TRANSPORT OF GLUTATHIONE, GLUCURONATE, AND SULFATE CONJUGATES BY THE MRP GENE-ENCODED CONJUGATE EXPORT PUMP

Previous studies have identified the ATP-dependent export of glutathio ne conjugates as a physiological function of the multidrug resistance protein (MRP). The involvement of MRP in the transport of endogenous a nd xenobiotic conjugates was investigated further using membrane vesic les from MRP-transfected HeLa cells. The ATP-dependent transport of th e glutathione conjugates [H-3]leukotriene C-4, S-(2,4-dinitrophenyl)-[ H-3]glutathione, and H-3-labeled oxidized glutathione was characterize d by determination of the transport efficiency V-max:K-m amounting to 1031, 114, and 7.1 ml x mg protein(-1) x min(-1), respectively. Additi onal endogenous substrates for MRP-mediated transport included the ste roid conjugate 17 beta-glucuronosyl [H-3]estradiol and the bile salt c onjugates [6 alpha-C-14]glucuronosylhyodeoxycholate and 3 alpha-sulfat olithocholyl [H-3]taurine. The K-m value of MRP for 17 beta-glucuronos yl [H-3]estradiol was 1.5 +/- 0.3 mu M, with a V-max:K-m ratio of 42 m l x mg protein(-1) x min(-1), and a K-i value of 0.7 mu M for the leuk otriene receptor antagonist MK 571. MRP-mediated ATP-dependent transpo rt was observed for the anticancer drug conjugates glucuronosyl [H-3]e toposide and monochloro-mono[H-3]glutathionyl melphalan, but not for u nmodified [C-14]doxorubicin, [H-3]daunorubicin, or [H-3]vinblastine. O ur results establish that MRP functions as an ATP-dependent export pum p not only for glutathione conjugates but also for glucuronidated and sulfated endogenous as well as exogenous compounds.