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