We have isolated a novel liver-specific organic anion transporter, LST- 1, that is expressed exclusively in the human, rat, and mouse liver. LST-1 is a new gene family located between the organic anion transporter family and prostaglandin transporter. LST-1 transports taurocholate (K(m) = 13.6 μM) in a sodium-independent manner. LST-1 also shows broad substrate specificity. It transports conjugated steroids (dehydroepiandrosterone sulfate, estradiol- 17β-glucuronide, and estrone-3-sulfate), eicosanoids (prostaglandin E2, thromboxane B2, leukotriene C4, leukotriene E4), and thyroid hormones (thyroxine, K(m) = 3.0 μM and triiodothyronine, K(m) = 2.7 μM), reflecting hepatic multispecificity. LST-1 is probably the most important transporter in human liver for clearance of bile acids and organic anions because hepatic levels of another organic union transporter, OATP, is very low. This is also the first report of the human molecule that transports thyroid hormones.
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