Membrane Dependence of Uridine Diphosphate Glucuronyltransferase: Effect of the Membrane on Kinetic Properties

s of Communications Meeting of the Biochemical Society Pharmacological Biochemistry Group, St. Mary3 Hospital Medical School, London 13 September 1974 GLUCURONIDATION MECHANISMS : a Colloquium organized by P. T. Nowell (Liverpool) Membrane Dependence of Uridine Diphosphate Glucuronyltransferase : Effect of the Membrane on Kinetic Properties DAVID ZAKIM and DONALD A. VESSEY Departments of Medicine and Biochemistry, University of California, San Francisco, and the Molecular Biology Division, Veterans Administration Hospital, San Francisco, Calif. 94121, U.S.A. The activities of some membrane-bound enzymes are enhanced by treatments which modify the lipid phase of the membrane. For example, treatment of liver microsomal fractions with purified phospholipase A or detergent activates UDP-glucuronyltransferase, assayed withp-nitrophenol as aglycone (Vessey & Zakim, 1971). For all species studied, the maximal potential activity of UDP-glucuronyltransferase is constrained by the environment within the untreated, or native, microsomal membrane. Variations in the manner in which the lipid phase is modified, as by sonication, detergent, or brief or exhaustive treatment with phospholipase A, produce forms of UDP-glucuronyltransferase with different kinetic parameters, though each is activated compared with the untreated enzyme. Rather than ‘all or none’ types of activations lipid-protein interactions appear therefore to have complex effects on the activity of UDP-glucuronyltransferase. The activity of UDP-glucuronyltransferase in untreated microsomal preparations is relatively low (Vessey & Zakim, 1972a,b), and the enzyme systemcan be overloaded quite easily in intact animals. The question of the physiological significance of constraint on the activity of UDP-glucuronyltransferase in native microsomal preparations is therefore an important problem. Insight into why the activity of UDP-glucuronyltransferase is less than the maximal potential activity in untreated microsomal preparations depends on an understanding of the regulatory characteristics of this enzyme, and the conditions under which it is presumed to function in vivo. The concentration of UDP-glucuronic acid in intact liver is small compared with other UDP-sugars (Keppler et al., 1970). For efficient function in vivo UDP-glucuronyltransferase must have high selectivity for the binding of UDP-glucuronic acid. The untreated form of UDP-glucuronyltransferase fulfils this requirement in that only UDPglucuronic acid, of all nucleotide sugars examined, binds to the enzyme. The untreated form of UDP-glucuronyltransferase is inhibited extensively, however, by relatively small concentrations of UDP, a product of the reaction (Vessey & Zakim, 1972a). Since the concentration of UDP in vivo is nearly equal to that of UDP-glucuronic acid (Keppler et al., 1970), end-product inhibition by UDP could limit severely the rate of glucuronidation reactions in intact animals. (Also, UTP, which is present in large