Transporter-mediated uptake of UDP-glucuronic acid by human liver microsomes: assay conditions, kinetics, and inhibition.
This study characterized the kinetics, variability, and factors that affect UDP-glucuronic acid (UDP-GlcUA) uptake by human liver microsomes (HLM). Biphasic kinetics were observed for UDP-GlcUA uptake by HLM. Uptake affinities (assessed as Kd) of the high- and low-affinity components differed by more than an order of magnitude (13 ± 6 vs. 374 ± 175 µM), but were comparable in terms of the maximal rate of uptake, with mean Vmax values differing less than 2.3-fold (56 ± 26 vs. 131 ± 35 pmol/min per mg). Variability in total intrinsic transporter activity (Uint) for microsomal UDP-GlcUA uptake across 12 livers was less than 4-fold. Experiments performed to optimize the conditions for microsomal UDP-GlcUA uptake demonstrated that both components were trans-stimulated by preloading (luminal addition) with an alternate UDP-sugar, and essentially abolished by the thiol-alkylating agent N-ethylmaleimide. Furthermore, interaction studies undertaken with a panel of drugs, alternate UDP-sugars, and glucuronide conjugates, at low (2.5 μM) and high (1000 μM) UDP-GlcUA concentrations, demonstrated that both components were inhibited to varying extents. Notably, the nucleoside analogs zidovudine, stavudine, lamivudine, and acyclovir inhibited both the high- and low- affinity components of microsomal UDP-GlcUA uptake by >45% at an inhibitor concentration of 100 μM. Taken together, these data demonstrate that human liver microsomal UDP-GlcUA uptake involves multiple protein-mediated components, and raises the possibility of impaired in vivo glucuronidation activity resulting from inhibition of UDP-GlcUA uptake into the endoplasmic reticulum membrane by drugs and other compounds.
Department of Clinical Pharmacology, School of Medicine, Flinders University, Adelaide, Australia andrew.rowland@flinders.edu.au.
Andrew
A
Rowland
Department of Clinical Pharmacology, School of Medicine, Flinders University, Adelaide, Australia.
Peter I
PI
Mackenzie
Department of Clinical Pharmacology, School of Medicine, Flinders University, Adelaide, Australia.
John O
JO
Miners
D001692:Biological Transport / Q000502:physiology; D004721:Endoplasmic Reticulum / Q000378:metabolism; D020719:Glucuronides / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008099:Liver / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000378:metabolism*; D008862:Microsomes, Liver / Q000378:metabolism*; D014535:Uridine Diphosphate Glucuronic Acid / Q000378:metabolism*
D020719:Glucuronides; D026901:Membrane Transport Proteins; D014535:Uridine Diphosphate Glucuronic Acid