Electrochemical investigation of flavonolignans and study of their interactions with DNA in the presence of Cu(II)

Tapinarof and its structure-activity relationship for redox chemistry and phototoxicity on human skin keratinocytes

Tapinarof (3,5-dihydroxy-4-isopropylstilbene) is a therapeutic agent used in the treatment of psoriasis (VTAMA®). In this study, we examined the redox behaviour, (photo)stability, (photo)toxicity and (bio)transformation of tapinarof in the context of a structure-activity relationship study. Selected derivatives of the structurally related tapinarof were investigated, namely resveratrol, pterostilbene, pinosylvin and its methyl ether. Tapinarof undergoes electrochemical oxidation in a neutral aqueous medium at a potential of around +0.5 V (vs. Ag|AgCl|3M KCl). The anodic reaction of this substance is a proton-dependent irreversible and adsorption-driven process. The pKa value of tapinarof corresponds to 9.19 or 9.93, based on empirical and QM calculation approach, respectively. The oxidation potentials of tapinarof and its analogues correlate well with their HOMO (highest occupied molecular orbital) energy level. The ability to scavenge the DPPH radical decreased in the order trolox ≥ resveratrol > pterostilbene > tapinarof > pinosylvin ≫ pinosylvin methyl ether. It was also confirmed that tapinarof, being a moderate electron donor, is able to scavenge the ABTS radical and inhibit lipid peroxidation. The 4'-OH group plays a pivotal role in antioxidant action of stilbenols. During the stability studies, it was shown that tapinarof is subject to spontaneous degradation under aqueous conditions, and its degradation is accelerated at elevated temperatures and after exposure to UVA (315-399 nm) radiation. In aqueous media at pH 7.4, we observed an ∼50 % degradation of tapinarof after 48 h at laboratory temperature. The main UVA photodegradation processes include dihydroxylation and hydration. In conclusion, the phototoxic effect of tapinarof on a human keratinocytes cell line (HaCaT) was evaluated. Tapinarof exhibited a clear phototoxic effect, similar to phototoxic standard chlorpromazine. The IC50 values of the cytotoxicity and phototoxic effects of tapinarof correspond to 27.6 and 3.7 μM, respectively. The main HaCaT biotransformation products of tapinarof are sulfates and glucuronides.

Natural phenolic antioxidants electrochemistry: Towards a new food science methodology

Natural phenolic compounds are abundant in the vegetable kingdom, occurring mainly as secondary metabolites in a wide variety of chemical structures. Around 10,000 different plant phenolic derivatives have been isolated and identified. This review provides an exhaustive overview concerning the electron transfer reactions in natural polyphenols, from the point of view of their in vitro antioxidant and/or pro-oxidant mode of action, as well as their identification in highly complex matrixes, for example, fruits, vegetables, wine, food supplements, relevant for food quality control, nutrition, and health research. The accurate assessment of polyphenols' redox behavior is essential, and the application of the electrochemical methods in routine quality control of natural products and foods, where the polyphenols antioxidant activity needs to be quantified in vitro, is of the utmost importance. The phenol moiety oxidation pathways and the effect of substituents and experimental conditions on their electrochemical behavior will be reviewed. The fundamental principles concerning the redox behavior of natural polyphenols, specifically flavonoids and other benzopyran derivatives, phenolic acids and ester derivatives, quinones, lignins, tannins, lignans, essential oils, stilbenes, curcuminoids, and chalcones, will be described. The final sections will focus on the electroanalysis of phenolic antioxidants in natural products and the electroanalytical evaluation of in vitro total antioxidant capacity.

