Demethylation of Methylmercury in Bird, Fish, and Earthworm

Toxicity of methylmercury (MeHg)
to wildlife and humans results
from its binding to cysteine residues of proteins, forming MeHg-cysteinate
(MeHgCys) complexes that hinder biological functions. MeHgCys complexes
can be detoxified in vivo, yet how this occurs is
unknown. We report that MeHgCys complexes are transformed into selenocysteinate
[Hg­(Sec)4] complexes in multiple animals from two phyla
(a waterbird, freshwater fish, and earthworms) sampled in different
geographical areas and contaminated by different Hg sources. In addition,
high energy-resolution X-ray absorption spectroscopy (HR-XANES) and
chromatography-inductively coupled plasma mass spectrometry of the
waterbird liver support the binding of Hg­(Sec)4 to selenoprotein
P and biomineralization of Hg­(Sec)4 to chemically inert
nanoparticulate mercury selenide (HgSe). The results provide a foundation
for understanding mercury detoxification in higher organisms and suggest
that the identified MeHgCys to Hg­(Sec)4 demethylation pathway
is common in nature.