Elsevier

Food Chemistry

Volume 135, Issue 3, 1 December 2012, Pages 1999-2004
Food Chemistry

o-Quinone involvement in the prooxidant tendency of a mixture of quercetin and caffeic acid

https://doi.org/10.1016/j.foodchem.2012.06.094Get rights and content

Abstract

The oxidation products of a previously tested prooxidant mixture of quercetin (Q) and caffeic acid (CA) at 1:2 ratio were analysed by LC–MS. The UV–Vis and MS spectra of three chromatographic peaks eluting at tR = 9.11 min, tR = 14.36 min and tR = 30.30 min were studied further. The structures of the tentatively identified compounds indicate polymeric molecules. A pentamer formed by 3 units of quercetin and 2 units of caffeic acid was attributed in the case of peaks tR = 9.11 min and tR = 14.36 min. The quercetin quinone (QQ) – a polymerization intermediate – is a fragment of the compound identified in the MS spectrum with the m/z = 323 coming from Q o-quinone m/z = 300 plus 23 from Na+. According to the UV–Vis spectrum, we suggest a different intermolecular arrangement which gives a more extended e-delocalisation. At tR = 30.30 min, the spectra helped us to tentatively identify this oxidation product as being a polymer of 4 CA units and 1 QQ.

Highlights

o-Quinones are involved in the prooxidant action of a Q and CA mixture. ► Polymeric structures of Q o-Quinone and CA were identified as oxidation products. ► The polymers are formed from 3 units of Q and 2 units of CA and of 4 CA and 1 QQ.

Introduction

Quercetin (Q) and caffeic acid (CA) take part in redox reactions in which they can act as either antioxidant (electron donor) or prooxidant (electron acceptors), depending on their environment. Quercetin (3,3′,4′,5,7-pentahydroxyflavone) is an important dietary flavonoid, present in different vegetables, fruits, seeds, nuts, tea and red wine (Beecher et al., 1999, Formica, 1995, Hollman and Katan, 1999). Quercetin has been considered for several decades as a multipotent bioflavonoid with great potential for the prevention and treatment of disease (Bischoff, 2008). Its documented impact on human health includes cardiovascular protection, anticancer, antiviral, anti-inflammatory activities, antiulcer effects and cataract prevention (Lugli et al., 2009).

Caffeic acid (3,4-dihydroxycinnamic acid), one of the most common phenolic acids, frequently occurs in fruits, grains and dietary supplements for human consumption as simple esters with quinic acid or saccharides (Jiang et al., 2005). The antioxidant effects of caffeic acid include three aspects: (i) anti-lipid-peroxidation; (ii) radical-scavenging; and (iii) antioxidation of low density lipoprotein. Caffeic acid protects against immunoregulation diseases, asthma and allergic reactions (Koshihara et al., 1984). Caffeic acid has been reported to decrease the risk of chronic diseases, such as inflammation, cardiovascular disease and cancer (Park, 2009).

Even though a balanced diet provides a sufficient amount of antioxidants, some people regularly take antioxidant supplements in the hope of slowing the biological oxidative processes that contribute to ageing and disease risk. Thus, the possibility exists that, in an environment resulting in prooxidant activity by dietary antioxidants, which can interfere with maintaining desired levels of reactive oxygen species (ROS), antioxidant supplementation may actually cause harm in terms of increased risk of new disease or interference. Toxicity can occur at very high intake levels of some commonly consumed antioxidants. Much remains to be learned about the differential effects of antioxidant dose levels, including levels that are “intermediate” between high and low doses (Seifried, McDonald, Anderson, Greenwald, & Milner, 2003).

It is still unknown exactly what amounts of antioxidants are needed to have a beneficial antioxidant effect and which dose reflects the safe and appropriate limit for use. Many of the antioxidant vitamins and other antioxidants can cause pathological changes to the exposed tissues and other organs by initiating different mechanisms. These harmful and undesired effects are caused by prooxidant, antioxidant or some other unknown pathways.

In vitro evidence suggests that phytochemical antioxidants may have unpredictable effects that may be cell type-specific and concentration-dependent. They can either scavenge the reactive oxidant species or generate more oxidative stress (Loo, 2003). In the case of flavonoids, pro-oxidant activity is thought to be directly proportional to the total number of hydroxyl groups. It has been demonstrated (Hanasaki, Ogawa, & Fukui, 1994) that a series of mono- and dihydroxyflavonoids showed no detectable pro-oxidant activity, while multiple hydroxyl groups, especially in the B-ring, significantly increased the production of hydroxyl radicals in a Fenton system. This prooxidant effect is responsible for the cytotoxic and proapoptotic effects of flavonoids isolated from various herbal medicines (Ueda et al., 2002).

