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.
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 [(14)C]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 ( K (m)) of MeHg-L-cysteine uptake by LAT1 and LAT2 (98+/-8 and 64+/-8 microM respectively) were comparable with those for methionine (99+/-9 and 161+/-11 microM), whereas the V (max) values were higher for MeHg-L-cysteine, indicating that it may be a better substrate than the endogenous amino acid. Uptake and efflux of [(3)H]methionine and [(14)C]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, [(3)H]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.
Department of Environmental Medicine, University of Rochester School of Medicine, 575 Elmwood Avenue, Box EHSC, Rochester, NY 14642, U.S.A.
Tracey A
TA
Simmons-Willis
Albert S
AS
Koh
Thomas W
TW
Clarkson
Nazzareno
N
Ballatori
D027182:Amino Acid Transport System y+; D000596:Amino Acids / Q000378:metabolism; D000704:Analysis of Variance; D000818:Animals; D001692:Biological Transport; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D003545:Cysteine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000494:pharmacology*; D018076:DNA, Complementary / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D027301:Fusion Regulatory Protein 1, Light Chains / Q000378:metabolism*; D006801:Humans; D007477:Ions; D007700:Kinetics; D027283:Large Neutral Amino Acid-Transporter 1 / Q000378:metabolism*; D008715:Methionine / Q000378:metabolism / Q000494:pharmacology; D008767:Methylmercury Compounds / Q000494:pharmacology*; D009498:Neurotoxins / Q000494:pharmacology*; D009865:Oocytes / Q000378:metabolism; D011485:Protein Binding; D018075:RNA, Complementary / Q000378:metabolism; D013997:Time Factors; D014982:Xenopus laevis / Q000378:metabolism
D027182:Amino Acid Transport System y+; D000596:Amino Acids; D018076:DNA, Complementary; D027301:Fusion Regulatory Protein 1, Light Chains; D007477:Ions; D027283:Large Neutral Amino Acid-Transporter 1; D008767:Methylmercury Compounds; D009498:Neurotoxins; D018075:RNA, Complementary; C420646:SLC7A8 protein, human; C030589:methylmercury cysteine; D008715:Methionine; D003545:Cysteine