Regulation of Microsomal Enzymes by Phospholipids

Treatment of bovine liver microsomes with a partially purified preparation of phospholipase A from Naja naja venom leads to activation of UDP-glucuronyltransferase with p-nitrophenol as glucuronyl acceptor. This stimulation of activity is due to a 6-fold increase in activity at Vmax. As activity at Vmax increases, there is a progressive decrease in binding affinity of the enzyme for both substrates, and although the enzyme remains stable at 23°, it becomes unstable at 37°. This unstable form of UDP-glucuronyltransferase decays to another stable form with a maximum activity 2.5-fold greater than that of untreated enzyme. As with phospholipase A, treatment with phospholipase C also activates UDP-glucuronyltransferase, but to a lesser extent. In addition to phospholipases other agents which can alter microsomal lipids also activate UDP-glucuronyltransferase. Triton X-100, sonication, and exposure to pH 9.8 increased activity at Vmax and had variable effects on the binding constants for UDP-glucuronic acid and p-nitrophenol. Maximal activation by these treatments was less than that obtained with phospholipase A; no two treatments had similar effects on all kinetic parameters of the enzyme. Nevertheless, additive effects could not be demonstrated. Although Triton stimulated glucuronidation of p-nitrophenol by the enzyme, it was without effect on the reverse reaction. This fact plus the other data indicate that the activation of UDP-glucuronyltransferase in these experiments cannot be attributed to compartmentation of the enzyme but is due to phospholipid-induced alterations of enzyme conformation.