A novel ICP-MS strategy identifies arsenite as an inhibitor of selenocysteine-tRNASec charging

Arsenite (As(III)) is a widespread environmental contaminant that increases susceptibility to oxidative stress. We recently reported that As(III) suppresses the induction of glutathione peroxidases (GPx) by various selenium sources in cultured cells; however, its underlying mechanism remains unclear. GPx contains a selenocysteine (Sec) residue essential for catalytic activity, and Sec biosynthesis requires multiple steps of selenium metabolism. Selenite is directly incorporated into the Sec biosynthetic pathway via selenophosphate synthetase 2 (SEPHS2) and utilized for Sec-tRNASec formation. Because Sec-tRNASec decodes UGA codons, impaired synthesis of Sec-tRNASec leads to nonsense-mediated decay or truncated translation of selenoprotein mRNAs. Here, we developed an inductively coupled plasma (ICP)-MS based method to evaluate Sec-tRNASec and found that As(III) inhibits Sec charging of tRNA. As(III) markedly suppressed GPx protein induction with minimal effects on mRNA abundance. As(III) did not affect total tRNASec levels; however, As(III) significantly decreased RNA-bound selenium released by deacylation, indicating reduced Sec-tRNASec formation. These results suggest that As(III) impairs selenoprotein translation by inhibiting Sec charging of tRNA.