Flavonoids form a large group of plant polyphenols, comprised of more than 5,000 molecules. The group is subdivided into anthocyanins, which are responsible for the colors from blue to red in flowers and fruits, and the colorless or white-to-yellow flavones, flavonols and isoflavones.
Many of these flavonoids are biologically active compounds that may have disease-preventing properties. Studies have been carried out mainly on cancer, cardiovascular disease and postmenopausal problems. In addition to their potent antioxidant properties, their mechanisms of action may include specific inhibition of enzymes or binding to receptors (Figure 1).
Most of the flavonoids in plants are found in the glycoside form, which means they are attached to a sugar residue. As such, they are water-soluble and can be kept in the aqueous plant vacuoles. The glycosides are usually biologically inactive in humans because of low cellular uptake and because they do not fit into the binding sites of enzymes and receptors. After oral application, transformation into active molecules occurs through the action of hydrolytic enzymes in the intestine.
Flavonoids have become very popular as cosmetic ingredients, with claimed skin benefits such as prevention of lipid oxidation, stimulation of fibroblast proliferation, reduction of collagen breakdown, and inhibition of 5α-reductase. The prevalent isoflavone in soy is the glycoside genistin. Its aglycone (the molecular form without sugar residues), genistein, is a well-known inhibitor of protein tyrosine kinases.