IDENTIFICATION OF GLUTATHIONE AS A DRIVING-FORCE AND LEUKOTRIENE C-4 AS A SUBSTRATE FOR OATP1, THE HEPATIC SINUSOIDAL ORGANIC SOLUTE TRANSPORTER

oatp1 is an hepatic sinusoidal organic anion transporter that mediates uptake of various structurally unrelated organic compounds from blood , The driving force for uptake on oatp1 has not been identified, altho ugh a role for bicarbonate has recently been proposed. The present stu dy examined whether oatp1-mediated uptake is energized by efflux (coun tertransport) of intracellular reduced glutathione (GSH), and whether hydrophobic glutathione S-conjugates such as leukotriene C-4 (LTC4) an d S-dinitrophenyl glutathione (DNP-SG) form a novel class of substrate s for oatp1, Xenopus laevis oocytes injected with the complementary RN A for oapt1 demonstrated higher uptake of 10 nM [H-3]LTC4 and 50 mu M [H-3]DNP-SG, and higher efflux of [H-3]GSH (2.5 mM endogenous intracel lular GSH concentration). The oatp1-stimulated LTC4 and DNP-SG uptake was independent of the Na+ gradient, cis-inhibited by known substrates of this transport protein and by 1 mM GSH, and was saturable, with ap parent K-m values of 0.27 +/- 0.06 and 408 +/- 95 mu M, respectively. Uptake of [H-3]taurocholate, an endogenous substrate of oatp1, was com petitively inhibited by DNP-SG, Of significance, oatp1-mediated tauroc holate and LTC4 uptake was cis-inhibited and trans-stimulated by GSH, and [H-3]GSH efflux was enhanced in the presence of extracellular taur ocholate or sulfobromophthalein, indicating that GSH efflux down its l arge electrochemical gradient provides the driving force for uptake vi a oatp1, The stoichiometry of GSH/taurocholate exchange was 1:1, These findings identify a new class of substrates for oatp1 and provide evi dence for GSH-dependent oatp1-mediated substrate transport.