Effects of cadmium, lead, copper and their combinations on selected biomarkers in Xenopus laevis tadpoles

The aim of this study was to determine the effects of cadmium (Cd), lead (Pb), copper (Cu), and their mixtures on Xenopus laevis tadpoles. For this aim, the application concentrations of metals were determined according to their acceptable limit values in drinking water set forth by the European Union (EU) and the 96-hour median letal concentration values (LC50). Given these, tadpoles in stage 46 were exposed to 0.005 (EU limit value), 0.52 (LC50/10), 2.59 (LC50/2) and 5.18 mg/L (LC50) Cd; 0.01 (EU limit value), 12.3 (LC50/10), 61.53 (LC50/2) and 123.05 mg/L (LC50) Pb; 0.01, 0.085 (LC50/10), 0.425 (LC50/2) and 0.85 mg/L (LC50) Cu and the metals were applied as double (1:1) and triple (1:1:1) metal mixtures in 96-hour static renewal test system. In addition to the 96-hour tests, tadpoles were also exposed to metal mixtures for 24-hours using LC50/2 and LC50 values. Effects of metals on glutathione S-transferase (GST), glutathione reductase (GR), acethylcholinesterase (AChE), carboxylesterase (CaE), glutathione peroxidase (GPx), catalase (CAT) levels and amount of metallothionein (MT) in tadpoles were determined. The results from 96-hour metal or metal mixture applications showed that all tadpoles died when exposed to LC50 value mixtures of Pb and Cd. Also, all tadpoles died when exposed to LC50/2 or LC50 value of metals in Pb+Cu, Cd+Cu and Pb+Cd+Cu mixtures. The LC50/2 and LC50 values of metals and the lowest concentrations of metal mixtures significantly affected selected enzymatical biomarkers. Also, the amount of MT significantly increased with exposure concentrations. The GST, GR and CAT activities were significantly affected and amount of MT significantly increased with the exposure to mixtures of metals using the application concentrations of LC50/2 or LC50 of 96-hour exposure time for 24-hour exposure for all tadpoles. The results indicated that metal and metal mixtures have toxic potential on X. laevis tadpoles in stage 46. Furthermore, when applied as mixtures, the metals showed a toxic effect on tadpoles at concentrations much lower than their LC50/2 or LC50 values. Also, our results showed that X. laevis tadpoles can be used as a suitable test organism and the selected biochemical markers may also serve well for studies with the aim of determining metal pollution in aquatic ecosystems. KEY WORDS: Cadmium, Lead, Copper, Metal toxicity, Xenopus laevis, Enzyme.