Two kinetically-distinct components of UDP-glucuronic acid transport in rat liver endoplasmic reticulum

Abstract

Previous studies have documented the presence of protein-mediated transport of UDP-glucuronic acid (UDP-G1cUA) in rat liver endoplasmic reticulum (ER). Measurement of uptake at varying concentrations of high specific activity [β- 32P]UDP-G1cUA has revealed the presence of a two component UDP-G1cUA transporting system. Transport at low substrate concentrations occurred predominantly via a high affinity component ( K m = 1.6 μM), whereas a low affinity component ( K m = 38 μM) predominated at high substrate concentrations. The K m for the high affinity system is in agreement with that previously published, while the low affinity component is a new finding. The uptake of UDP-G1cUA was temperature-sensitive, time dependent, and saturable for both components. The high affinity transport was affected by trans-stimulation and cis-inhibition by UDP- N-acetylglucosamine (UDP-G1cNAc); however, the same concentrations of UDP-G1cNAc had less effect on the low affinity system. In order to further study the two transport components, various inhibitors of anion transport carriers were tested. The high affinity component was strongly inhibited by 4-acetamido-4′-isothiocyanatostilbene-2,2′-disulfonic acid (SITS) and furosemide, while the low affinity system was less sensitive to these reagents. Dose-dependent inhibition by 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS) was found for both transport systems. Probenecid was found to be a weak inhibitor of both components of the UDP-G1cUA uptake. Finally, the major metabolite of 3′-azido-3′-deoxythymidine, 3′-azido-3′-deoxythymidine monophosphate (AZTMP), was able to inhibit the uptake of UDP-GlcUA by both components. The results indicate the presence of two carrier-mediated UDP-glucuronic acid transporting components in rat liver ER.