Membrane translocation and regulation of uridine diphosphate-glucuronic acid uptake in rat liver microsomal vesicles.
OBJECTIVE
Hepatic glucuronidation is quantitatively the most important conjugation reaction by which an array of endogenous compounds and xenobiotics undergo biotransformation and detoxification. The active site of the uridine diphosphate (UDP) glucuronosyltransferases, which catalyze glucuronidation reactions, has been postulated to reside in the lumen of the endoplasmic reticulum. The aim of this study was to characterize the process whereby UDP glucuronic acid (UDP-GlcUA), the cosubstrate for all glucuronidation reactions, is transported into microsomal vesicles.


METHODS
The uptake process was analyzed using rapid filtration techniques, radiolabeled UDP-GlcUA, and rat liver microsomes.


RESULTS
Uptake was saturable with respect to time and concentration, inhibited by 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid and 4-acetamido-4'-isothio-cyanatostilbene-2-2'-disulfonic acid, and was osmotically sensitive. Transport was stimulated by Mg2+ and guanosine triphosphate (50 mumol/L) but not guanosine 5'-O-(3-thiotriphosphate) or adenosine triphosphate. Luminal UDP-N-acetylglucosamine (1 mmol/L) produced enhanced uptake of UDP-GlcUA (trans stimulation). In contrast to nucleotide sugar transport in the Golgi apparatus, trans uridine monophosphate and UDP did not alter UDP-GlcUA transport in microsomes, indicating distinct processes.


CONCLUSIONS
These data provide unambiguous evidence for the existence of a unique, substrate-specific, regulated, carrier-mediated process that transports UDP-GlcUA into the lumen of hepatocyte microsomes. This transporter may regulate glucuronidation in vivo.
Berg
C L
CL
Gastroenterology Division, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts.
Radominska
A
A
Lester
R
R
Gollan
J L
JL
D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D007425:Intracellular Membranes / Q000378:metabolism*; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism*; D009995:Osmosis; D051381:Rats; D017207:Rats, Sprague-Dawley; D013379:Substrate Specificity; D014530:Uridine Diphosphate / Q000494:pharmacology; D014535:Uridine Diphosphate Glucuronic Acid / Q000493:pharmacokinetics*; D014542:Uridine Monophosphate / Q000494:pharmacology
D014535:Uridine Diphosphate Glucuronic Acid; D014530:Uridine Diphosphate; D014542:Uridine Monophosphate