Metallothionein and mercury tolerance in the killifish, fundulus heteroclitus

Synthesis of metallothionein (MT) as a possible mechanism by which Fundulus heteroclitus adapts to heavy metal pollution has been investigated in relation to responses to methylmercuric chloride (meHg) and HgCl₂(Hg²⁺) at different life stages.Clutches of embryos from unpolluted sites exhibit teratogenic responses to a standard dose of meHg or of Hg²⁺ which range from little or no effect (tolerant) to severely affected or almost complete inhibition of development (susceptible). At the time of hatching, untreated embryos from tolerant clutches had twice as much of a protein which co-migrates in SDS-PAGE gels with Fundulus MT as do those of susceptible clutches. This difference could be a mechanism for conveying tolerance to Hg²⁺ and also to meHg if demethylation occurs. However, eggs at the time of deposition had virtually no MT, suggesting that the protein is synthesized too late to provide tolerance in the early stages of development which are critical in the etiology of malformations. Treatment of embryos with non-teratogenic doses of either meHg or Hg²⁺ did not increase the amount of MT-like protein. This may indicate that the thionein gene is already fully active, and would also explain observations that Hg²⁺ pretreatment of embryos does not enhance tolerance to Hg²⁺ in the larval stage.MT, isolated from livers of Hg²⁺- or Cu²⁺-dosed or control adults, is associated with Cu and Zn but not with Hg, although it can bind Hg²⁺in vitro. Fundulus, like other fish, methylates much (ca. 75 %) of its Hg burden, and meHg is considered by some not to react with MT. Fundulus liver has its Hg burden distributed between homogenate sediment (25 %), the cytosol (60 %) and the remainder in lipid. The liver responds to Hg²⁺ treatment by increasing lipid content. This may allow sequestration of some lipid-soluble meHg. Of the cytosolic Hg, most is associated with larger proteins and some (ca. 25 %) is dialyzable.These data suggest that metallothionein is not a significant factor in Hg tolerance in Fundulus adults, and has a questionable role in embryonic and larval tolerance to Hg²⁺ and none to the more toxic meHg.