The function of metallothionein in Caenorhabditis elegans: Is detoxification of copper or of cadmium more important?

Much of the literature on metallothioneins (MT) focuses on their role in protecting against Cd toxicity. Here we compare the effects of Cu, Hg, Ag, Cr, Pb, Zn and Cd on survival, growth and reproduction of wild type and mtl-2 deleted individuals of Caenorhabditis elegans. The largest difference was in the survival of animals exposed to Cu, where mtl-2 deleted worms were 100 times more sensitive. LC50 values for Pb did not differ and were between 1.4 and 5 times greater for wild type worms exposed to other metals. The two strains showed only small (less than 3-fold) differences in the effects of metals on reproduction and growth. After 72 hours of exposure, the LC for copper was 0.06 µM for 50 mtl-2 deleted worms. Cu concentrations in excess of this are regularly found in soil pore water, indicating that a major role of MT in wild populations of C elegans is the prevention of acute Cu toxicity. By contrast, effects of Cd on survival, reproduction and growth occur only at concentrations that are between 1400 and 80000 higher than those reported in soil pore water.
This suggests that the reduction of Cd toxicity is not an important function of MT in C. elegans, even though MT gene expression is very responsive to Cd.