Isoflavones structures

As a whole, flavonoids and isoflavones cover almost the entire spectmm of redox potentials inherent to the ETC [2,16,17]. Therefore, on electrochemical grounds, it is feasible that these polyphenolic compounds can interact with mitochondrial redox centers and divert or impair electron flux through the ETC (Fig. 1). There is also the potential for complex-specific interactions depending on their redox potential. Extensive in vitro and ex vivo structure-activity studies were performed to determine the effect of a plethora of flavonoids and isoflavones on mitochondrial electron transport and to correlate potency with their redox potential [16,18—21]. These and other studies were extensively reviewed in the first volume of this series [2]. The reader is referred to the first volume for more detailed information only the key findings are summarized here. AU the polyphenohc compounds tested were either inhibitory or ineffective toward complex I and complex 11, defined as NADH-oxidase and succino-oxidase activity, respectively [2]. None was found to exert a stimulatory effect. From the onset, it should be noted that the multitude and complexity of the flavonoid and isoflavone structures allow only the formulation of generalized structure-activity mles. 

Many higher plants synthesize flavanes, flavanones, flavones, and isoflavones with a wide range of structural complexity. They make a significant contribution to the food intake of both herbivores and humans, and they have aroused particular interest on account of their degradation by mammals that are mediated by intestinal bacteria. Most of them exist naturally as glycosides and these are readily hydrolyzed to the aglycones. 

The basic structures of flavanones, flavones, and isoflavones together with coumestrol, an intermediate in the phenylpropane metabolism, are given in Fig. 2. The 3,5,7,3 -tetrahydroxy-4 -methoxyflavanone is a nod gene inducer in Rhizo-bium leguminosarum bv. viciae the 3, 4, 5,7-tetrahydroxyflavone, in Rhizobium ineliloti and 4,7-dihydroxyisoflavone, in Bradyrhizobium japonicum. Coumestrol, an intermediate in phenylpropane metabolism, is only a weak inducer (7). 

These data established the methoxy group at 4 and the structure of xenognosin B as 2 This sample also proved to be identical by NMR, MS, and biological activity to a synthetic sample of 2, 7-dihydroxy-4 -methoxy isoflavone (generously provided by Professor Paul Dewick, University of Nottingham, U.K.). 

Isoflavones also have a diphenylpropane structure in which the B ring is located in the C3 position. They have structural analogies to estrogens, such as estradiol, with hydroxyl groups at the C7 and C4 positions (Shier and others 2001). 

Flavonoids are a complex group of polyphenolic compounds with a basic C6-C3-C6 structure that can be divided in different groups flavonols, flavones, flavanols (or flavan-3-ols), flavanones, anthocyanidins, and isoflavones. More than 6,000 flavonoids are known the most widespread are flavonols, such as quercetin flavones, such as lu-teolin and flavanols (flavan-3-ols), such as catechin. Anthocyanidins are also bioactive flavonoids they are water-soluble vegetable pigments found especially in berries and other red-blue fruits and vegetables. 

The Japanese diet differs in many aspects from its Western counterpart, not only in consumption of soy but also of fish. To date, however, soy alone appears to have attracted the attention of researchers. The structural similarity between the isoflavones contained in soy and estrogen has perhaps made it conceptually easier to associate soy consumption and the low incidence of hot flushes in Japanese women. This may prove to be too simplistic as it is highly likely that a combination of nutrients, rather than just one compound, may determine the favorable health effects of the diet. 

Min T, Kasahara H, Bedgar DL et al (2003) Crystal structures of pinoresinol-lariciresinol and phenylcoumaran benzylic ether reductases and their relationship to isoflavone reductases. J Biol Chem 278 50714-50723... 

Xiao, H. B., Krucker, M., Putzhach, K., and Albert, K., Capillary liquid chromatography-microcoil IH nuclear magnetic resonance spectroscopy and liquid chromatography-ion trap mass spectrometry for on-hne structure elucidation of isoflavones in Radix astragali. Journal of Chromatography A 1067(1-2), 135-143, 2005. 

This chapter, therefore, aims to present a brief unified summary of general techniques, with reference to the different categories of structure flavones and flavonols (and their glycosides), isoflavones, flavanones, chalcones, anthocyanins, and proanthocyanidins. 

---