Redox properties of individual quercetin moieties

Quercetin is one of the most prominent and widely studied flavonoids. Its oxidation has been previously investigated only indirectly by comparative analyses of structurally analogous compounds, e.g. dihydroquercetin (taxifolin). To provide direct evidence about the mechanism of quercetin oxidation, we employed selective alkylation procedures for the step-by-step blocking of individual redox active sites, i.e. the catechol, resorcinol and enol C-3 hydroxyls, as represented by newly prepared quercetin derivatives 1-3. Based on the structure-activity relationship (SAR), electrochemical, and computational (density functional theory) studies, we can clearly confirm that quercetin is oxidized in the following steps: the catechol moiety is oxidized first, forming the benzofuranone derivative via intramolecular rearrangement mechanism; therefore the quercetin C-3 hydroxy group cannot be involved in further oxidation reactions or other biochemical processes. The benzofuranone is oxidized subsequently, followed by oxidation of the resorcinol motif to complete the electrochemical cascade of reactions. Derivatization of individual quercetin hydroxyls has a significant effect on its redox behavior, and, importantly, on its antiradical and stability properties, as shown in DPPH/ABTS radical scavenging assays and UV-Vis spectrophotometry, respectively. The SAR data reported here are instrumental for future studies on the oxidation of biologically or technologically important flavonoids and other polyphenols or polyhydroxy substituted aromatics. This is the first complete and direct study mapping redox properties of individual moieties in quercetin structure.

Interaction of isolated silymarin flavonolignans with iron and copper

Silymarin, the standardized extract from the milk thistle (Silybum marianum), is composed mostly of flavonolignans and is approved in the EU for the adjuvant therapy of alcoholic liver disease. It is also used for other purported effects in miscellaneous nutraceuticals. Due to polyhydroxylated structures and low systemic bioavailability, these flavonolignans are likely to interact with transition metals in the gastrointestinal tract. The aim of this study was to analyze the interactions of pure silymarin flavonolignans with copper and iron. Both competitive and non-competitive methods at various physiologically relevant pH levels ranging from 4.5 to 7.5 were tested. Only 2,3‑dehydrosilybin was found to be a potent or moderately active iron and copper chelator. Silybin A, silybin B and silychristin A were less potent or inactive chelators. Both 2,3‑dehydrosilybin enantiomers (A and B) were equally active iron and copper chelators, and the preferred stoichiometries were mainly 2:1 and 3:1 (2,3‑dehydrosilybin:metal). Additional experiments showed that silychristin was the most potent iron and copper reductant. Comparison with their structural precursors taxifolin and quercetin is included as well. Based on these results, silymarin administration most probably affects the kinetics of copper and iron in the gastrointestinal tract, however, due to the different interactions of individual components of silymarin with these transition metals, the biological effects need to be evaluated in the future in a much more complex study.

Effects of 2,3-Dehydrosilybin and Its Galloyl Ester and Methyl Ether Derivatives on Human Umbilical Vein Endothelial Cells

The effects in vitro of 2,3-dehydrosilybin and several galloyl esters and methyl ethers on the viability, proliferation, and migration of human umbilical vein endothelial cells (HUVECs) were evaluated. The monogalloyl esters were synthesized by a chemoselective esterification method or by Steglich esterification of suitably protected 2,3-dehydrosilybin (1) with protected gallic acid. 2,3-Dehydrosilybin (1) displayed more potent cytotoxic, antiproliferative, and antimigratory activities (IC50 12.0, 5.4, and 12.2 μM, respectively) than silybin. The methylated derivatives were less active, with the least potent being 3,7-di-O-methyl-2,3-dehydrosilybin (6). On the other hand, galloylation at C-7 OH and C-23 OH markedly increased the cytotoxicity and the effects on the proliferation and migration of HUVECs. The most active derivative was 7-O-galloyl-2,3-dehydrosilybin (13; IC50 value of 3.4, 1.6, and 4.7 μM in the cytotoxicity, inhibition of proliferation, and antimigratory assays, respectively). Overall, this preliminary structure-activity relationship study demonstrated the importance of a 2,3-double bond, a C-7 OH group, and a galloyl moiety in enhancing the activity of flavonolignans toward HUVECs.

