The biochemistry, chemistry and physiology of the isoflavones in soybeans and their food products

Abstract

In this review of the chemistry, absorption, metabolism, and mechanisms of action of plant isoflavones, emphasis is placed on the isoflavones in soy and the food products derived from them. Soybeans have been part of food history in Asia for several millennia but did not reach the Americas and Europe until the eighteenth century. In the twentieth century, there was a tremendous increase in the cultivation of soybeans in the United States and more recently in South America. Soy foods have entered the U.S. food supply in ever-increasing amounts both in the form of traditional products (soy milk, tofu) and in more subtle ways in dairy and bread/cake products. The isoflavones in non-fermented foods are for the most part in the form of glycoside conjugates. These undergo changes due to different processing procedures. Isoflavones and their metabolites are well absorbed and undergo an enterohepatic circulation. They are often termed phytoestrogens because they bind to the estrogen receptors although weakly compared to physiologic estrogens. This estrogenicity is not the only mechanism by which isoflavones may have bioactivity-they inhibit tyrosine kinases, have antioxidant activity, bind to and activate peroxisome proliferator regulators alpha and gamma, inhibit enzymes in steroid biosynthesis, strongly influence natural killer cell function and the activation of specific T-cell subsets, and inhibit metastasis. These various properties may explain the much lower incidence of hormonally-dependent breast cancer in Asian populations compared to Americans and Europeans.

FIG. 1.

The pathway for isoflavone biosynthesis. First phenylalanine reacts with malonyl CoA to produce 4-hydroxycinnamoyl CoA. Under the catalytic control of chalcone synthase 4-hydroxycinnamoyl CoA condenses with three molecules of malonyl CoA to form a chalcone. Chalcone isomerase closes the heterocyclic ring to form naringenin. The B-ring is moved from the 2-position to the 3-position by isoflavone synthase. Isoflavone dehydratase removes water to generate the 2,3 double bond in the heterocyclic ring (see Figure 2 for the numbering scheme).

FIG. 2.

The numbering scheme of isoflavones. The scheme starts from the ethereal oxygen in the heterocyclic ring. The B-ring ring has a separate numbering system (1’-6’).

FIG. 3.

Soy food processing to commercially available items. The Asian products are largely on the left of the figure. There fermentation of soybeans is very common to make miso, soybean paste and tempeh, as well as soy sauce. Soybeans are used to manufacture full-fat soymilk and tofu. Tofu can be found in several forms depending how much water is squeezed out. Natto Is obtained from the surface layer when generating soymilk. The American products start with a solvent (hexane) extraction procedure to recover the oil from the soybean. The defatted protein-enriched soy flour (50% protein) is the source of soy protein concentrate (70% protein) and soy protein isolate (>90% protein). Soy protein isolate is used to make low-fat soymilk and tofu as well as a fermented isoflavone-protein enriched product.

FIG. 4.

Effect of processing on soy chemistry. The 6”-O-malonate ester of genistin is either hydrolyzed (hot water or aqueous solvent) to genistein or decarboxylated by dry heat to 6”-O-acetylgenistin. Fermentation to release genistein, the aglycone, can also be accompanied by 6- or 8-hydroxylation.

FIG. 5.

The C- and O-glucosides of daidzein. Daidzein undergoes conjugation with glucose either to form the 7-O-glucoside daidzin (as in soybeans) or the 8-C-glucoside puerarin.

FIG. 6.

Bacterial metabolism of daidzein. Daidzein is converted by intestinal microorganisms in most people to form dihydrodaidzein and O-desmethylangolensin. A limited group of people (∼30%) have microorganisms that reduce daidzein to the isoflavan S-(-)-equol. Each of the metabolites has a chiral center at C-3.

FIG. 7.

Orientation of the isoflavone genistein and 17β-estradiol. Genistein has been drawn using the convention of flavonoid investigators (A). This has the A-ring on the left and B-ring on the right. The keto group in the heterocyclic ring points down. Most investigators in isoflavone research have drawn genistein rotated so that the keto group faces up (B). The X-ray crystallographic studies, however, suggest that the correct orientation when comparing genistein to 17β-estradiol (D), is that obtained by rotating structure horizontally, with the B-ring on the left and the A-ring on the right (C).

FIG. 8.

Products of reaction of isoflavones with neutrophil oxidants. Three monochloroisomers (6-, 8- and 3’-Cl) and one mononitro (3’-) have been synthesized and tested in biological experiments.^, Chloronitrogenistein has also been observed. Daidzein R₁ = H; genistein R₁ = OH.

FIG. 9.

Product of the in vitro reaction between genistein and thyroid peroxidase. Similar to the chloroisoflavones (Fig. 8), thyroid peroxidase iodinates genistein in the 6-, 8- and 3’-positions.