in aged rats.

Tocopherol as singlet oxygen scavenger in photosystem II

Singlet oxygen is formed in the photosystem II reaction center in the quench of P680 triplets, and the yield is dependent on light intensity and the reduction level of plastoquinone. Singlet oxygen in PS II triggers the degradation of the D1 protein. We investigated the participation of tocopherol as a singlet oxygen scavenger in this system. For this purpose, we inhibited tocopherol biosynthesis at the level of the HPP-dioxygenase in the alga Chlamydomonas reinhardtii under conditions in which plastoquinone did not limit the photosynthesis rate. In the presence of the inhibitor and in high light for 2 h, photosynthesis in vivo and photosystem II was inactivated, the D1 protein was degraded, and the tocopherol pool was depleted and fell below its turnover rate/h. The inhibited system could be fully resuscitated upon the addition of a chemical singlet oxygen quencher (diphenylamine), and partly by synthetic cell wall permeable short chain alpha- and gamma-tocopherol derivatives. We conclude that under conditions of photoinhibition and extensive D1 protein turnover tocopherol has a protective function as a singlet oxygen scavenger.

Characterization of tocopherol cyclases from higher plants and cyanobacteria. Evolutionary implications for tocopherol synthesis and function

Tocopherols are lipophilic antioxidants synthesized exclusively by photosynthetic organisms and collectively constitute vitamin E, an essential nutrient for both humans and animals. Tocopherol cyclase (TC) catalyzes the conversion of various phytyl quinol pathway intermediates to their corresponding tocopherols through the formation of the chromanol ring. Herein, the molecular and biochemical characterization of TCs from Arabidopsis (VTE1 [VITAMIN E 1]), Zea mays (SXD1 [Sucrose Export Deficient 1]) and Synechocystis sp. PCC6803 (slr1737) are described. Mutations in the VTE1, SXD1, or slr1737 genes resulted in both tocopherol deficiency and the accumulation of 2,3-dimethyl-6-phytyl-1,4-benzoquinone (DMPBQ), a TC substrate. Recombinant SXD1 and VTE1 proteins are able to convert DMPBQ to gamma-tocopherol in vitro. In addition, expression of maize SXD1 in a Synechocystis sp. PCC6803 slr1737 knockout mutant restored tocopherol synthesis, indicating that TC activity is evolutionarily conserved between plants and cyanobacteria. Sequence analysis identified a highly conserved 30-amino acid C-terminal domain in plant TCs that is absent from cyanobacterial orthologs. vte1-2 causes a truncation within this C-terminal domain, and the resulting mutant phenotype suggests that this domain is necessary for TC activity in plants. The defective export of Suc in sxd1 suggests that in addition to presumed antioxidant activities, tocopherols or tocopherol breakdown products also function as signal transduction molecules, or, alternatively, the DMPBQ that accumulates in sxd1 disrupts signaling required for efficient Suc export in maize.

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.

Synergistic effects of a photooxidized polycyclic aromatic hydrocarbon and copper on photosynthesis and plant growth: evidence that in vivo formation of reactive oxygen species is a mechanism of copper toxicity

Heavy metals and polycyclic aromatic hydrocarbons (PAHs) are often cocontaminants in industrialized environments, yet little is known about either the extent or mechanisms of their cotoxicity. To address this shortfall, the combined effects of an oxygenated PAH, 1,2-dihydroxyanthraquinone (1,2-dhATQ), and a heavy metal, Cu2+, on photosynthesis and growth of the duckweed (Lemna gibba) were evaluated. Using assays of chlorophyll a fluorescence and photosystem I activity, 1,2-dhATQ inhibited electron transport at the cytochrome b6/f complex. Conversely, Cu2+ alone (at low concentrations) had little effect on photosynthesis. When Cu2+ was combined with 1,2-dhATQ, an increase in transient and steady-state chlorophyll a fluorescence quenching occurred relative to 1,2-dhATQ alone. Treatment of isolated thylakoid membranes with 1,2-dhATQ inhibited whole-chain linear electron transport, measured as O2 consumption using methyl viologen as the electron acceptor. However, Cu2+ plus 1,2-dhATQ resulted in active O2 consumption with or without methyl viologen as an electron acceptor. From these data, we conclude that 1,2-dhATQ renders the plastoquinone pool to a highly reduced state by inhibiting at cytochrome b6/f. Then, Cu2+ is able to mediate the transfer of electrons from reduced plastoquinone to O2, forming reactive oxygen species. At the whole-organism level, when Cu2+ and 1,2-dhATQ were mixed at concentrations that resulted in the above-mentioned impacts on photosynthesis, synergistic inhibition of plant growth was observed. This suggests a catalytic mechanism of toxicity for redox active metals, a process that could be instrumental in explaining their impacts at low concentrations.

