Background
Soy isoflavones (ISO) are secondary metabolites formed during the growth of soybeans. In nature, they mainly exist in two forms: free aglycones and bound glycosides. Aglycones account for 2% to 3% of the total, including genistein, daidzein and glycosides; glycosides account for 97% to 98% of the total, and there are 9 types of them, namely glucoside, malonyl glucoside and acetylglucoside of the above three aglycones.
Figure 1. Classification and structures of soy isoflavones. (José L. Peñalvo, et al.; 2004)
Soy isoflavones are distributed in the seed coat, hypocotyl and cotyledon of soybeans. The content of isoflavones in the hypocotyl is quite high, while the content in the cotyledon and seed coat is lower. Under normal circumstances, the content of isoflavones in the hypocotyl is 10 to 60 times that of the cotyledon, and its content accounts for 30% to 50% of the total content of soybean isoflavones. In addition, isoflavones exist in soybeans mainly in the form of malonyl glycosides; acetyl glycosides and aglucosides are rare.
Physical and Chemical Properties of Soya Isoflavones
Pure soy isoflavones are colourless, crystalline substances with a bitter taste. Soy isoflavones are readily soluble in polar solutions such as acetone, ethanol, methanol, ethyl acetate and dilute alkali, but insoluble in petroleum ether and n-hexane. The water solubility of soy isoflavones is related to their structure. Free soy isoflavones have the worst water solubility and are basically insoluble in water; bound soy isoflavones are generally soluble in water, but genistein is difficult to dissolve in water. In water with a temperature of 40~50°C, its solubility does not change. When the temperature is 70~90°C, its solubility increases significantly with the increase in temperature. Therefore, when soy isoflavones are extracted from aqueous solution, the extraction temperature should be higher than 70°C.
Only free aglycones in soy isoflavones have the highest biological activity. Under heating and alkaline conditions, acyl glucosides in soy isoflavones can be hydrolysed to remove malonyl and acetyl groups and converted to glucosides. The pH of alkaline hydrolysis conditions is 8-13 and the degree of hydrolysis increases with increasing pH and temperature. Under strong acid, high temperature or enzyme conditions, glucosides in soy isoflavones may be hydrolysed to remove glucose groups and converted to glucose ligand forms. The enzyme used in enzymatic hydrolysis is β-glucosidase and the optimum hydrolysis conditions are based on the highest enzyme activity.
Soybean Isoflavones Physiological Functions
Modern medicine has proven that soy isoflavones are closely related to human health and have many physiological functions, such as estrogen-like effects, anti-cancer effects, prevention of cardiovascular diseases, antioxidant activity, prevention of osteoporosis and relief of menopausal syndrome. At the same time, soy isoflavones can also improve the body's immune function.
Natural estrogen refers to a steroid compound containing 18 carbon atoms secreted by humans or mammals, such as estradiol and estrone, which plays an important role in the reproductive regulation of humans and mammals. Compared with natural estrogen secreted by glands, people call estrogen or estrogen-like substances in the outside world exogenous estrogens, among which exogenous estrogens from plants are called phytoestrogens, and soy isoflavones are one of the phytoestrogens. It is mainly found in leguminous plants, with the highest content in soybeans. At present, most researchers believe that the estrogenic effect of soy isoflavones is due to its similar structure to estradiol. However, the estrogenic effect of soy isoflavones is weak, about 1/1 000 of estradiol. Experiments have shown that soy isoflavones can bind to endogenous estrogen receptors, mainly to the β subtype of estrogen receptors (ERβ), and exert estrogen effects. When the endogenous estrogen level is low, it behaves as a weak estrogen agonist; when the estrogen level in the body is high, it competes with endogenous 17β-E2 to occupy the estrogen receptor, showing an anti-estrogen effect.
The main anti-cancer mechanisms of soy isoflavones that have been confirmed include the following aspects: 1. Similar to female estrogen and anti-estrogen effects; 2. Inhibit the activity of cancer-related enzymes, especially tyrosine kinase; 3. In the promotion stage of cancer cell proliferation, it has the effect of inhibiting angiogenesis; 4. Eliminate reactive oxygen, thus having an antioxidant effect; 5. Regulate the cell cycle; 6. Soft wood lignin has the effect of inhibiting the activity of some enzymes related to DNA cutting. In addition, in medical applications, experts generally believe that the use of soy isoflavones alone or in combination with chemotherapy is a relatively effective anti-cancer method.
