Heavy metal mediated inhibition of rBAT-induced amino acid transport

Abstract

rBAT, a protein which is located in the brush border membranes of intestine and renal proximal tubule cells, was recently shown to induce electrogenic countertransport of neutral and dibasic amino acids after its expression in Xenopus oocytes. Here, we studied the effects of heavy metals on rBAT induced amino acid transport in Xenopus oocytes to clarify a possible involvement of rBAT in heavy metal-induced aminoaciduria. The heavy metals Hg2+ and Pb2+ inhibited rBAT-induced amino acid transport with a different profile of action. The Pb2+ mediated inhibition occurred rapidly upon superfusion and was readily reversible upon washout. The maximal inhibition caused by Pb2+ was about 50% of the amino acid-induced currents at an apparent affinity (Km) of about 10 microM. In contrast, the Hg(2+)-mediated inhibition occurred rather slowly, depending on its concentration, and was not reversible during washout with control solution. However, the Hg(2+)-mediated amino acid transport inhibition could be reversed with Hg2+ chelating agents and reducing compounds. Other oxidative agents, such as the membrane permeable 2,2'-Dithio-bis(5-Nitropyridine) (DTNP), but not the membrane impermeable 5,5'-Dithio-bis (2-Nitrobenzoic acid) (DTNB), mimicked the effect of Hg2+, and their effect could similarly be reversed with 2,3-Dihydroxybutane-1,4-dithiol (DTE). In conclusion, Pb2+ and Hg2+ inhibit rBAT-induced amino acid transport in a noncompetitive, allosteric fashion. Blockade of rBAT-induced amino acid transport may be involved in aminoaciduria following mercury or lead intoxication.