Dietary antioxidants as a source of hydrogen peroxide

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M. Grzesik et al. Food Chemistry 278 (2019) 692–699 Glycitein, CID: 5317750 Hesperetin, CID: 72281 Hesperidin, CID: 10621 Hydrocinnamic acid (3-Phenylpropionic acid), CID: 107 D-Isoascorbic acid, CID: 54675810 Mangiferin, CID: 5281647 Melatonin, CID: 896 Metformin hydrochloride, CID: 14219 L-methionine, CID: 6137 Morin, CID: 5281670 Naringenin, CID: 932 Naringin, CID: 442428 Oxaloacetic acid, CID: 970 D-pantothenic acid hemicalcium, CID: 11306073 Propyl gallate, CID: 4947 Pyrogallol, CID: 1057 Pyruvic acid, CID: 1060 Quercetin, CID: 5280343 trans-Resveratrol, CID: 445154 Rutin, CID: 5280805 Sinapic acid, CID: 637775 Sodium ascorbate, CID: 23667548 Sodium succinate, CID: 9020 Trolox, CID: 40634 Vanillic acid, CID: 8468 1. Introduction From a chemical point of view, an antioxidant can be defined as a substance which, when present at a low concentration with respect to that of an oxidizable substrate, prevents or significantly delays oxida- tion of this substrate (Halliwell, 1990). Although in biological systems several other mechanisms of action of antioxidants can be dis- tinguished, including inhibition of oxidant-producing enzymes and chelation of metal ions catalyzing oxidation reactions, the main way of an antioxidant action is its sacrificial oxidation instead of the substrate. Such a reaction generates an oxidized form of the antioxidant com- pound and other reaction product(s). Under aerobic conditions, anti- oxidants are subject to oxidation by oxygen or reactive oxygen species (ROS), and this reaction may protect other substrates from oxidation. Other products of the reaction of antioxidant oxidation are reduced forms of oxygen: superoxide radical anion (O2%− ) in the case of one- electron oxidation or hydrogen peroxide (H2O2) in the case of two- electron oxidation. Eventually, dismutation of superoxide produces H2O2 so this product of oxygen reduction can be expected to accumu- late as a result of oxidation of antioxidant compounds. Generation of H2O2 due to oxidation of phenolic compounds, in- cluding gallic acid (Wee, Long, Whiteman, & Halliwell, 2003), (−)-epigallocatechin (EGC), (−)-epigallocatechin gallate (EGCG), (+)-catechin (C), and quercetin (Q) (Long, Clement, & Halliwell, 2000; Halliwell, Clement, Ramalingam, & Long, 2000), hydroxytyrosol, del- phinidin and rosmarinic acid (Long, Hoi, & Halliwell, 2010), ascorbic acid (Wee et al., 2003) as well as thiol compounds (cysteine, glu- tathione, N-acetylcysteine, gamma-glutamylcysteine, cysteinylglycine, cysteamine, homocysteine) in commonly used cell culture media, especially Dulbecco's Modified Eagle's Medium (DMEM), Roswell Park Memorial Institute (RPMI) 1640 Medium and Eagle’s Minimal Essential Medium (MEM) (Hua Long & Halliwell, 2001) has been documented. Green and black tea, coffee and red wine, beverages rich in catechins, also produce H2O2 when added to cell culture media (Chai, Long, & Halliwell, 2003; Akagawa, Shigemitsu, & Suyama, 2003; Long, Lan, Hsuan, & Halliwell, 1999). Tea and coffee but not cocoa were shown to generate H2O2 to achieve levels of over 100μM. Milk decreased net H2O2 production by beverages and showed some ability to remove H2O2 itself (Long et al., 1999). The production of H2O2 in these fluids was in good agreement with the content of phenolic compounds, suggesting that polyphenols are responsible for the generation of H2O2 in beverages (Akagawa et al., 2003). Generation of H2O2 in vivo due to oxidation of antioxidant com- pounds has also been demonstrated. Holding green tea solution in the mouth or chewing green tea produces micromolar concentrations of H2O2 in the mouth (Lambert, Kwon, Hong, & Yang, 2007). Higher levels of H2O2 were found in urine of coffee drinkers and attributed to ex- cretion of hydroxyhydroquinone from coffee and its oxidation in urine, resulting in H2O2 production (Hiramoto, Kida, & Kikugawa, 2002; Halliwell, Long, Yee, Lim, & Kelly, 2004). The H2O2 generated by au- toxidation of antioxidants may not only introduce artefacts in cell culture experiments (Halliwell et al., 2000), but also contribute to bactericidal action and to paradoxical genotoxic and mutagenic activ- ities of these substances, generally assumed to have beneficial effects on human health (Lluís et al., 2011; Gomes et al., 2018). In spite of literature reports, the generation of H2O2 in culture media is still an underappreciated problem in model studies of biolo- gical effects of food components and antioxidants. In many cases it is unclear if the effects observed in vitro, contributed by H2O2 generated by antioxidant autoxidation, are relevant to in vivo conditions where this autoxidation is absent or strongly attenuated. Moreover, data on autoxidation and H2O2 production are available only for a limited number of antioxidants and data on a broader spectrum of antioxidants are lacking. Similarly, our knowledge of the H2O2 in food, beverages and our body due to autoxidation of food components is still limited. The aim of this study was to compare the propensity of over 50 of commonly used antioxidants, especially of natural origin (present in food and beverages) for autoxidation and generation of H2O2, and to get an insight into the mechanism of this effect. 2. Materials and methods 2.1. Materials BDTM DifcoTM Yeast Nitrogen Base and BactoTM Peptone were pur- chased from Becton Dickinson Poland (Warsaw), D-(+)-glucose and Xylenol Orange were obtained from Polish Chemical Reagents (POCh, Gliwice, Poland), perchloric acid (HClO4) was purchased from Chempur (Piekary Śląskie, Poland), phosphate-buffered saline (PBS) and di- methyl sulfoxide (DMSO) were obtained from Lab Empire (Rzeszów, 693

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