Plant phenolics in human health and as antioxidants

It is evident that plant phenolic compounds constitute one of the most numerous and widely distributed groups of substances with more than 8000 phenolic structures currently known (28). In addition to stress linked phenolics coming only from the shikimate and phenylpropanoid pathways, a number of the phenolic compounds are found in plants, including the flavonoids that contribute to the characteristic flavor and fragrance of vegetables, fruits, tea, and wine. These compounds, which come from phenylpropanoid and polyketide (acetate-malonate) pathways, also have biological properties that are beneficial to human health. Flavonoids such as quercetin and catechin and isoflavonoids, genistein for example, are being investigated for properties which may reduce the incidence of cancer (22,23). Flavonoids and isoflavonoids are a class of phenolic compounds that have appeared sequentially during plant evolution and are simple aromatic compounds generated from both the phenypropanoid and acetate-malonate (polyketide) pathways (24). From a functional health point of view, it is suggested that such phenolics, for example, through the consumption of tea, may provide protection against certain cancers; soybeans may provide protection against breast cancer and osteoporosis (22). The Japanese and Chinese frequently used plants rich in polyphenol tannins in their folk medicines for the treatment of inflammation, liver injury, kidney ailments, hypertension, and ulcers. Oregano extracts have been shown to inhibit lipid peroxidation by their flavonoid fractions such as flavone apigenin, the flavanone, eriodictoyl, dihydrokaempferol, and dihydroquercetin (29). Rosmarinic acid-containing Ocimum sanctum (holy basil), derived from the phenylpropanoid pathway, is commonly used in India to reduce fevers and gastrointestinal disease (30). Essential oils from thyme (Thyme vulgaris L.) have phenolic linked antioxidant properties, which may result from the presence of free radical scavengers in these oils (31-33). These diverse phenolic compounds in thyme are unusually derived from phenylpropanoid, polyketide, and terpenoid pathways. Many other phenolic compounds have the ability to inhibit platelet aggregation, block calcium influx, and protect low density lipo-proteins (LDL) from oxidation (34). l-DOPA from the fava bean, that is the focus of this chapter, has been evaluated in Parkinson's management (35-37); it is derived from L-tyro-sine of the phenylpropanoid pathway (Figure 9.1).

Among many functional roles, the most important function assigned to phenolics is their antioxidant activity. Antioxidants may be defined as substances, which when present in low concentrations compared with those of an oxidizable substrate, such as proteins, carbohydrates, and fats, delay or prevent the oxidation of the substrate (38). Phenolic antioxidants from dietary plants can be useful to counter reactive oxygen species related to human diseases. Reactive oxygen species (ROS) are able to oxidize cellular components such as DNA, lipids, and proteins (23). Dysfunction of oxidative phosphorylation at the mitochondrion has been recognized as a major physiological source of ROS (39). Lipid peroxidation damages the structural integrity of the mitochondria, which can result in organelle swelling, resulting in increased permeability to cations, decreased membrane potential, and damage to electron transfer activities (40). This form of tissue damage can ultimately lead to some of the major diseases such as cancer, cardiovascular disease, immune dysfunction, diabetes, and neurodegeneration (23). To deal with ROS, the biological systems, including human cellular systems, have an effective defense system, which includes enzymes such as superoxide dismutase (SOD), catalase (CAT), high molecular weight antioxidants, and an array of low molecular weight antioxidants such as ascorbic acid, a-tocopherol, P-carotene, and glutathione (41). The endogenous antioxidant responses, both enzymatic and nonenzymatic could also be enhanced by dietary intake of

Pentose phosphate pathway

Glycolysis nadp nadph nadp nadph

Glucose

Glucose-6-P Glucose-6-P

G-6-P dehydrogenase (G6PDH) |

Fructose-6-P

Glucos

6-phosphoglucono-lactone

I 6-phosphogluconolactone j I dehydrogenase

Ribulose-5-P

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