The Role of the Microsomal Membrane in Control of Uridine Diphosphate Glucuronyltransferase Activity in vitro and in vivo

for phase separationwithin thelipidphase(Phillipsetal., 1970; Shimschick & McConnel, 1973) UDP-glucose inhibited the activity of UDP-glucuronyltransferase. There was no inhibition at any temperature above 16°C. Similarly, at temperatures below that for phase separation within the lipid portion of the membrane, UDP-glucuronyltransferase was insensitive to activation by UDP-N-acetylglucosamine. Interactions between UDPglucuronyltransferase and the lipid portion of untreated lipid microsomal preparations are hence not essential for catalytic activity, but they are critical for physiological function.Thisis truenot onlyforperturbationswhichincreasethefluidityofmembranelipids, but holds as well for decreases in the fluidity of membrane lipids. Since activation of UDP-glucuronyltransferase by phospholipases and detergents is not unique to this enzyme, the significance of the activation of other enzymes like UDP-glucuronyltransferase may lie also in control of enzyme functions other than the catalytic rate constant.