Flavonolignan 2,3-dehydroderivatives: Preparation, antiradical and cytoprotective activity

The protective constituents of silymarin, an extract from Silybum marianum fruits, have been extensively studied in terms of their antioxidant and hepatoprotective activities. Here, we explore the electron-donor properties of the major silymarin flavonolignans. Silybin (SB), silychristin (SCH), silydianin (SD) and their respective 2,3-dehydroderivatives (DHSB, DHSCH and DHSD) were oxidized electrochemically and their antiradical/antioxidant properties were investigated. Namely, Folin-Ciocalteau reduction, DPPH and ABTS(+) radical scavenging, inhibition of microsomal lipid peroxidation and cytoprotective effects against tert-butyl hydroperoxide-induced damage to a human hepatocellular carcinoma HepG2 cell line were evaluated. Due to the presence of the highly reactive C3-OH group and the C-2,3 double bond (ring C) allowing electron delocalization across the whole structure in the 2,3-dehydroderivatives, these compounds are much more easily oxidized than the corresponding flavonolignans SB, SCH and SD. This finding was unequivocally confirmed not only by experimental approaches, but also by density functional theory (DFT) calculations. The hierarchy in terms of ability to undergo electrochemical oxidation (DHSCH~DHSD>DHSB>>SCH/SD>SB) was consistent with their antiradical activities, mainly DPPH scavenging, as well as in vitro cytoprotection of HepG2 cells. The results are discussed in the context of the antioxidant vs. prooxidant activities of flavonolignans and molecular interactions in complex biological systems.

Sulfation modulates the cell uptake, antiradical activity and biological effects of flavonoids in vitro: An examination of quercetin, isoquercitrin and taxifolin

Quercetin 3'-O-sulfate is one of the main metabolites of the natural flavonoid quercetin in humans. This study was designed to prepare quercetin 3'-O-sulfate (1), isoquercitrin 4'-O-sulfate (2) and taxifolin 4'-O-sulfate (3) by the sulfation of quercetin, isoquercitrin (quercetin 3-O-glucoside) and taxifolin (2,3-dihydroquercetin) using the arylsulfate sulfotransferase from Desulfitobacterium hafniense, and to examine the effect of sulfation on selected biological properties of the flavonoids tested. We found that flavonoid sulfates 1-3 were weaker DPPH radical scavengers than the corresponding nonsulfated flavonoids, and that 1-3, unlike quercetin, did not induce the expression of either heme oxygenase-1 in RAW264.7 cells or cytochrome P450 1A1 in HepG2 cells. In both cell types, the cell uptake of compounds 1-3 was much lower than that of quercetin, but comparable to that of the glycoside isoquercitrin. Moreover, HPLC/MS metabolic profiling in HepG2 cells showed that flavonoid sulfates 1-3 were metabolized to a limited extent compared to the nonsulfated compounds. We conclude that sulfation of the tested flavonoids reduces their antiradical activity, and affects their cell uptake and biological activity in vitro.

Silymarin as a Natural Antioxidant: An Overview of the Current Evidence and Perspectives

Silymarin (SM), an extract from the Silybum marianum (milk thistle) plant containing various flavonolignans (with silybin being the major one), has received a tremendous amount of attention over the last decade as a herbal remedy for liver treatment. In many cases, the antioxidant properties of SM are considered to be responsible for its protective actions. Possible antioxidant mechanisms of SM are evaluated in this review. (1) Direct scavenging free radicals and chelating free Fe and Cu are mainly effective in the gut. (2) Preventing free radical formation by inhibiting specific ROS-producing enzymes, or improving an integrity of mitochondria in stress conditions, are of great importance. (3) Maintaining an optimal redox balance in the cell by activating a range of antioxidant enzymes and non-enzymatic antioxidants, mainly via Nrf2 activation is probably the main driving force of antioxidant (AO)  action of SM. (4) Decreasing inflammatory responses by inhibiting NF-κB pathways is an emerging mechanism of SM protective effects in liver toxicity and various liver diseases. (5) Activating vitagenes, responsible for synthesis of protective molecules, including heat shock proteins (HSPs), thioredoxin and sirtuins and providing additional protection in stress conditions deserves more attention. (6) Affecting the microenvironment of the gut, including SM-bacteria interactions, awaits future investigations. (7) In animal nutrition and disease prevention strategy, SM alone, or in combination with other hepatho-active compounds (carnitine, betaine, vitamin B₁₂, etc.), might have similar hepatoprotective effects as described in human nutrition.

