Flavonoids and Polyphenols

A wide variety of compounds collectively known as flavonoids (or sometimes as bioflavonoids) occur in plants as glycosides; some function to defend the plants against attack, others are pigments in flowers and fruits. Many of the glycosides are hydrolyzed by intestinal bacterial glycosidases, and the aglycones are absorbed. Figure 14.10 shows the six main types of flavonoid aglycone.

In the 1940s, the flavonoids were known as vitamin P (for permeability), because they were shown to have effects on the permeability of blood capillaries. They were not shown to be dietary essentials. By the 1970s, they were regarded as hazardous mutagens and potential carcinogens, because they can undergo redox cycling reactions and generate oxygen radicals. By the 1990s, they were regarded as potentially protective compounds with three types of action:

1. Although they can undergo redox cycling, they can also act as radical-trapping antioxidants.

2. They reduce the activation of many procarcinogens to the active proximate carcinogen by lowering the activity of microsomal cytochrome P450 enzymes.

Figure 14.10. Major classes of flavonoids.

3. They increase the metabolic clearance of potential carcinogens and their metabolites by inducing the enzymes involved in conjugation for excretion.

Some of the flavonoids also have estrogenic and antiestrogenic actions (Section 14.7.4).

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