A unique multifunctional transporter translocates estradiol-17beta -glucuronide in rat liver microsomal vesicles

A wide array of drugs, xenobiotics, and endogenous compounds undergo detoxification by conjugation with glucuronic acid in the liver via the action of UDP-glucuronosyltransferases. The mechanism whereby glucuronides, generated by this enzyme system in the lumen of the endoplasmic reticulum (ER), are exported to the cytosol prior to excretion is unknown. We examined this process in purified rat liver microsomes using a rapid filtration technique and (3)H]estradiol-17beta-d-glucuronide ((3)H]E(2)17betaG) as model substrate. Time-dependent uptake of intact (3)H]E(2)17betaG was observed and shrinkage of ER vesicles by raffinose lowered the steady-state level of (3)H]E(2)17betaG accumulation. In addition, rapid efflux of (3)H]E(2)17betaG from rat liver microsomal vesicles suggested that the transport process is bidirectional. Microsomal uptake was saturable with an apparent K(m) and V(max) of 3.29 +/- 0.58 microm and 0.19 +/- 0.02 nmol.min(-1).mg protein(-1), respectively. Transport of (3)H]E(2)17betaG was inhibited by the anion transport inhibitors 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and probenecid. Specificity of the transport process was investigated by studying the cis-inhibitory effect of anionic metabolites, as well as substrates of the plasma membrane multidrug resistance-associated proteins on the uptake of (3)H]E(2)17betaG. Collectively, these data are indicative of a novel multifunctional and bidirectional protein carrier for E(2)17betaG and other anionic compounds in the hepatic ER. This intracellular membrane transporter may contribute to the phenomenon of multidrug resistance.