title: Uridine diphosphoxylose enhances hepatic microsomal UDP-glucuronosyltransferase activity by stimulating transport of UDP-glucuronic acid across the endoplasmic reticulum membrane.
abstract: The UDP-glucuronosyltransferase (UGT) system fulfils a pivotal role in the biotransformation of potentially toxic endogenous and exogenous compounds. Here we report that the activity of UGT in rat liver is stimulated by UDP-xylose. This stimulation was found in native microsomal vesicles as well as in the intact endoplasmic reticulum (ER) membrane, as studied in permeabilized hepatocytes, indicating the potential physiological importance of UDP-xylose in the regulation of UGT. We present evidence that UDP-xylose enhances UGT activity by stimulation of (i) the uptake of UDP-glucuronic acid across the ER membrane and (ii) the elimination of the UDP and/or UMP reaction product out of the ER lumen. UDP-xyloe produced a marked trans-stimulation of microsomal UDP-glucuronic acid uptake when it was present within the lumen of the ER. When UDP-xylose was presented at the cytosolic side of the ER, it acted as a weak inhibitor of UDP-glucuronic acid uptake. Likewise, cytosolic UDP-glucuronic acid strongly trans-stimulated efflux of intravesicular UDP-xylose, whereas cytosolic UDP-xylose was inefficient in trans-stimulating efflux of UDP-glucuronic acid. Microsomal UDP-xylose influx was markedly stimulated by UMP and UDP. Such stimulation was only apparent when microsomes had been preincubated and thereby preloaded with UMP or UDP, indicating that UMP and UDP exeted their effect on UDP-xylose uptake by trans-stimulation from the luminal side of the ER membrane.
authors.affiliation: Laboratory of Biological Chemistry, Department of Molecular Cell Biology, Catholic University of Leuven, Belgium.
authors.forename: X
authors.initials: X
authors.lastname: Bossuyt
authors.forename: N
authors.initials: N
authors.lastname: Blanckaert
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D018870:Endoplasmic Reticulum, Rough / Q000378:metabolism*; D014453:Glucuronosyltransferase / Q000187:drug effects* / Q000378:metabolism*; D008297:Male; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology*; D051381:Rats; D017208:Rats, Wistar; D013268:Stimulation, Chemical; D014530:Uridine Diphosphate / Q000378:metabolism; D014535:Uridine Diphosphate Glucuronic Acid / Q000493:pharmacokinetics*; D014540:Uridine Diphosphate Xylose / Q000494:pharmacology*; D014542:Uridine Monophosphate / Q000378:metabolism
chemical_list: D014535:Uridine Diphosphate Glucuronic Acid; D014540:Uridine Diphosphate Xylose; D014530:Uridine Diphosphate; D014542:Uridine Monophosphate; D014453:Glucuronosyltransferase