Study On The Induction Of Metallothionein Synthesis In Ruditapes Philippinarum Exposed To Rare Earth Elements

Metallothioneins (MTs) are a class of protein characterized by a low molecular weight, thiol-rich, high cysteine content, and could be induced by certain metals. There is growing evidence that MT in aquatic organisms can be induced easily by heavy metals, and there has a significant correlation between this induction and metal concentrations.At present, bivalves such as Mytilus galloprovincialis and Ruditapes decussates are mostly used in monitoring heavy metal pollution according to MT levels.China is rich in rare earth resources as well as consumption. The developing use in agriculture, animal husbandry, industrial and modern biomedicine have made more and more rare earth elements(REEs) releasing in the environment. Through surface runoff,dry deposition and wet precipitation, REEs can transfer to the ocean and impact their distribution in the coastal water. High concentration of REEs present metal ions characteristic. Whether rare earth ions can induce MT synthesis in marine organisms may limit the use of MT as a biomarker of heavy metal pollution in seawater. In ecotoxicology there are some arguments that whether REEs can produce similar MT induction effect as heavy metals, and for marine bivalves there have no reports yet.Using Ruditapes philippinarum as experimental animal, the influences of exposure concentration and duration are studied in relation to MT content in the gills and viscera after rare earth ion (La3+,Ce3+,Nd3+)-exposed, according to results above, the possibility whether they can induce MT synthesis can be analysised. And then, to obtain the optimal induction conditions, the change of MT.levels with exposure time, salinity, pH value are measured. Furthermore, base on this condition, the qualitative researches are conducted by UV absorption spectrum scanning and SDS-PAGE, to find the difference between MT induced by REEs and heavy metals.At last, to discuss the inducing mechanisms of MT by REEs, the competitive combination with rare earth ions and MT induced by heavy metal is explored in vitro experiments.The main conclusions are listed as follows:(1)La3+ and Nd3+ exposure can induce MT synthesis, and there was certain limit. For gills, MT production was accelerated by La3+ of 1×10-1～1×102μg/L obviously,for viscera, all La3+ treatments from 1×10-1μg/L to 1×104μg/L had significant induction of MT which contents were almost constant, MT content reached the highest level after exposed to La3+(1×102μg/L) for 5d; All Nd3+ treatments from 1×10-1μg/L to 10μg/L had significant induction of MT, MT content reached the highest level after exposed to Nd3+(lμg/L) for 7d; There are no significant inducement of MT in all Ce3+ treatments, most of them show decrease indeed. It was noticeable that the concentration of La3+ and Nd3+ in some coastal water of China had reached the level to induce MT synthesis in bivalves.(2) Abiotic factors such as exposure temperature, salinity, pH value can influence the MT induction. Under the deal (1×102μg/L La3+ expose for 5d,1μg/L Nd3+ for 7d), the optimal induction conditions are:temperature 10℃,salinity 20 (La3+) or 25 (Nd3+), pH value 8.5.(3)The qualitative research on MT induced by La3+and Nd3+ showed that there is certain different between MT induced by REEs and heavy metals:the characteristic peaks of MT-like induced by La3+ was at 258nm,and it is expressed mainly as dimer form and trimer form which the molecular was 13.1kD and 18.3kD;Be similar to La3+, MT-like induced by Nd3+ also have a characteristic peaks at 258nm, and it is expressed mainly as dimer form and trimer form which the molecular was 13.0kD and 18.1kD.The characteristic peaks of MT induced by heavy metals are:Zn-MT 220nm, Cd-MT 250nm,Cu-MT 275nm. The inducement of rare earth ions can limit the use of MT as a biomarker of heavy metal pollution in seawater, however, the diversity of characteristic peak may provide a avenue to avoid these interferences.(4) La3+ and Nd3+can substitute small amount of Cd2+ from Cd-MT in vitro,but this substituting effects do not make significant changes in the structure.All above results show that rare earth ions can induce MT synthesis in marine bivalves, which can limit the use of MT as a biomarker of heavy metal pollution, however, the diversity in quality can lead a way to eliminate the interference.