Evidence that UDP-glucuronyltransferase in liver microsomes at 37 degrees C is in a gel phase lipid environment.

UDP-glucuronyltransferase (EC 2.4.1.17) in intact, untreated microsomes from pig liver is activated by relatively low concentrations of UDP-N-acetylglucosamine. This property is absent after treatment of microsomes with detergents or phospholipases, and also is not a characteristic of pure, delipidated enzyme. Sensitivity to activation by UDP-N-acetylglucosamine was reconstituted, however, by incorporation of pure, delipidated enzyme into unilamellar bilayers of phosphatidylcholine that were in a gel phase. Warming of these bilayers to just above the temperature for the gel to liquid crystalline phase transition led to an abrupt loss of sensitivity of UDP-glucuronyltransferase to activation by UDP-N-acetylglucosamine. These experiments establish that sensitivity to activation by UDP-N-acetylglucosamine is an inherent property of UDP-glucuronyltransferase. The data also suggest that the lipid environment of UDP-glucuronyltransferase in intact, untreated microsomes can modulate the sensitivity of this enzyme to allosteric activation by UDP-N-acetylglucosamine, and that this lipid environment is in a gel phase in intact microsomes at 37 degrees C.