Study On Toxicity Of Cadmium And Zinc On Gills In Freshwater Crab Sinopotamon Henanense

Cadmium (Cd2+) is a heavy metal which is toxic and easy to accumulate in the body. Exposure to Cd2+ can damage the body because it causes the body to produce reactive oxygen species (ROS). Zinc (Zn2+) is an essential element of the organism and many important enzymes are zinc enzymes which maintain the structure and function of biological protein in the cell. Zn2+ and Cd2+ are often coexisted in the aquatic environment and prone to demonstrate the substitution effect because of the similar chemical properties. Cd2+ affects the metabolism of Zn2+ on its toxicity while Zn2+ shows the synergistic or antagonistic effects on Cd2 toxicity. At present the mechanism of single heavy metal has been understood well, but the mechanism of joint toxicity of heavy metals in the environment needs to be further studied.The low-concentration sub-chronic toxicity-testing method is introduced in this paper. The damage of Cd2+ and the joint effect of Cd2+ and Zn2+ on the freshwater crab(Sinopotamon henanense) gills are researched carefully, which include the following three parts.(1)Laboratory-reared Sinopotamon henanense is exposed to the heavy metals for 14d and 28d, and the activity of antioxidant enzymes included superoxide dismutase (SOD) and catalase (CAT) and the content of the lipid peroxidation products (MDA) and metallothionein (MT) in the gills are also measured accurately. The results showed that the activity of SOD and CAT in gills increased significantly under the combined exposure of Cd2+ and Zn2+ compared with the same concentration of Cd2+. However, the effect is opposite with the high concentrations of Cd2+. The content of MDA decreases significantly and the MT demonstrates the rising trend compared with the same concentration of Cd.(2) To investigate the effects of cadmium and zinc exposure on the bioaccumulation of cadmium in the gills of the freshwater crab, laboratory-reared Sinopotamon henanense were exposed to either cadmium alone or the combination of cadmium and zinc at different concentrations and sampled at 14 days and 28 days. The collected gills were homogenized and by using the subcellular fractionation method separated into the biological detoxified metals (BDM), biologically active metals (BAM) and the cellular debris (CD). The cadmium accumulation in each part was measured by the flame atomic absorption spectrophotometry (AAS). The results demonstrated that BDM contains 50% of cadmium in all the parts,suggesting it is an important fraction for cadmium accumulation. The heat-stable protein (HSP) and metal-rich granules (MRG) consist of biological detoxified metals (BDM). Either high (1000 μg·L-1) or low zinc (100μg·L-1) promotes cadmium accumulation in the HSP and MRG at low exposure concentration of cadmium, while inhibiting its accumulation at high exposure concentration. In BAM, either high or low zinc plays promoting effects at certain degrees on cadmium accumulation, with low concentration of Zn showing better performance than the high Zn.(3) The uptake pathways of cadmium and Zinc in Sinopotamon henanense which is an important environmental monitoring species is very important for the joint toxicity mechanism research of cadmium and Zinc. Three Cd2+ exposure concentrations (50μg·L-1、100μg·L-1、500μg·L-1) and two Zn2+ concentrations (100μg·L-1、1000μg·L-1) are designed and the Ca2+ channel inhibitor LaCl3 and thiol inhibitor N-ethyl-maleimide (NEM) are used in this paper. The Cd2+ and Zn2+ absorption mechanism of Sinopotamon henanense is researched based on the subcellular fractionation methods.The results showed that the calcium channel inhibitors could significantly inhibit the absorption of Cd2+ while NEM hads a significant inhibitory effect on Cd2+ and Zn. The subcellular analysis shows that LaCl3 significantly inhibit Cd2+ distribution in’the cell debris (CD) and organelles (ORG) while NEM significantly inhibited the Cd2+ distribution in the heat-stable proteins (HSP). NEM inhibited the Zn2+ distribution in the heat denatured protein (HDP) and the two inhibitors can both promote the Zn2+ metabolism.The conclusions of this paper are as follows.(1) Zn2+ significantly inhibits the oxidative damage of Cd2+ on the organism. The protective effect of Zn2+ on the organism is related to the activity protection of Zn2+ on SOD and CAT and also by inducing the synthesis of MT.(2) Zn2+ has a certain effect on the accumulation and distribution of Cd2+ under the combined exposure of Cd2+ and Zn2+ in the gills of Sinopotamon henanense. At low exposure concentrations of Cd2+, Zn2+ promotes the accumulation of Cd2+ in biological detoxification portion (BDM) while at high concentrations and long processing time, Zn2+ gradually shows the inhibitory effection. Zn2+ promotes the accumulation of Cd2+ in BAM and the promotion of low Zn2+ exposure concentration is more predominant.(3) Cd2+ and Zn2+ interact with each other mainly by the mercapto channel and Zn2+ has a strong ability of self-regulation.