Received for publication June 7, 2005.
Revised August 1, 2005.
Accepted for publication August 2, 2005.

Handling of the Homocysteine S-Conjugate of Methylmercury by Renal Epithelial Cells: Role of OAT1 and Amino Acid Transporters

Abstract

Recently, the activity of the organic anion transporter 1 (OAT1) protein has been implicated in the basolateral uptake of inorganic mercuric species in renal proximal tubular cells. Unfortunately, very little is known about the role of OAT1 in the renal epithelial transport of organic forms of mercury, such as methylmercury (CH₃Hg⁺). Homocysteine (Hcy) S-conjugates of methylmercury (CH₃Hg-Hcy) have been identified recently as being potentially important biologically relevant forms of mercury. Thus, the present study was designed to characterize the transport of (CH₃Hg-Hcy) in Madin-Darby canine kidney (MDCK) cells (which are derived from the distal nephron) that were transfected stably with the human isoform of OAT1 (hOAT1). Data on saturation kinetics, time-dependency, substrate specificity and temperature-dependency demonstrated that (CH₃Hg-Hcy) is a transportable substrate of hOAT1. However, substrate-specificity data from the control MDCK cells also showed that (CH₃Hg-Hcy) is a substrate of one or more transporter(s) that is/are not hOAT1. Additional findings indicated that at least one amino acid transport system was likely responsible for this transport. Interestingly, the activity of amino acid transporters accounted for the greatest level of uptake of (CH₃Hg-Hcy) in the hOAT1-expressing cells. Furthermore, rates of survival of the hOAT1-transfected MDCK cells were significantly lower than those of corresponding control MDCK cells when they were exposed to cytotoxic concentrations of (CH₃Hg-Hcy). Collectively, the present data indicate that (CH₃Hg-Hcy) is a transportable substrate of OAT1 and amino acid transporters, and thus, is likely a transportable mercuric species taken up in vivo by proximal tubular epithelial cells.

Key words: Amino Acid Transporters, Homocysteine, Homocysteine S-conjugates, Methylmercury, OAT1, Transport