Bone is an indispensable part of the human body, playing a role in supporting and protecting the human body. At the same time, bones are in a state of renewal throughout the life process. The hormones that affect bone metabolism are mainly parathyroid hormone, calcitonin and 1.25-(OH)2- D3, and estrogen is also involved in the regulation of bone metabolism. Estrogen receptors exist in both osteoblasts and osteoclasts of the human body. Estrogen can directly act on osteoblasts to enhance the bone formation process. The estrogen level in menopausal women decreases, and the osteoblast-promoting effect of estrogen also decreases. At the same time, the osteoclast effect remains unchanged, causing bone metabolism to be in a negative balance state, causing bone calcium to continue to be lost, thus leading to osteoporosis. Therefore, postmenopausal women with osteoporosis often use estrogen supplementation, i.e. estrogen replacement therapy. However, long-term use of estrogen will bring side effects. Soy isoflavones have the effect of estrogen without the side effects of using estrogen.
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Prevention of Cardiovascular Disease
Soy isoflavones have a significant preventive effect on cardiovascular disease. The main mechanisms of action are: 1. soy isoflavones can upregulate LDL receptors, increase LDL receptor activity, accelerate cholesterol clearance and have anti-thrombotic effects; 2. soy isoflavones can reduce plasma total cholesterol and LDL concentrations, promote thyroid hormone secretion, bile acid excretion and have estrogen-like effects; 3. softwood lignin blocks the production of platelet-derived growth factor by inhibiting tyrosine kinase. Softwood lignin blocks platelet-derived growth factor production by inhibiting tyrosine kinase, thereby inhibiting thrombin-induced platelet activation and aggregation, and through inhibition reduces systemic thrombosis associated with atherosclerosis.
Recent research suggests that one of the disease-preventing mechanisms of soy isoflavones is that they have a strong antioxidant effect. Soy isoflavones can increase the activity of antioxidant enzymes in blood and tissues and reduce the levels of lipid peroxides. The mechanism of action may be that soy isoflavones inhibit lipid peroxidation and the generation of reactive oxygen species, and have a significant inhibitory effect on hydrogen peroxide produced by tumor cells induced by the carcinogens TPA and PMA. It can also inhibit 8-OHAG produced by DNA oxidation caused by the fentoa reaction or ultraviolet irradiation, thus preventing DNA molecules from mutating.
Research shows that oestrogen is a key neuroprotective agent with effects against neurodegenerative diseases. Lack of estrogen protection predisposes to Alzheimer's or Parkinson's disease.
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Antibacterial and Anti-Inflammatory
Research shows that soy isoflavones at 0.05% have significant antifungal activity. Soy isoflavones have strong inhibitory effects on bacteria (such as Staphylococcus aureus, Micrococcus luteus, Bacillus cereus, Bacillus subtilis, Listeria monocytogenes, Bacillus pumilus, etc.) and some fungi.
Application of Soya Isoflavones
Soy isoflavones are widely used in general foods (e.g. beverages and chocolate). An American company has developed a concentrate containing 32.2 mg/g soy isoflavones. It is heat stable and can be used not only in beverages but also as a material for health food.
In health food, soy isoflavones are most commonly used in the form of ligands, especially in the Japanese market where such products are more common. Soy isoflavones may prevent osteoporosis and regulate bone metabolism. In addition, the combination of soya isoflavones with minerals, vitamins, etc. has been found to be particularly effective. This type of health food is particularly suitable for post-menopausal women and can also be used as a calcium supplement for young women. Research on the anti-cancer function of soy isoflavones has also made progress, so people are looking forward to the development of various health foods based on the anti-cancer function of soy isoflavones.
Soy isoflavones have the functions of inhibiting cancer, preventing osteoporosis and treating menopausal syndrome in women, and have no side effects. Therefore, the development of soy isoflavones in the pharmaceutical industry has great prospects.
Soy isoflavones can also be used in cosmetics without side effects. Once the sugar moiety of soy isoflavones is hydrolysed, it becomes a lipophilic bioactive compound that can penetrate the skin like oestrogen. Therefore, the use of soy isoflavones can prevent and delay skin ageing.
References
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José L. Peñalvo, et al.; Simplified HPLC method for total isoflavones in soy products. Food Chemistry. 2004, 87(2):297-305.
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Nakai S, et al.; Health Promotion Effects of Soy Isoflavones. J Nutr Sci Vitaminol (Tokyo). 2020, 66(6):502-507.
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Hu C, et al.; Biochemistry and use of soybean isoflavones in functional food development. Crit Rev Food Sci Nutr. 2020, 60(12):2098-2112.
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Wang SY, et al.; Occurrence of isoflavones in soybean sprouts and strategies to enhance their content: A review. J Food Sci. 2022, 87(5):1961-1982.
