Isoflavones are a type of often naturally occurring isoflavonoids, many of which act as phytoestrogens in mammals. Some are termed antioxidants because of their ability to trap singlet oxygen. Isoflavones are produced almost exclusively by the members of the Fabaceae (i.e., Leguminosae, or bean) family.
Isoflavones (and closely related phytoestrogens) have grown popular as dietary supplements, but there are few studies showing any benefits from these compounds, and their use is viewed within the scientific and medical community as pseudoscience. Some studies have also identified significant risks from isoflavones.
Organic chemistry and biosynthesis
Isoflavones of nutritional interest are substituted derivatives of isoflavone, being related to the parent by the replacement of two or three hydrogen atoms with hydroxyl groups. The parent isoflavone is of no nutritional interest.
Isoflavone differs from flavone (2-phenyl-4H-1-benzopyr-4-one) in location of the phenyl group.
Isoflavones are produced via a branch of the general phenylpropanoid pathway that produces flavonoid compounds in higher plants. Soybeans are the most common source of isoflavones in human food; the major isoflavones in soybean are genistein and daidzein. The phenylpropanoid pathway begins from the amino acid phenylalanine, and an intermediate of the pathway, naringenin, is sequentially converted into the isoflavone genistein by two legume-specific enzymes, isoflavone synthase, and a dehydratase. Similarly, another intermediate naringenin chalcone is converted to the isoflavone daidzein by sequential action of three legume-specific enzymes: chalcone reductase, type II chalcone isomerase, and isoflavone synthase. Plants use isoflavones and their derivatives as phytoalexin compounds to ward off disease-causing pathogenic fungi and other microbes. In addition, soybean uses isoflavones to stimulate soil-microbe rhizobium to form nitrogen-fixing root nodules.
Most members of the Fabaceae family contain significant quantities of isoflavones. Analysis of levels in various species has found that the highest levels of genistein and daidzein in psoralea (Psoralea corylifolia). Various legumes including soybean (Glycine max L.), green bean (Phaseolus vulgaris L.), alfalfa sprout (Medicago sativa L.), mung bean sprout (Vigna radiata L.), cowpea (Vigna unguiculata L.), kudzu root (Pueraria lobata L.), and red clover blossom and red clover sprout (Trifolium pratense L.) have been studied for their estrogenic activity. Highly processed foods made from legumes, such as tofu, retain most of their isoflavone content, with the exception of fermented miso, which has increased levels.
In plant tissue, they most often occur as glycosides or their respective malonates or acetyl conjugates, rendering them even more water-soluble (see isoflavone-7-O-beta-glucoside 6"-O-malonyltransferase). The latter forms are unstable and are transformed, e.g. by decarboxylation. Often when leguminose plants are challenged with viral or fungal infections, the water-soluble transport forms are hydrolyzed to the respective aglycones at the target site.
Potential health effects
The consumption of isoflavones-rich food is under study for its potential association with lower rates of postmenopausal cancer. Use of soy isoflavone dietary supplements may be associated with reduction of hot flashes in postmenopausal women.
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