A Comparison Of Tri-And Hexavalent Chromium On Genetic Damage

In the process of industrialization, heavy metals are widely used and released into the environment. These metals are difficult to be degraded biochemically, and thus threaten the health of human beings through the food chain. Therefore, increasing attention has been paid worldwide to heavy metal pollution problems. Internationally, Cr is considered to be one of the four major pollutants together with Hg, Cd, and Pb, which exists mostly in two valence states in nature: hexavalent chromium [chromium(Ⅵ)] and trivalent chromium [chromium(Ⅲ)]. Cr(Ⅵ) species are considered as the most toxic form of chromium, and confirmed as a human carcinogen by the International Agency for Research on Cancer (IARC) in1987. Cr(Ⅲ) is acknowledged much less toxic, and have the therapeutic effect on diabetes. However, Cr(Ⅲ) as essential trace elements remains controversial. The special biochemical role of chromium has yet to be identified.In this study, by using yeast SUP4-o mutagenesis assay system, the mutation rate of the yeast strain SJR576-p induced by Cr(Ⅲ) and Cr(Ⅵ) was detected. The result indicated that150uM CrCl3and300μM CrO3induced similar mutation frequencies. It is about2.5×10-5and almost five times of spontaneous mutation frequency. It suggests that the degree of genetic damage to the yeast cells of Cr(Ⅲ) is not less than Cr(Ⅵ). Furthermore, intracellular DNA damage was detected in yeast cells and animal cells (Jurkat) treated with different valence states of chromium salts by Single Cell Gel Electrophoresis. The result showed that DNA damadge of Cr(Ⅲ) is stronger than that of Cr(Ⅵ) in both cells..Previous studies have demonstrated that glutathione is an important intracellular reductive agent of Cr(Ⅵ). Here we investigated the effect of three kinds chromium salts (potassium dichromate, chromium trioxide, chromium chloride) on intracellular GSH content. The results showed that potassium dichromate and chromium trioxide, while not chromium chloride, reduced the intracellular GSH content. It indicated that there are different intracellular biochemical actions between Cr(Ⅲ) and Cr(Ⅵ) and their detoxification mechanisms are also different.In vitro, three kinds of chromium salts was used to study the cleaving damage of the supercoiled and linearized plasmid DNA. It showed that K2Cr2O7almost have no damage to double helix structure of plasmid DNA, while80μM CrO3induced partial damage and80μM CrCl3almost completely degraded the plasmid. Therefore, Cr(Ⅲ) caused the maximum damage. By using Circular dichroism, EB and SYBR competitive binding assay, and similar chemical bond phosphorus ester bond cleavage induction experiments, the interactions of the two kinds of valence states of chromium with DNA were analyzed. It demosntrated that Cr(Ⅲ) may combine to the minor groove and phosphate backbone of supercoiled DNA, thereby inducing phosphorus ester bond cleavage, DNA fragmentation, and blocking EB-bound DNA. While Cr(Ⅵ) induced DNA damage mainly through the reactive oxygen species (ROS) generated by the valence change.In conclusion, the genetic damage induced by Cr(Ⅲ) is not less than that of Cr(Ⅵ) either in vivo or in vitro. Therefore, the environmental endangerment caused by Cr(Ⅲ) can not be ignored. Our study provided the basic data for the genetic damage mechanism of Cr(Ⅲ) and Cr(Ⅵ), and help us to further understand the action mode of Cr(Ⅲ) in genetic damage.