Transport of Glutathione, Glucuronate, and Sulfate Conjugates by the MRP Gene-encoded Conjugate Export Pump

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 [(3)H]leukotriene C(4), S-(2,4-dinitrophenyl)-[(3)H]glutathione, and (3)H- labeled oxidized glutathione was characterized by determination of the transport efficiency V(max):K(m) amounting to 1031, 114, and 7.1 ml multiplied by min(-1), respectively. Additional endogenous substrates for MRP-mediated transport included the steroid conjugate 17 beta- glucuronosyl [(3)H]estradiol and the bile salt conjugates [6 alpha-(14)C]glucuronosylhyodeoxycholate and 3 alpha-sulfatolithocholyl [(3)H]taurine. The K(m) value of MRP for 17-beta-glucuronosyl [(3)H]estradiol was 1.5 +/- 0.3 microM, with a V(max):K(m) ratio of 42 ml multiplied by mg protein(-1) multiplied by min(-1), and a K(i) value of 0.7 microM for the leukotriene receptor antagonist MK 571. MRP-mediated ATP-dependent transport was observed for the anticancer drug conjugates glucuronosyl [(3)H]etoposide and monocholoro-mono[(3)H]glutathionyl melphalan, but not for unmodified [(14)C]doxorubicin, [(3)H]daunorubicin, or [(3)H]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.