Fukumoto and Mazza (2000) noted dual antioxidant and prooxidant activities for a variety of plant-derived polyphenols, including gallic acid, protocatechuic acid, syringic acid, vanillic acid, ellagic acid, caffeic acid, coumaric acid, chlorogenic acid, ferulic acid, myricetin, quercetin, rutin, kaempferol, (+)-catechin, (−)-epicatechin, delphinidin, and malvidin. The volume of research on the antioxidant properties of polyphenols, as related to their biological effects, greatly overshadows the fewer studies on the biological consequences of the prooxidant nature of polyphenols (Babich, Schuck, Weisburg, & Zuckerbraun, 2011). When interpreting cellular responses to a polyphenol, attention must be focused on the effect evoked by the polyphenol per se, as distinct from the effect evoked through its generation of significant levels of ROS (Babich et al., 2011).

Because flavonoids, such as quercetin, and oxidases (i.e. lipoxygenases) are present simultaneously in fruits and vegetables, the generation of quinoid derivatives in biological systems is plausible (Pinto & Macias, 2005). This process is of great relevance from a biological point of view, because the conversion of supposed beneficial antioxidants, such as flavonoids, to electrophilic prooxidants may constitute a possible toxicological risk (Boersma et al., 2000).

Quercetin is considered an excellent free radical-scavenging antioxidant, owing to the high number of hydroxyl groups and conjugated π orbitals by which quercetin can donate electrons or hydrogen, and scavenge H2O2 and superoxide anion (O2radical dot) (Heijnen, Haenen, Van Acker, Van Der, & Bast, 2001). The reaction of quercetin with O2radical dot leads to the generation of the semiquinone radical and H2O2 (Metodiewa, Jaiswal, Cenas, Dickancaite, & Segura-Aguilar, 1999). The first oxidation product of quercetin is a semiquinone radical (Metodiewa et al., 1999). This radical is unstable and rapidly undergoes a second oxidation reaction that produces another quinone (quercetin-quinone, QQ) (Metodiewa et al., 1999). Since QQ can react with proteins, lipids and DNA, it is responsible for protein and DNA damage, as well as lipid peroxidation. In a previous study (Chedea, Braicu, & Socaciu, 2010) we have shown that the antioxidant/prooxidant effect of a grape seed extract is mediated by o-quinones and oxidised polyphenol polymers. This work presents LC–MS evidence for the involvement and fate of QQ in the prooxidant behaviour of a mixture Q and CA.

Section snippets

Chemicals

Boric acid was purchased from Applichem, Ethylene diaminetetraacetic acid (EDTA), caffeic acid (CA) and quercetin (Q) were purchased from Sigma (Germany). Cobalt(II) chloride hexahydrate, hydrogen peroxide, H2O2 (30%) and methanol were purchased from Merck (Germany).

LC–MS analysis

The mixture Q + CA, in the ratio 1:2, was screened by LC–MS (ESI+). The solutions of Q and CA and their mixture were in methanol at a total concentration of 200 μM. The compounds were mixed at molar ratios of (v/v) 1:2 (Q:CA) in a

Results and discussion

In one of our recent studies, a screening by chemiluminescence of different mixtures of polyphenols at different molar ratios indicated that Q and CA, combined at a ratio of 1:2 (Q:CA), have the highest prooxidant tendency (Choueiri, Chedea, Calokerinos, & Kefalas, 2012). For this mixture, LC–MS analyses were done in order to determine the oxidation products involved in the observed prooxidant action.

The chromatogram of Q + CA at the ratio 1:2, at 278 nm is presented in Fig. 1.Three peaks were

Conclusions

An attempt was made to bring evidence of o-quinone involvement in the prooxidant tendency of a mixture of two model antioxidants, Q and CA in a ratio of 1:2. LC–MS analysis revealed that polymeric molecules of the QQ and CA units were formed during the oxidation.

Acknowledgement

V.S. Chedea is a Japan Society for the Promotion of Science (JSPS) postdoctoral fellow.

References (24)

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These authors contributed equally to this work.

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