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.

Determination of hydroxyl free radical formation in human platelets using high-performance liquid chromatography with electrochemical detection

The formation of the hydroxyl free radical (HFR) can be quantified indirectly, by measuring two products of the hydroxylation of salicylic acid, 2,3-dihydroxybenzoate (2,3-DHB) and 2,5-dihydroxybenzoate (2,5-DHB). In this study, we used reversed-phase high-performance liquid chromatography with electrochemical (coulometric) detection to measure 2,3-and 2,5-DHB levels in human platelets. The limits of detection of the method were 10 and 5 fmol on column for 2,3-DHB and 2,5-DHB, respectively. We tested the technique by measuring increases in dihydroxybenzoate levels after exposure of platelets to experimentally induced oxidative stress. Then, we measured platelet levels of 2,3- and 2,5-DHB in patients with Parkinson's disease, under therapy with L-DOPA, and in normal subjects. We also measured platelet concentrations of L-DOPA and its major metabolite, 3-O-methyldopa (3-OMD). Parkinsonian patients showed increased levels of both 2,3- and 2,5-DHB. Platelet levels of 2,3-DHB were positively correlated with platelet levels of L-DOPA and 3-OMD. The technique we describe proved simple and extremely sensitive and may represent a useful tool for the study of oxidative stress in humans.

Total antioxidant potential of resinous exudates from Heliotropium species, and a comparison of the ABTS and DPPH methods

Total reactive antioxidant potential (TRAP) of resinous exudates from Heliotropium species was evaluated by measuring the bleaching of stable free radicals. The antioxidant capacity of the resinous exudates in Trolox equivalents, evaluated from the bleaching of ABTS derived radical cations, ranged from 2.0 M (H. huascoense) to 5.2 M (H. stenophyllum), indicating a very high concentration of phenolic compounds. Considerably smaller values were obtained by measuring the bleaching of DPPH radicals. The ratio between the values obtained employing ABTS derived radicals and DPPH, ranged from 37 (H. megalanthum) to 4.5 (H. chenopodiaceum variety typica). The magnitude of the difference can be considered as an indication of the relative reactivity of the antioxidants present in the exudates. Similar ratios were observed when stoichiometric coefficients were evaluated for representative purified flavonoids obtained from the resinous exudates.

Lipoxygenase inhibitors counteract protein kinase C mediated events in human T lymphocyte proliferation

Four structurally unrelated inhibitors of lipoxygenase (LO), i.e. nordihydroguaiaretic acid (NDGA), Esculetin, AA861 and 5,8,11,14-eicosatetraynoic acid (ETYA) suppressed mitogen induced proliferation of human peripheral blood lymphocytes in a dose-dependent manner. The degree of suppression was influenced by the type of the mitogenic stimulus. Receptor mediated stimulation, i.e. through phytohemagglutinin or the anti-CD3 antibody OKT3, was overall less susceptible, whereas proliferation initiated by direct activation of protein kinase C (PKC), i.e. through phorbol myristate acetate or indolactam V, was profoundly suppressed (up to 90%). The effect of the LO inhibitors was not due to non-specific interference with intracellular radical intermediates, because AA861 and ETYA showed no radical scavenging activity. Two PKC inhibitors, H-7 and H-8, similarly suppressed lymphocyte proliferation and showed essentially the same suppressive pattern as LO inhibitors. The results clearly indicate that LO product(s) participate in signal transduction mechanisms in T lymphocytes, possibly via stimulation of PKC activity and cell proliferation.