Previous studies have identified the ATP-dependent export of glutathione conjugates as a physiological function of the multidrug resistance protein (MRP). The involvement of MRP in the transport of endogenous and xenobiotic conjugates was investigated further using membrane vesicles from MRP-transfected HeLa cells. The ATP-dependent transport of the glutathione conjugates [3H]leukotriene C4, S-(2,4-dinitrophenyl)-[3H]glutathione, and 3H-labeled oxidized glutathione was characterized by determination of the transport efficiency Vmax:Km amounting to 1031, 114, and 7.1 ml × mg protein-1 × min-1, respectively. Additional endogenous substrates for MRP-mediated transport included the steroid conjugate 17β-glucuronosyl [3H]estradiol and the bile salt conjugates [6α-14C]glucuronosylhyodeoxycholate and 3α-sulfatolithocholyl [3H]taurine. The Km value of MRP for 17β-glucuronosyl [3H]estradiol was 1.5 ± 0.3 µm, with a Vmax:Km ratio of 42 ml × mg protein-1 × min-1, and a Ki value of 0.7 µm for the leukotriene receptor antagonist MK 571. MRP-mediated ATP-dependent transport was observed for the anticancer drug conjugates glucuronosyl [3H]etoposide and monochloro-mono[3H]glutathionyl melphalan, but not for unmodified [14C]doxorubicin, [3H]daunorubicin, or [3H]vinblastine. Our results establish that MRP functions as an ATP-dependent export pump not only for glutathione conjugates but also for glucuronidated and sulfated endogenous as well as exogenous compounds.

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This work was supported in part by the Deutsche Forschungsgemeinschaft through SFB 352 (Heidelberg, Germany).

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