HEAVY-METAL MEDIATED INHIBITION OF RBAT-INDUCED AMINO-ACID-TRANSPORT

rBAT, a protein which is located in the brush border membranes of inte stine and renal proximal tubule cells, was recently shown to induce el ectrogenic countertransport of neutral and dibasic amino acids after i ts expression in Xenopus oocytes [1]. Here, we studied the effects of heavy metals on rBAT induced amino acid transport in Xenopus oocytes t o clarify a possible involvement of rBAT in heavy metal-induced aminoa ciduria. The heavy metals Hg2+ and Pb2+ inhibited rBAT-induced amino a cid transport with a different profile of action. The Pb2+ mediated in hibition occurred rapidly upon superfusion and was readily reversible upon washout. The maximal inhibition caused by Pb2+ was about 50% of t he amino acid-induced currents at an apparent affinity (K-m) of about 10 mu M. In contrast, the Hg2+-mediated inhibition occurred rather slo wly, depending on its concentration, and was not reversible during was hout with control solution. However, the Hg2+-mediated amino acid tran sport inhibition could be reversed with Hg2+ chelating agents and redu cing compounds. Other oxidative agents, such as the membrane permeable 2,2'-Dithio-bis(5-Nitropyridine) (DTNP), but not the membrane imperme able 5,5'-Dithio-bis (2-Nitrobenzoic acid) (DTNB), mimicked the effect of Hg2+, and their effect could similarly be reversed with 2,3-Dihydr oxybutane-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 amin oaciduria following mercury or lead intoxication.