Historically, the oxidation of fatty acids has been perceived as a deleterious process resulting in the generation of off-flavors in commercial produce. The process of oxidation can be considered as either autoxidation, a chemical reaction that usually takes place at ambient temperatures between atmospheric oxygen and an organic compound, or as an enzyme-catalyzed reaction. The complexity of the autoxidation reaction has been well documented (106). The insertion of oxygen is also accompanied by the movement of double bonds resulting in the formation of a conjugated diene with a maximal absorbance at 234 nm. Spectral measurements at this wavelength have been used to monitor hydroper-
oxide formation (107). An important feature of autoxidation is that it is autocatalytic, the rate of oxidation being slow at the start (the induction period) and increasing as the reaction progresses. In common with other radical chain reactions, autoxidation can be divided into three separate processes: initiation, propagation, and termination. Initiation is perhaps the process most difficult to define because of the low concentration of radicals and the likelihood of there being more than one process, since a large number of different radicals can abstract hydrogen from RH to form R" + H. The initiating species may be, for example, a transition metal ion, a radical generated by photolysis or high-energy irradiation, or a radical generated by decomposition of a hydroperoxide. In the propagative phase, further reactive species are generated as in the generation of a radical R" from the alkene RH and its subsequent reaction with oxygen. In the termination phase there is an interaction of radicals to produce non-initiating and nonpropagating products.
Autoxidation is facilitated by pro-oxidants and inhibited by antioxidants. Pro-oxidants, such as metals or other radical initiators, operate by promoting the initiation step or else they may inhibit the activity of antioxidants. Antioxidants are frequently added to fats and fat-containing foodstuffs to prolong shelf life (108). These are often phenolic compounds which act by interferring with the propagation sequence by conserving propagating radicals into nonpropagating species (109, 110). Their effectiveness is often increased by compounds such as citric acid, ascorbic acid, or phosphoric acid (called synergists). In this regard, there is considerable evidence for a contributory, antioxidant role for vitamins E and C and the carotenoids—constituents of fruit, vegetables, beverages, and grains—in the maintenance of health and the protection from coronary heart disease, cataracts, and certain cancers (111).
Recent work indicates possible important roles of polyphenolic components (the flavonoids, phenylpro-panoids, and phenolic acids) as contributors to antiox-idant activities and also as agents of other mechanisms contributing to cardioprotective and anticarcinogenic actions. Epidemiological evidence suggests a negative correlation between dietary antioxidants and coronary arterial disease. Such an association may involve a mechanism whereby antioxidants delay the onset or progression of the disease by reducing oxidative reactions, downregulating thrombic mechanisms, and attenuating endothelial dysfunction.
A large range of flavonoids and phenylpropanoids found in fruit, vegetables, and beverages are more effective antioxidants in in vitro systems than vitamin C or vitamin E (111). Examination of the antioxidant status of a number of consumables gave a hierarchy of red wine>black tea = green tea>red grape juice = blackcurrant juice>white grape juice = orange juice>white wine = apple juice. Indeed, in what has become known as the "French paradox'' (their relatively high fat consumption but relatively low coronary disease) has been attributed, in part at least, to the beneficial effects derived from the consumption of red wine containing the flavonoid constituents derived from the black grape skins.
There is also increasing epidemiological evidence for an association between dietary intake of carote-noid-containing fruit and vegetables and a decreased incidence of coronary heart disease and certain cancers (111, 112). Recent studies (113) have shown that dietary intake of lycopene-rich foods was inversely associated with a risk of prostate cancer. These effects have been ascribed in part to the antioxidant activity of carotenoids. In addition to direct radical scavenging properties of some compounds given above, others such as the glucosinolates (114) (present in high amounts in vegetables such as brocolli, Brussels sprouts, and cabbage) or cysteine sulfoxides (115) in Allium (garlic and onion) act by stimulating the animals' own enzymic defenses (116, 117). This appears to operate via the antioxidant responsive element present in the 5 '-flanking region of many antioxidant enzymes such as glutathione S-trans-ferases, glutathione peroxidases, quinone reductase, and superoxide dismutase.
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