The Protective Effect And Mechanism Of Threonine On Cadmium-exposed Yeast And Mice

As a prevalent toxic pollutant in the food chain,cadmium can cause a series of tissue and organs damages during prolonged and high-dose exposure,which does harm to human health.Hence,the way to effectively protect body from the damages induced by cadmium exposure has always been a focus of the food safety all the time.Amino acids are the fundamental substances of dietary protein.Some relative foods,including milk,are always considered to have the capacity of alleviating toxicity of heavy metals effectively.However,the mechanism remains unknown.This study intends to respectively determine the effects of 20 common amino acids on the growth,cadmium accumulation,and oxidative damage in cadmium-exposed yeast.Additionally,the effects of amino acids on weight gain,cadmium accumulation,organ damages and oxidative damages etc were measured in acute-cadmium-exposed mice.Finally,the results revealed the protective mechanism of amino acids on the body under cadmium exposure in vitro experiments.The main contents are as follows:(1)The cadmium exposure experiment in S.cerevisiae cells,threonine is the amino acid with the strongest resistance to cadmium-induced cytotoxicity among the20 basic amino acids.At the same concentration,threonine showed the best alleviation effect on the growth inhibition of cadmium-exposed yeast and the lowest cell death rate.Meanwhile,compared with other amino acid synthesis-deficient strains,threonine synthesis-deficient strains(thr1?,thr4?)showed the more obvious cadmium sensitivity,and after the supplement of threonine,the cadmium sensitivity of thr1? and thr4? is reduced.In the process of revealing the protective effect of threonine on yeast,this project further analyzed the effects of threonine on cadmium accumulation and membrane damages,and we found that the protective effect of threonine was more related to its mitigation effect of membrane damage on cadmium-exposed yeast.Through further analysis of membrane lipid oxidation,superoxide free radical generation,and the comparison of different protective effects of threonine on antioxidant enzymes SOD and CAT under cadmium and hydrogen peroxide exposure.It is found that threonine could reduce the oxidative damage caused by cadmium probably via maintaining the enzyme activity of SOD under cadmium exposure in S.cerevisiae cells.(2)The acute cadmium exposure experiment in mice,threonine has the best effect of alleviating the oxidative damage of organs among the 20 amino acids.This study demonstrated that when the intake concentration of amino acids reached to 40?M/g·day and above,it could significantly increase the weight gain index of acute cadmium-exposed mice,reduce the accumulation of cadmium in various organs,and promote the excretion of cadmium.More importantly,compared to other amino acids,threonine shows a significant alleviation effect on the oxidative damage of various organs in mice exposed to acute cadmium.And threonine significantly improves the MDA level of lipid peroxidation and superoxide anion production in organs of mice exposed to acute cadmium,especially in liver and kidney,which were basically consistent with the experimental results of the eukaryotic cell model.From further analysis,we found that the protective effect of threonine on SOD enzyme was better than that on CAT enzyme in acute cadmium-exposed mice.Among various organs,threonine has the best protective effect on SOD enzyme activity in heart and kidney.Threonine can also relieve cadmium-induced decrease in catalase activity in four organs except the lung to a certain extent.From the histopathological images of hepatocytes,it is found that threonine could also improve liver cell damages caused by acute cadmium exposure,and inhibit the increase of serum AST and ALT activities induced by cadmium.It was further confirmed at the animal level that threonine had a strong protective effect on the body under cadmium exposure,and its protective effect was closely related to its protective to the SOD enzyme.(3)Based on the strong protective effect of threonine on cadmium-exposed yeast cells and mice,this project further explored the anti-cadmium and antioxidant effect of threonine in vitro.In the vitro experiments,threonine significantly improved the decline of SOD enzyme activity caused by cadmium exposure,and the declining results SOD enzyme activity were the same in yeast and animal.Threonine can also maintain the ?-helical structure of animal SOD enzymes.Inducing free radical production is an important pathway for cadmium to reduce SOD enzyme level.From further analysis,compared with Vc,threonine did not show the stronger scavenging ability of DPPH and ABTS oxygen free radical.After analyzing the blocking study of threonine on cadmium-binding SOD enzyme,we found that compared with Vc,threonine had a stronger affinity to cadmium ions,but it had a weaker affinity to other metal ions i.e Cu,Zn required by SOD enzyme.It showed that threonine could block the inhibitory effect of cadmium on SOD enzyme activity by binding to cadmium,thereby reducing the oxidative damages caused by cadmium.In this study,by exploring the protective effects and mechanisms of different amino acids in exposed yeast and mice,we found that threonine could block the inhibitory effect of cadmium on SOD antioxidant enzyme activity by binding to cadmium,and reduced oxidative damages of cells and liver and other organs caused by cadmium exposure,having the protective effect on the boby when exposed to cadmium.These findings will fully reveal the mechanism of amino acid to prevent and control cadmium toxicity,provide important reference data,and provide a new way to effectively protect the body from cadmium pollution in food.