## 2 The Role of Phytochemical in Oxidative Stress
### Phytochemicals as Pro-Oxidant Agents: Friend or Foe? Besides the already stated phytochemicals with antioxidant effects, it has also been shown that some of these molecules may possess pro-oxidant effects. In particular, flavonoids have been found to be mutagenic in vitro [61; 62]. In fact, as described above, the chemical structure of flavonoids exerts a key role in determining their antioxidant activities, as well as their copper-initiated pro-oxidant activities. As long as the OH substitution is necessary for the antioxidant activity, the subclasses of flavone and flavanone, which have no OH substitutions and provide the basic chemical structures for flavonoids, do not show antioxidant or copper-initiated pro-oxidant activities [61]. Higher OH substitutions are associated with stronger redox activity. O-Methylation, probably as well as other O-modifications of the flavonoid OH substituents, inactivates both the antioxidant and pro-oxidant effects of flavonoids. In general, flavonoids occur in foods as O-glycos with sugars bound at the C3 position. Methylation or glycosidic modification of the OH substitutions leads to inactivation of the transition metal-initiated pro-oxidant activity of a flavonoid [63]. On the other hand, flavonoids, such as quercetin and kaempfer, induce nuclear DNA damage and lipid peroxidation in the presence of transition metals. The in vivo copper-initiated pro-oxidant action of flavonoids is generally not considered significant, as copper ions are largely sequestered in tissues, except in the case of metal toxicity. In this context, flavonoids, such as catechins and quercetin, possess chelating properties, inhibiting their reactivity [64; 65]. Besides, polyphenols are well-known as scavenger/chain breaking phytochemicals (Figure 3). Despite the well-known ability of flavonoids in preventing iron-induced lipid peroxidation in hepatocytes, including quercetin [66], it has been suggested that the pro-oxidant activity of some polyphenols is more prominent in vitro under particular conditions, such as high pH or the presence of high concentrations of transition metal ions and oxygen molecules [2]. Specifically, small phenolic molecules, which are easily oxidized, such as quercetin and gallic acid, have a known pro-oxidant activity, while, on the other hand, phenols with a high molecular weight (i.e., tannins), have little or no pro-oxidant activity [15]. The pro-oxidant properties of polyphenols may derive from different possible mechanisms, including metal reduction,copper ion mobilization, chemical instability, cellular glutathione (GSH) depletion, and sulfhydryl (SH) interactions [67]. Although reduction and antioxidant capacity are related, it must be considered that the hydroxyl radical, the lipid peroxidation initiator, is produced from the reaction between reduced iron or copper and hydrogen peroxide (Figure 3). Moreover, some dietary polyphenols, such as resveratrol and caffe acid, have been shown to trigger DNA damages through the mobilization of endogenous copper ions, suggestive of chromatin binding, leading to ROS production [68]. The preferential cytotoxicity towards tumor cells is probably due to the high levels of copper ions, and not of iron ions in cells and in tumor tissues. The mechanism through which copper concentration is increased in the tumor is not yet clear [68; 69]. However, it has been shown that the copper transporter 1, with high affinity in humans, is over-expressed in malignant cells, resulting in the increased absorption and accumulation of the metal. Also, it has been hypothesized that copper may be necessary for ceruoplasmin expression, which is the main protein that binds it, which is over-expressed in cancer cells, and thus it has been proposed to be an endogenous stimulator of angiogenesis. Nonetheless, further in vivo studies are necessary to better understand these processes.