Antioxidants, polyphenols antioxidant reaction

Biological Antioxidant Models. Tea extracts, tea polyphenol fractions, and purified catechins have all been shown to be effective antioxidants in biologically-based model systems. A balance between oxidants and antioxidants is critical for maintenance of homeostasis. Imbalances between free radicals and antioxidants may be caused by an increased production of free radicals or decreased effectiveness of the antioxidants within the reaction system. These imbalances can be caused by the radicals overwhelming the antioxidants within the system, or by an excess of antioxidants leading to a prooxidant functionaHty (105—118). When antioxidant defense systems are consistently overwhelmed by oxidative reactions, significant damage can... 

Based on chemical, thermodynamic, and kinetic data, this chapter will discuss the mechanisms proposed for polyphenol biological actions, especially antioxidant reactions. Although most of the points addressed in this chapter are valid for an important number of plant polyphenols, we will focus the analysis on flavonoids and, in particular, on flavanols. 

Curcumin is a polyphenol antioxidant derived from the turmeric root. Its hydrogenated form tetrahydrocurcumin (THC) was found to significantly lighten skin color [ 151]. In a local double-blind placebo-con-troUed study done at Research Institute for Tropical Medicine, Philippines, the depigmenting effects of 0.25% THC and 4% HQ were shown to be comparable throughout the 4 week trial in 50 subjects. No adverse reactions were noted from 0.25% THC whereas reactions were mild to moderate with 4% HQ [119]. 

Chemical Antioxidant Systems. The antioxidant activity of tea extracts and tea polyphenols have been determined using in vitro model systems which are based on hydroxyl-, peroxyl-, superoxide-, hydrogen peroxide-, and oxygen-induced oxidation reactions (109—113). The effectiveness of purified tea polyphenols and cmde tea extracts as antioxidants against the autoxidation of fats has been studied using the standard Rancimat system, an assay based on air oxidation of fats or oils. A direct correlation between the antioxidant index of a tea extract and the concentration of epigallocatechin gallate in the extract was found (107). 

The total antioxidant activity of teas and tea polyphenols in aqueous phase oxidation reactions has been deterrnined using an assay based on oxidation of 2,2 -azinobis-(3-ethylbenzothiazoline-sulfonate) (ABTS) by peroxyl radicals (114—117). Black and green tea extracts (2500 ppm) were found to be 8—12 times more effective antioxidants than a 1-mAf solution of the water-soluble form of vitamin E, Trolox. The most potent antioxidants of the tea flavonoids were found to be epicatechin gallate and epigallocatechin gallate. A 1-mAf solution of these flavanols were found respectively to be 4.9 and 4.8 times more potent than a 1-mAf solution of Trolox in scavenging an ABT radical cation. 

There has been some evidence of a higher antioxidant effect when both flavonoids and a-tocopherol are present in systems like LDL, low-density lipoproteins (Jia et al., 1998 Zhu et al, 1999). LDL will incorporate a-tocopherol, while flavonoids will be present on the outside in the aqueous surroundings. A similar distribution is to be expected for oil-in-water emulsion type foods. In the aqueous environment, the rate of the inhibition reaction for the flavonoid is low due to hydrogen bonding and the flavonoid will not behave as a chain-breaking antioxidant. Likewise, in beer, none of the polyphenols present in barley showed any protective effect on radical processes involved in beer staling, which is an oxidative process (Andersen et al, 2000). The polyphenols have, however, been found to act synergistically... 

Cervellati R, Renzulli C, Guerra MC and Speroni E. 2002. Evaluation of antioxidant activity of some natural polyphenolic compounds using the Briggs-Rauscher reaction method. J Agric Food Chem 50(26) 7504-7509. 

Antioxidant activity of flavonoids has already been shown about 40 years ago [90,91]. (Early data on antioxidant flavonoid activity are cited in Ref. [92].) Flavonoids are polyphenols, and therefore, their antioxidant activity depends on the reactivity of hydroxyl substituents in hydrogen atom abstraction reactions. As in the case of vitamins E and C, the most studied (and most important) reactions are the reactions with peroxyl radicals [14], hydroxyl radicals [15], and superoxide [16]. 

Polyphenols can act as antioxidants by a number of potential pathways. The most important is likely to be by free radical scavenging, in which the polyphenol can break the radical chain reaction. Polyphenols are effective antioxidants in a wide range of chemical oxidation systems, being capable of scavenging peroxyl radicals, alkyl peroxyl radicals, superoxide, hydroxyl radicals, nitric oxide and peroxynitrate in aqueous and organic environments [121]. This activity is due to the ability of donating an H atom from an aromatic hydroxyl group to a free radical, and the major ability of an aromatic structure to support an unpaired electron by delocalization around the 7i-electron system. Phenolic acids... 

The analysis of the thermodynamic and kinetic characteristics of these reactions will allow a better understanding of the biological importance of chain breaking in the antioxidant actions of polyphenols. 

Polyphenols could act as effective chain-breaking antioxidants (AH = POH) through the one-electron transfer reactions 4 and 5 if they produce a stable and relatively nonreactive antioxidant radical (A = PO) [Jovanovic et al., 1994], Reactions 4 and 5 can be represented by the reaction 8, where L or LOO represents the oxidant free radical. This reaction can be decomposed in two half-reactions one reduction (reaction 9) and one oxidation (reaction 10) ... 

The reactions between free radicals and flavonoids (or polyphenols) are assumed to form aroxyl radicals (PO) (reaction 8). The stability of these secondary radical species is an important element to be considered in their antioxidant actions. Flavonoids with similar reduction potentials can originate radicals with very different reactivity toward other molecules present in biological systems. While a stable and relatively nonreactive PO is also nonreactive to propagate the chain reaction, a high reactive PO would propagate rather than interrupt a chain reaction. 

In association with well-known health benefits related to the consumption of fruit- and vegetable-rich diets, research on the protective effects of plant-derived phenolic compounds (polyphenols) has developed notably in recent years. In particular, their antioxidant properties have been the objective of extensive research. However these phenolics are the target of an array of chemical reactions that, if confirmed to occur in vivo, would contribute to their health promoting effects. It is now emerging that both parent compounds and their metabolites produced after ingestion can regulate cell and tissue functions by both antioxidant and nonantioxidant mechanisms. This volume provides the latest evidence supporting these concepts. 

Other antioxidant species are synthesized by cells like uric acid, ubiquinol or thiols (cystein, homocystein, etc.). In addition, many compounds found in food display antioxidant properties retinol (vitamin A) and its precursor /(-carotene, and polyphenols (flavonoids, etc.). Figure 8.2 shows the apparent standard potential of some LMWA and ROS explaining the spontaneous oxido-reduction reactions at the origin of the antioxidant protection system. 

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