Chemo-enzymatic synthesis of silybin and 2,3-dehydrosilybin dimers

Divalent or multivalent molecules often show enhanced biological activity relative to the simple monomeric units. Here we present enzymatically and chemically prepared dimers of the flavonolignans silybin and 2,3-dehydrosilybin. Their electrochemical behavior was studied by in situ and ex situ square wave voltammetry. The oxidation of monomers and dimers was similar, but adsorption onto the electrode and cell surfaces was different. A 1,1-diphenyl-2-picrylhydrazyl (DPPH) and an inhibition of microsomal lipoperoxidation assay were performed with same trend of results for silybin and 2,3-dehydrosilybin dimers. Silybin dimer showed better activity than the monomer, while on the contrary 2,3-dehydrosilybin dimer presented weaker antioxidant/antilipoperoxidant activity than its monomer. Cytotoxicity was evaluated on human umbilical vein endothelial cells, normal human adult keratinocytes, mouse fibroblasts (BALB/c 3T3) and human liver hepatocellular carcinoma cell line (HepG2). Silybin dimer was more cytotoxic than the parent compound and in the case of 2,3-dehydrosilybin its dimer showed weaker cytotoxicity than the monomer.

Quantum-chemical study on relationship between structure and antioxidant properties of hepatoprotective compounds occurring in Cynara scolymus and Silybum marianum

Accurate quantum computations based on the density functional theory have been performed to study the relationship between the electronic geometry and antioxidant capacity of chlorogenic acid, silybin and all geometric stereoisomers of cynarine, isolated from plant extract of Cynara scolymus and Silybum marianum. To elucidate their antioxidant activity, the HOMO orbital distribution, adiabatic ionization potential (AIP), spin density in free radicals, homolytic dissociation enthalpies (BDE), and proton dissociation enthalpies (PDE) of the O – H bonds have been calculated. For minimum energy conformations, the antioxidative parameters were quantitatively analyzed at the B3LYP/6-311G(d,p) level of theory. The results have shown that the hydrogen transfer mechanism is more preferable in nonpolar medium than in water. From the results obtained we can conclude that SET-PT (single electron transfer followed by proton transfer) is the most preferred mechanism in water medium. The catechol moiety and planar geometry of trans-stilbene and cis-stilbene moieties in cynarine stereoisomers, chlorogenic acid and their phenoxy radicals strongly contribute to enhancement of the antioxidant activity of these compounds. Trans,trans-cynarine appears to be the best candidate for proton and hydrogen atom donor. It has been predicted that cis,cis, trans,cis and cis,trans stereoisomers of cynarine show antioxidant capacity. Our study shows that all of the investigated compounds reveal strong antioxidant activity.

Mitochondrial dysfunctions and altered metals homeostasis: new weapons to counteract HCV-related oxidative stress

The hepatitis C virus (HCV) infection produces several pathological effects in host organism through a wide number of molecular/metabolic pathways. Today it is worldwide accepted that oxidative stress actively participates in HCV pathology, even if the antioxidant therapies adopted until now were scarcely effective. HCV causes oxidative stress by a variety of processes, such as activation of prooxidant enzymes, weakening of antioxidant defenses, organelle damage, and metals unbalance. A focal point, in HCV-related oxidative stress onset, is the mitochondrial failure. These organelles, known to be the "power plants" of cells, have a central role in energy production, metabolism, and metals homeostasis, mainly copper and iron. Furthermore, mitochondria are direct viral targets, because many HCV proteins associate with them. They are the main intracellular free radicals producers and targets. Mitochondrial dysfunctions play a key role in the metal imbalance. This event, today overlooked, is involved in oxidative stress exacerbation and may play a role in HCV life cycle. In this review, we summarize the role of mitochondria and metals in HCV-related oxidative stress, highlighting the need to consider their deregulation in the HCV-related liver damage and in the antiviral management of patients.

