Methylmercury (MeHg) readily crosses cell membrane barriers to reach its target tissue, the brain. Although it is generally assumed that this rapid transport is due to simple diffusion, recent studies have demonstrated that MeHg is transported as a hydrophilic complex, and possibly as an l-cysteine complex on the ubiquitous L-type large neutral amino acid transporters (LATs). To test this hypothesis, studies were carried out in Xenopus laevis oocytes expressing two of the major L-type carriers in humans, LAT1—4F2 heavy chain (4F2hc) and LAT2—4F2hc. Oocytes expressing LAT1—4F2hc or LAT2—4F2hc demonstrated enhanced uptake of [14C]MeHg when administered as the l-cysteine or d,l-homocysteine complexes, but not when administered as the d-cysteine, N-acetyl-l-cysteine, penicillamine or GSH complexes. Kinetic analysis of transport indicated that the apparent affinities (Km) of MeHg—l-cysteine uptake by LAT1 and LAT2 (98±8 and 64±8μM respectively) were comparable with those for methionine (99±9 and 161±11μM), whereas the Vmax values were higher for MeHg—l-cysteine, indicating that it may be a better substrate than the endogenous amino acid. Uptake and efflux of [3H]methionine and [14C]MeHg—l-cysteine were trans-stimulated by leucine and phenylalanine, but not by glutamate, indicating that MeHg—l-cysteine is both a cis- and trans-substrate. In addition, [3H]methionine efflux was trans-stimulated by leucine and phenylalanine even in the presence of an inwardly directed methionine gradient, demonstrating concentrative transport by both LAT1 and LAT2. The present results describe a major molecular mechanism by which MeHg is transported across cell membranes and indicate that metal complexes may form a novel class of substrates for amino acid carriers. These transport proteins may therefore participate in metal ion homoeostasis and toxicity.
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October 2002
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Research Article|
October 01 2002
Transport of a neurotoxicant by molecular mimicry: the methylmercury–l-cysteine complex is a substrate for human L-type large neutral amino acid transporter (LAT) 1 and LAT2 Available to Purchase
Tracey A. SIMMONS-WILLIS;
Tracey A. SIMMONS-WILLIS
Department of Environmental Medicine, University of Rochester School of Medicine, 575 Elmwood Avenue, Box EHSC, Rochester, NY 14642, U.S.A.
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Albert S. KOH;
Albert S. KOH
Department of Environmental Medicine, University of Rochester School of Medicine, 575 Elmwood Avenue, Box EHSC, Rochester, NY 14642, U.S.A.
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Thomas W. CLARKSON;
Thomas W. CLARKSON
Department of Environmental Medicine, University of Rochester School of Medicine, 575 Elmwood Avenue, Box EHSC, Rochester, NY 14642, U.S.A.
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Nazzareno BALLATORI
Nazzareno BALLATORI
1
Department of Environmental Medicine, University of Rochester School of Medicine, 575 Elmwood Avenue, Box EHSC, Rochester, NY 14642, U.S.A.
1To whom correspondence should be addressed (e-mail [email protected]).
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Publisher: Portland Press Ltd
Received:
May 29 2002
Revision Received:
July 02 2002
Accepted:
July 15 2002
Accepted Manuscript online:
July 15 2002
Online ISSN: 1470-8728
Print ISSN: 0264-6021
The Biochemical Society, London ©2002
2002
Biochem J (2002) 367 (1): 239–246.
Article history
Received:
May 29 2002
Revision Received:
July 02 2002
Accepted:
July 15 2002
Accepted Manuscript online:
July 15 2002
Citation
Tracey A. SIMMONS-WILLIS, Albert S. KOH, Thomas W. CLARKSON, Nazzareno BALLATORI; Transport of a neurotoxicant by molecular mimicry: the methylmercury–l-cysteine complex is a substrate for human L-type large neutral amino acid transporter (LAT) 1 and LAT2. Biochem J 1 October 2002; 367 (1): 239–246. doi: https://doi.org/10.1042/bj20020841
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