Phenolic compounds are the most widely distributed secondary metabolites in plants, and constitute several thousands of compounds. Phenolics in plants are primarily responsible for their protection from free radical stress under photosynthetic conditions and ultraviolet light, and act against herbivores and pathogens (Shahidi, 2000a). They also contribute to the variety of color and taste of foods containing them (Shahidi and Naczk, 1995). In addition, they serve as wound-healing agents in plants as a result of their oxidation and subsequent condensation with free amino acids and proteins.
Structurally, phenolics are derived, as a first step, from phenylalanine and in a small number of plants from tyrosine via the action of ammonia lyase. These compounds generally contain at least one aromatic ring with one hydroxyl group (phenols) or more (polyphenols). In oilseeds, they exist as low-molecular-weight compounds which occur universally in higher plants with only some species specificity, and oligomeric and polymeric forms. The resultant compounds from the action of ammonia lyase on aromatic amino acids, known as phenylpropanoids, may then be subjected to a variety of modifications in plants, including hydroxylation and methy-lation, to afford a wide range of C6 - C3 compounds which are derivatives of trans-cinnamic acid (Figure 10.1). These compounds may lose a two-carbon moiety to yield benzoic acid derivatives. Condensation of C6 - C3 compounds with malonyl coenzyme A affords chalcones which may subsequently cyclize, under acidic conditions, to produce flavonoids and isoflavonoids as well as related compounds with C6 - C3 - C6 units (Figure 10.2), among others (Shahidi, 2000b).
Phenolic acids, phenylpropanoids, and flavonoids/isoflavonoids in foods may occur in the free form, but are often glycosylated with sugars, especially glucose. While the presence of sugars in such compounds is responsible for their specific
characteristics and transport in the plants and/or body fluids, they do not have any significant effect on the biological activity of compounds involved once ingested. Nonetheless, when measuring total antioxidant capacity of oilseeds and their extracts, it might be necessary to hydrolyze them to free their phenolic hydroxyl groups that are responsible for their antioxidant behavior in vivo. Phenolic acids may also be present in the esterified as well as bound forms (Naczk and Shahidi, 1989).
Oilseed phenolics may exist in both simple and complex forms; the latter group consists of both hydrolyzable and condensed tannins. Condensed tannins are produced via polymerization of flavonoids, and are abundantly present in woody plants and seed coats, but are distinctly absent in herbal species. However, hydrolyzable tannins are formed by the reaction of gallic acid with hexose molecules and are more selectively present in 15 out of the 40 orders of dicotyledons. Thus, oilseeds contain a cocktail of different phenolics that may act cooperatively and synergistically with one another to exert their effects, both in terms of antioxidative action (Shukla et al., 1997) and health promotion and disease prevention.
In the body, oxidation products and reactive oxygen species (ROS) may lead to a number of diseases and tissue injuries such as those of the lungs, heart, kidney, liver, gastrointestinal tract, blood, eye, skin, muscle, and brain, as well as the aging process. In healthy individuals, ROS are neutralized by the action of antioxidant and antioxidant enzymes. However, when the action of the enzyme system is inadequate because of illness and during infancy or due to aging, the oxidation process is not controlled naturally, and augmentation may provide the necessary means to combat degenerative diseases and other ailments caused by ROS. The manner in which antioxidants intervene is by their effect in a multistage process and may involve prevention of lipid oxidation, protein cross-linking, and DNA mutation, among others (Shahidi, 1997).
In terms of neutralizing free radicals, phenolics are well known to protect cells and their components against cancer development. Simple phenolic acids and tocopherols have been shown to be potent inhibitors of formation of carcinogens such as N-nitroso compounds (Kuenzig et al., 1984). Meanwhile, inhibition of benzo (a) pyrene-induced neoplasia in the forestomach of mice fed various plant phenolics has been reported by Wattenberg et al. (1980). Chromosomal aberrations induced by polycyclic aromatic hydrocarbons were inhibited by caffeic acid (Raj et al., 1983), while chloro-genic acid blocked chemically induced carcinogens in the large intestine of hamsters (Mori et al., 1986). Chang et al. (1985) have demonstrated antitumor-promoting activity of ellagic acid and quercetin. Flavonoids, including catechins, were also found to reduce hyperlipedemia in animals (Choi et al., 1991). In the case of heart disease, inhibition of LDL cholesterol oxidation helps in prevention of foam cells formation and lipid streaks development. Oxidized LDL cholesterol is more atherogenic than native LDL and is also known to affect tissue factor expression. Several recent studies provide compelling evidence that dietary intake of antioxidants can lower the production of atherogenic oxidized LDL cholesterol and thus may decrease the risk of cardiovascular disease (Steinberg et al., 1989; Niki, 1991, Niki et al., 1995).
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