Evidence for the intraluminal positioning of p-nitrophenol UDP-glucuronosyltransferase activity in rat liver microsomal vesicles.
Addition of p-nitrophenol and UDP-glucuronic acid to rat hepatic microsomes enhanced the MgATP-stimulated Ca2+ sequestration. This stimulatory effect was more explicit in the presence of the activator of glucuronidation, UDP-N-acetylglucosamine. The stimulation of Ca2+ uptake was dependent on the p-nitrophenol concentration and showed a good correlation with the rate of p-nitrophenol glucuronidation. The stimulation of Ca2+ sequestration was probably due to its coaccumulation with the intraluminar Pi originated during glucuronidation. The increase in extravesicular osmolarity due to the addition of UDP-glucuronic acid to microsomes resuspended in an hyposmotic medium caused a rapid and prolonged shrinking as revealed by light-scattering measurements. This indicates a poor permeability of microsomal membrane to UDP-glucuronic acid. The subsequent addition of the pore-forming compound alamethicin resulted in an immediate swelling of vesicles indicating a rapid entry of UDP-glucuronic acid. Alamethicin also caused an about 15-fold increase in p-nitrophenol UDP-glucuronosyltransferase activity. These results support the hypothesis of the intravesicular compartmentation of the microsomal UDP-glucuronosyltransferase catalytic site.
Fulceri
R
R
Istituto di Patologia Generale, University of Siena, Italy.
Bánhegyi
G
G
Gamberucci
A
A
Giunti
R
R
Mandl
J
J
Benedetti
A
A
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000408:Alamethicin / Q000494:pharmacology; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects; D014453:Glucuronosyltransferase / Q000378:metabolism*; D008297:Male; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology*; D009596:Nitrophenols / Q000378:metabolism* / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D014535:Uridine Diphosphate Glucuronic Acid / Q000378:metabolism / Q000494:pharmacology
D009596:Nitrophenols; D014535:Uridine Diphosphate Glucuronic Acid; D000408:Alamethicin; D000255:Adenosine Triphosphate; D014453:Glucuronosyltransferase; D002118:Calcium; C024836:4-nitrophenol