Direct antioxidants, plant phenolic compounds

DNA damage in the presence of cytochrome P450s (CYPs) and Cu(II) in vitro or in cells [Yoshino et al., 2004 Ahsan et al., 1999]. At higher concentrations curcumin exhibited genotoxicity and cytotoxicity in different systems [Cao et al., 2007]. At last, in vivo intervention trials using typical antioxidants such as ascorbate or carotenes yielded mostly disappointing results in terms of protection against oxidation-related diseases, sometime even increased the incidences of diseases [Stevenson and Hurst, 2007 Dinkova-Kostova and Talalay, 2008]. Obviously, the direct antioxidant capacity of plant phenolic compounds cannot explain their cytoprotective activities. 

The impact of plant products on the metabolism of synthetic dmgs results from the inhibition or activation of cytochrome P-450 (CYP) enzymes. Evaluation of the potential activation of CYP by administration of natural plant products or dietary supplements is important for prediction of interactions between their components and dmgs. Therefore, attention is directed to research on the impact of products available on the food market known as natural non-nutritive substances on dmg absorption. Non-nutritive dietary components are mainly secondary plant metabolites, which include, among others, phenolic compounds such as phenolic acids and flavonoids. The health effects of non-nutritive substances are not yet known. So far, there is no answer on the extent to which they are absorbed and metabolized by the body, and there is no information on the permitted daily intake for these compounds. This information is particularly important because certain non-nutritive natural substances are simultaneously considered to be anti-nutritional factors, mainly because they inhibit digestion and reduce the bioavailability of nutrients or dmgs. It is also possible that they form undesirable interactions with dmgs. The positive health effects of non-nutritive natural substances are not only attributed to their antioxidant properties. These substances are involved in various metabolic... 

Phenols or phenol groups are a common structure of several secondary plant metabolites. Characterized by at least one hydroxyl function attached to a benzene ring, such molecules can be found in numerous variations in plants, e.g., in flavonoids (Fig. 4) or tocopherols (Fig. 2). The antioxidant capacity of phenolic compounds includes not only direct scavenging of ROS, but also originates from the abihty of compounds carrying two or more hydroxyl groups to bind transition metal ions. Such chelation strongly decreases the transition metals capability to induce environmental oxidative stress. 

Type III polyketide synthases are particularly relevant to this chapter because they catalyze the formation of phenolic compounds. This group of polyketide synthases consists of CHSs, stilbene synthase (STS), and curcuminoid synthase (CUS), which perform decarboxylative condensations between a starter unit, either p-coumaroyl-CoA 19 or cinnamoyl-CoA 18, and an extender unit, malonyl-CoA 10. CHS, STS, and CUS convert the substrate molecules into flavo-noids (Cg-Cs-Cg), stilbenoids 8 (Cg-C2-Cg), and curcuminoids 9 (Cg-C7-C6), respectively [59]. Stilbenoids 8 and curcuminoids 9 are out of the scope of this chapter but possess medicinal properties as well resveratrol is a well-known stilbenoid 8 associated with longevity, and curcumin is a common curcuminoid 9 that is responsible for the yellow color in turmeric and can be utilized as a natural pigment possessing antioxidant and anti-inflammatory properties [60-63]. For an in-depth treatment of plant polyketide production in microbes, the reader is directed to a recent comprehensive review by Boghigian et al. [64]. 

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