Vol. 62, Issue 4, 921-926, October 2002

Identification of a Mechanism by Which the Methylmercury Antidotes N-Acetylcysteine and Dimercaptopropanesulfonate Enhance Urinary Metal Excretion: Transport by the Renal Organic Anion Transporter-1

Albert S. Koh, Tracey A. Simmons-Willis, John B. Pritchard, Steven M. Grassl, and Nazzareno Ballatori

Department of Environmental Medicine, University of Rochester School of Medicine, Rochester, NY (A.S.K., T.A.S.-W., N.B.); Laboratory for Pharmacology and Chemistry, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina (J.B.P.); and Department of Pharmacology, State University of New York Upstate Medical University, Syracuse, New York (S.M.G.)

N-Acetylcysteine (NAC) and dimercaptopropanesulfonate (DMPS) are sulfhydryl-containing compounds that produce a dramatic acceleration of urinary methylmercury (MeHg) excretion in poisoned animals, but the molecular mechanism for this effect is unknown. NAC and DMPS are themselves excreted in urine in high concentrations. The present study tested the hypothesis that the complexes formed between MeHg and these anionic chelating agents are transported from blood into proximal tubule cells by the basolateral membrane organic anion transporters (Oat) 1 and Oat3. Xenopus laevis oocytes expressing rat Oat1 showed increased uptake of [¹⁴C]MeHg when complexed with either NAC or DMPS but not when complexed with L-cysteine, glutathione, dimercaptosuccinate, penicillamine, or -glutamylcysteine. In contrast, none of these MeHg complexes were transported by Oat3-expressing oocytes. The apparent K_m values for Oat1-mediated transport were 31 ± 2 µM for MeHg-NAC and 9 ± 2 µM for MeHg-DMPS, indicating that these are relatively high-affinity substrates. Oat1-mediated uptake of [¹⁴C]MeHg-NAC and [¹⁴C]MeHg-DMPS was inhibited by prototypical substrates for Oat1, including p-aminohippurate (PAH), and was trans-stimulated when oocytes were preloaded with 2 mM glutarate but not glutamate. Conversely, efflux of [³H]PAH from Oat1-expressing oocytes was trans-stimulated by glutarate, PAH, NAC, DMPS, MeHg-NAC, MeHg-DMPS, and a mercapturic acid, indicating that these are transported solutes. [³H]PAH uptake was competitively inhibited by NAC (K_i of 2.0 ± 0.3 mM) and DMPS (K_i of 0.10 ± 0.02 mM), providing further evidence that these chelating agents are substrates for Oat1. These results indicate that the MeHg antidotes NAC and DMPS and their mercaptide complexes are transported by Oat1 but are comparatively poor substrates for Oat3. This is the first molecular identification of a transport mechanism by which these antidotes may enhance urinary excretion of toxic metals.