Summary
Isoflavones are substituted derivatives of isoflavone, a type of naturally occurring isoflavonoids, many of which act as phytoestrogens in mammals. Isoflavones are produced almost exclusively by the members of the bean family, Fabaceae (Leguminosae). Although isoflavones and closely related phytoestrogens are sold as dietary supplements, there is little scientific evidence for either the safety of long-term supplementation or of health benefits from these compounds. Some studies have identified potential risks from high intake of isoflavones, such as in women with a history of breast cancer, but this concern has not been substantiated with high-quality clinical research. Isoflavone is an isomer of flavone, which is chromone substituted with a phenyl group in the 2-position. In isoflavone, the phenyl group is in the 3-position. Substituted isoflavone derivatives are related to the parent by the replacement of two or three hydrogen atoms with hydroxyl groups. 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.
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Lignan
The lignans are a large group of low molecular weight polyphenols found in plants, particularly seeds, whole grains, and vegetables. The name derives from the Latin word for "wood". Lignans are precursors to phytoestrogens. They may play a role as antifeedants in the defense of seeds and plants against herbivores. Lignans and lignin differ in their molecular weight, the former being small and soluble in water, the latter being high polymers that are undigestable. Both are polyphenolic substances derived by oxidative coupling of monolignols.
Genistein
Genistein (C15H10O5) is a naturally occurring compound that structurally belongs to a class of compounds known as isoflavones. It is described as an angiogenesis inhibitor and a phytoestrogen. It was first isolated in 1899 from the dyer's broom, Genista tinctoria; hence, the chemical name. The compound structure was established in 1926, when it was found to be identical with that of prunetol. It was chemically synthesized in 1928. It has been shown to be the primary secondary metabolite of the Trifolium species and Glycine max.
Phytoestrogen
A phytoestrogen is a plant-derived xenoestrogen (see estrogen) not generated within the endocrine system, but consumed by eating plants or manufactured foods. Also called a "dietary estrogen", it is a diverse group of naturally occurring nonsteroidal plant compounds that, because of its structural similarity to estradiol (17-β-estradiol), have the ability to cause estrogenic or antiestrogenic effects. Phytoestrogens are not essential nutrients because their absence from the diet does not cause a disease, nor are they known to participate in any normal biological function.
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