Isoflavone
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The chemical structure of the isoflavone backbone (3-phenyl-4H-1-benzopyr-4-one)

Isoflavones are a class of organic compounds and biomolecules related to the flavonoids.^[1] Many act as phytoestrogens in mammals. They are also very strong antioxidants.^{citation needed} Several selected isoflavones and isoflavone-rich foods possess activity against cancer, including certain types of breast and prostate cancer .^[2] Isoflavone (3-phenyl-4H-1-benzopyr-4-one) differs from flavone in the following: in isoflavones, the position of the phenyl group on the 4H-1-benzopyr-4-one skeleton is in position 3 relative to the oxygen of the ring, whereas in flavones it is in position 2. Isoflavones are polyphenolic compounds produced almost exclusively by the members of the Fabaceae/Leguminosae (bean) family.

Because of the large chemical and structural complexity of the isoflavones, effects on human and animal health show differences in direction.^[2] Early evidence that plants produced estrogen-like compounds was observed in the infertility among sheep eating large amounts of clover in Australia.^{citation needed} Similarly, Californian Quails that fed on the leaves of high-isoflavone desert annuals during periods of food shortage had reduced fertility.^[3] Studies using chemically pure isoflavones or plant materials with known concentrations of these compounds have indicated both positive and negative effects of isoflavones on disease progression and fertility. (Reviewed in ^[2])

Biosynthesis

Isoflavones are produced from a branch of the general phenylpropanoid pathway which produces all 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 in to 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.

Occurrence

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 were found in psoralea (Psoralea corylifolia). Various legumes including kudzu (Pueraria lobata), lupine (Lupinus spp), fava bean (Vicia faba), and soy (Glycine max) contained substantial amounts of isoflavones according to this analysis. Highly processed foods made from legumes, such as tofu, retained most of their isoflavone content, with the exception of fermented miso, which actually has increased levels.

Other dietary sources of isoflavones include chick pea (biochanin A), alfalfa (formononetin and coumestrol), and peanut (genistein).

Research

Isolated phytoestrogen-like isoflavones are an active research area. A paper in the Proceedings of the National Academy of Sciences studied the effect of the isolated soy isoflavones genistein and daidzein (commonly found in dietary supplements and infant formulas) on adult mice with their ovaries removed. The study found the mice had thymic and immune system abnormalities and reduction in immune system activity. The article suggests further research into human phytoestrogen response is warranted.^[4]

"Researchers Daniel Doerge and Daniel Sheehan, two of the FDA's experts on soy, signed a letter of protest, which points to studies that show a link between soy and health problems in certain animals. The two say they tried in vain to stop the FDA approval of soy because it could be misinterpreted as a broader general endorsement beyond benefits for the heart."^[5]

References

1. ^ Kaufman PB, Duke JA, Brielmann H, Boik J, Hoyt JE (1997) “A comparative survey of leguminous plants as sources of the isoflavones genistein and daidzein: Implications for human nutrition and health” J Altern Compl Med 3:7-12
2. ^ ^{a b c} Dixon, RA (2004). "Phytoestrogens". Annu Rev Plant Biol. 55: 225–61. doi:10.1146/annurev.arplant.55.031903.141729. PMID 15377220. 
3. ^ Phytoestrogens:Adverse effect on reproduction in California quail PMID 1246602
4. ^ "The phytoestrogen genistein induces thymic and immune changes: A human health concern?" Proceedings of the National Academy of Sciences May 28, 2002
5. ^ FDA Scientists Against Soy. The anti-soy website "Soy Online Service" has the original letter in PDF. [1]

External links

Isoflavones info