Weiwei Zhang,#1,2Xuezhen Jiang,#2Shuang Zhao,#3Xiaojie Zheng,2Jin Lan,1Hexiang Wang,2 and Tzi Bun Ng4
Mercury is considered to be “a global pollutant” and raises concern worldwide. Once mercury enters the body, it will be distributed all over the body but will accumulate in the brain, kidney and liver. To date, no substance originating from edible fungi capable of adsorbing mercury has been reported. We found that the mushroom Grifola frondosa exhibited mercury adsorption capacity. A polysaccharide-peptide (GFPP), displaying the unique N-terminal amino acid sequence of APPGMHQKQQ and 7 partial sequences with high reliability obtained by LC-MS/MS, was isolated by hot-water extraction of its fruiting bodies followed by ion exchange chromatography and gel filtration chromatography. Two rat models were employed to determine the dose and the duration of HgCl2 treatment (given by acute administration or continuous treatment) to test if G. frondosa could promote mercury elimination. For rats subjected to acute treatment with HgCl2, both GFPP and G. frondosa fruiting bodies (GFFF) could accelerate the decline of blood mercury level, which fell precipitously by 50% on the second day. GFPP and GFFF also promoted elimination of the burden of mercury in the liver and kidneys. For rats receiving continuous HgCl2 treatment, G. frondosa prevented the progressive increase of blood mercury level, and kept the blood mercury level within a relatively stable range.
Introduction
Mercury is an element in the natural environment. It existed in natural water, atmosphere and soil at very low concentrations before the rapid development of industries. Due to its wide industrial applications, a large amount of mercury was discharged into the air, water and soil as a pollutant which threatens health. Mercury with different valences can be present in solid, liquid and gas forms in the environment, and mercury in different states can enter the body through different channels, especially via respiration and dietary intake. In organisms, mercury exists in two forms, inorganic and organic1. Mercury mainly exists in the inorganic form in plants but in the organic form in animals and microbes.
According to the clinical manifestations, mercury could bring about various health hazards including neurotoxicity, reproductive toxicity and injury to the renal, hepatic, respiratory and immune systems2. Due to the high lipid solubility, organic mercury can traverse the blood brain barrier and accumulates in the brain3,4. Even when exposed to low levels of mercury the nervous system can be damaged. The symptoms of neurotoxicity include not only mental, language, and behavioral disorders, but also sensory abnormalities, mental retardation and ataxia. Organic mercury can also damage the fetus after traversing the placental barrier and affect the reproductive function after passing through the blood-testis barrier5. The reproductive effects of mercury on male animals are produced by affecting the sex gland and endocrine function. The nucleic acid and subcellular structure of the reproductive cells treated by mercury are adversely affected to different extents, leading to cytotoxicity, interfering with supply to the cells, reduced sperm count and decreased sperm motility6. The reproductive effects of mercury on female animals are mainly observed in an extended estrus cycle, resulting in an ovulation barrier and delayed corpus luteum formation7,8. Exposure to mercury can disrupt DNA synthesis in reproductive cells, retard cell division, alter subcellular structure, induce abnormal energy metabolism, destroy fertilizing ability, and undermine reproductive ability. The liver and kidney are the organs in which mercury accumulates resulting in acute injury and oxidative stress9.
Mercury poisoning has been studied in humans and a diversity of animals encompassing monkeys, pigs, sheep, cows, horses, dogs, cats, rabbits, mice, chickens, ducks, quails, fish, and other animals. Among these animals, rats were the main subjects. There are also numerous studies on mercury removal from the body, in which two main categories of mercury drugs were used, chemicals and natural extracts. Chemical drugs against mercury include antioxidants like buthionine sulfoximing (BSO), glutathione (GSH), vitamin C, taurine and metal chelating agents such as 2, 3-dimercapto-1-propanol (BAL), 2, 3-dimercaptosuccinic acid (DMSA) and sodium 2, 3-dimercato-1-propaneulfonate (DMPS)10–12. DMSA and DMPS are good clinical drugs for removing organic and inorganic mercury, respectively. The side effects of chemical drugs have been proven. However, considering the toxic and side effects of chemical drugs, natural extracts for mercury removal have attracted considerable interest. Lycopene, procyandin and sulforaphane are antioxidants which protect against the oxidative damage and renal injury caused by mercury13–15. Luteolin, widespread in plants, not only curtailed the accumulation of renal mercury and promoted mercury excretion but also attenuated oxidative stress, inflammation and apoptosis16–18. Besides, Smilax glabra, schisandrin b and Hippophae rhamnoides oil were also reported to have a mercury-removing effect.
Grifola frondosa, a wild mushroom known worldwide, with a nice external appearance a delicious taste, and an abundance of nutrients, has a high value for food and medicinal purposes. The major ingredients of G. frondosa are polysaccharides. As an effective immune regulator, G. frondosa polysaccharide activates immune cells such as macrophages, NK cells and T lymphocytes, and also promotes secretion of cytokines including IL-2, IL-8, IL-12, TNF-α and others19–21. These immunoregulatory effects enable G. frondosa polysaccharide to have antitumor and antivirus capabilities. The hypoglycemic22,23, hypolipidemic24, hypotensive25,26, antioxidant27, hepatoprotective28, anti-radiation29, and antibacterial effects30 of G. frondosa polysaccharide have also been reported.
In this study, a polysaccharide-peptide was isolated from fruiting bodies of G. frondosa which was proved to have a mercury eliminating effect both in vitro and in vivo. This suggests that this edible mushroom could serve as a new resource for developing antidotes for heavy metal intoxication which can be used with safety on a long term basis.