System B0,+and the Transport of Thiol-S-Conjugates of Methylmercury

Methylmercury (CH₃Hg⁺) is a clinically relevant toxicant that is the most abundant form of mercury found in the environment. After exposure, it accumulates in the kidneys, liver, and central nervous system. The mechanisms by which this toxicant is taken up by target cells are only now beginning to be understood. Some experimental data support a hypothesis involving molecular mimicry, whereby thiol conjugates of methylmercury (especially a cysteine S-conjugate) mimic one or more amino acids and are transported into target cells by amino acid transporters. In the present study, we tested the hypothesis that Cys and homocysteine (Hcy) S-conjugates of methylmercury (CH₃Hg-S-Cys and CH₃Hg-S-Hcy, respectively) mimic one or more amino acids at the site of the Na⁺-dependent amino acid transporter, system B^0,+. In the kidneys, system B^0,+ is situated on the luminal plasma membrane of proximal tubular epithelial cells. To test our hypothesis, we measured uptake of CH₃Hg-S-Cys and CH₃Hg-S-Hcy in Xenopus laevis oocytes injected with water or capped RNA encoding mouse ATB^0,+. Analyses of time course, substrate specificity, and saturation kinetics showed that the uptake of CH₃Hg-S-Cys and CH₃Hg-S-Hcy was 5- to 10-fold greater in oocytes expressing ATB^0,+ than in corresponding water-injected controls. Moreover, the transport of CH₃Hg-S-Cys and CH₃Hg-S-Hcy was inhibited by substrates transported by system B^0,+. Finally, our data indicate that CH₃Hg-S-Cys and CH₃Hg-S-Hcy may mimic of one or more amino acids (e.g., methionine) that are normally transported by system B^0,+. To our knowledge, this is the first report implicating system B^0,+ in the transport of any mercuric species.