System B^0,+and the Transport of Thiol-S-Conjugates of Methylmercury

Abstract

Methylmercury (CH 3 Hgϩ ) 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 (CH 3 Hg-S-Cys and CH 3 Hg-S-Hcy, respectively) mimic one or more amino acids at the site of the Na ϩ -dependent amino acid transporter, system B 0,ϩ . In the kidneys, system B 0,ϩ is situated on the luminal plasma membrane of proximal tubular epithelial cells. To test our hypothesis, we measured uptake of CH 3 Hg-S-Cys and CH 3 Hg-S-Hcy in Xenopus laevis oocytes injected with water or capped RNA encoding mouse ATB 0,ϩ . Analyses of time course, substrate specificity, and saturation kinetics showed that the uptake of CH 3 Hg-S-Cys and CH 3 Hg-S-Hcy was 5-to 10-fold greater in oocytes expressing ATB 0,ϩ than in corresponding water-injected controls. Moreover, the transport of CH 3 Hg-S-Cys and CH 3 Hg-S-Hcy was inhibited by substrates transported by system B 0,ϩ . Finally, our data indicate that CH 3 Hg-S-Cys and CH 3 Hg-S-Hcy may mimic of one or more amino acids (e.g., methionine) that are normally transported by system B 0,ϩ . To our knowledge, this is the first report implicating system B 0,ϩ in the transport of any mercuric species.

Inactivation by Hg2+ and methylmercury of the glutamine/amino acid transporter (ASCT2) reconstituted in liposomes: Prediction of the involvement of a CXXC motif by homology modelling

“…HgCl 2 and methylmercury are the most common forms of mercury to which humans are exposed [4,[6][7]; mersalyl has been used as a prototypal hydrophilic mercury compound which is frequently used as a specific reagent for Cys residues of proteins [35]. Preliminary studies performed with hydrophilic SH reagents, suggested that the reconstituted transporter contains thiol groups of cysteine exposed towards the extraliposomal compartment, which corresponds to the extracellular environment; these data correlated well with the predicted hydropathy profile of ASCT2 [27,40].…”

“…14 Differently from other amino acid or anion transporters [2,5,[7][8][9] ASCT2 is not involved in the absorption of mercurials nor as free molecules neither as conjugates. M a n u s c r i p t 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 …”

“…[7]. The homology structural model of the ASCT2 was built using the glutamate transporter homologue from P. horikoshii crystal structure (1XFH) as the template.…”

“…It was reported that the amino acid transport systems LAT1, LAT2 and B°, + , as well as organic anion transporters are able to trasport thiol conjugates of methylmercury, especially as cysteine-S-conjugates [2,5,[7][8][9]. To test whether ASCT2 could transport the mercuric reagents as conjugates with cysteine or N-acetylcysteine, the efflux procedure was used.…”

“…Indeed, it was reported that some transport systems catalyze the transport of mercuric compounds through biological membranes [2,[7][8][9]. Furthermore, interaction of mercuric compounds with transport systems of plasma membrane [1,[10][11][12][13][14][15] and also of mitochondrial membrane [16][17] have been reported to cause alterations of function.…”