Antioxidant, metal-binding and DNA-damaging properties of flavonolignans: a joint experimental and computational highlight based on 7-O-galloylsilybin

Besides the well-known chemoprotective effects of polyphenols, their prooxidant activities via interactions with biomacromolecules as DNA and proteins are of the utmost importance. Current research focuses not only on natural polyphenols but also on synthetically prepared analogs with promising biological activities. In the present study, the antioxidant and prooxidant properties of a semi-synthetic flavonolignan 7-O-galloylsilybin (7-GSB) are described. The presence of the galloyl moiety significantly enhances the antioxidant capacity of 7-GSB compared to that of silybin (SB). These findings were supported by electrochemistry, DPPH (2,2-diphenyl-1-picrylhydrazyl) scavenging activity, total antioxidant capacity (CL-TAC) and DFT (density functional theory) calculations. A three-step oxidation mechanism of 7-GSB is proposed at pH 7.4, in which the galloyl moiety is first oxidized at Ep,1=+0.20V (vs. Ag/AgCl3M KCl) followed by oxidation of the 20-OH (Ep,2=+0.55V) and most probably 5-OH (Ep,3=+0.95V) group of SB moiety. The molecular orbital analysis and the calculation of O-H bond dissociation enthalpies (BDE) fully rationalize the electrooxidation processes. The metal (Cu(2+)) complexation of 7-GSB was studied, which appeared to involve both the galloyl moiety and the 5-OH group. The prooxidant effects of the metal-complexes were then studied according to their capacity to oxidatively induce DNA modification and cleavage. These results paved the way towards the conclusion that 7-O-galloyl substitution to SB concomitantly (i) enhances antioxidant (ROS scavenging) capacity and (ii) decreases prooxidant effect/DNA damage after Cu complexation. This multidisciplinary approach provides a comprehensive mechanistic picture of the antioxidant vs. metal-induced prooxidant effects of flavonolignans at the molecular level, under ex vivo conditions.

Metabolic profiling of phenolic acids and oxidative stress markers after consumption of Lonicera caerulea L. fruit

This study investigated the effect of one-week consumption of 165 g/day fresh blue honeysuckle berries (208 mg/day anthocyanins) in 10 healthy volunteers. At the end of intervention, levels of benzoic (median 1782 vs 4156), protocatechuic (709 vs 2417), vanillic (2779 vs 4753), 3-hydroxycinnamic (143 vs 351), p-coumaric (182 vs 271), isoferulic (805 vs 1570), ferulic (1086 vs 2395), and hippuric (194833 vs 398711 μg/mg creatinine) acids by LC/MS were significantly increased in the urine. Clinical chemistry safety markers were not altered. Oxidative stress markers, erythrocyte glutathione peroxidase (0.73 vs 0.88 U/g Hb) and catalase (2.5 vs 2.8 μkat/g Hb) activities, and erythrocyte/plasma thiobarbituric acid reactive substance (522 vs 612/33 vs 38 μmol/g Hb/protein) levels were significantly increased, without change in plasma antioxidant status. Nonsignificant changes of advanced oxidation protein products and oxidized LDL were observed. The results provide a solid base for further study of metabolite excretion and antioxidant parameters after ingestion of anthocyanins.

Electrochemical sensing of total antioxidant capacity and polyphenol content in wine samples using amperometry online-coupled with microdialysis

This work describes the method for total antioxidant capacity (TAC) and/or total content of phenolics (TCP) analysis in wines using microdialysis online-coupled with amperometric detection using a carbon microfiber working electrode. The system was tested on 10 selected wine samples, and the results were compared with total reactive antioxidant potential (TRAP), oxygen radical absorbance capacity (ORAC), and chemiluminescent determination of total antioxidant capacity (CL-TAC) methods using Trolox and catechin as standards. Microdialysis online-coupled with amperometric detection gives similar results to the widely used cyclic voltammetry methodology and closely correlates with ORAC and TRAP. The problem of electrode fouling is overcome by the introduction of an electrochemical cleaning step (1-2 min at the potential of 0 V vs Ag/AgCl). Such a procedure is sufficient to fully regenerate the electrode response for both red and white wine samples as well as catechin/Trolox standards. The appropriate size of microdialysis probes enables easy automation of the electrochemical TAC/TCP measurement using 96-well microtitration plates.