Uridine Diphosphate Glucuronyltransferase Activity in Nuclei and Nuclear Envelopes of Rat Liver and its Apparent Induction by Phenobarbital

The presence of UDP-glucuronyltransferase (EC 2.4.1.17) activity in chick-embryo liver nuclei and nuclear envelope and its inducibility by phenobarbital has been previously reported (Fry & Wishart, 1976). This enzyme has also been found in adult rat liver, with 64-76 % in the microsomal and 14-19 % in the unpuri6ed nuclear fractions (Amar-Costesec et ul., 1974). In view of the heterogeneity of such unpurified nuclear fractions and the high microsomal activity, much, if not all, of such apparent nuclear activity could be due to microsomal contamination. In the present study a reliable estimate of nuclear and nuclearenvelope activity was sought. Purified nuclear preparations have been prepared, with the degree of contamination of these quantified by electronmicroscope morphometry (Weibel & Bolender, 1972), and the nuclear envelopes were isolated from these fractions as described previously (Fry & Wishart, 1976). The results on the nuclear and nuclearenvelope preparations were compared with those on standard microsomal preparations and on microsomal preparations subjected to the nuclear-envelope-isolation procedures (‘treated microsomes’). As the microsomal enzyme is latent in brokencell preparations, such comparison has little meaning without some standardization of the degree to which the latent enzyme has been activated (L.eakey &Donald, 1976). All fractions were therefore assayed for glucuronyltransferase activity, by the procedure described by Winsnes (1969) with o-aminophenol as substrate, at a range of digitonin concentrations, so that estimates of maximal activity could be made. Female Wistar rats (3 months old) were used for all experiments and the effect of phenobarbital was determined by comparing results on phenobarbital-treated and control rats sampled on the same day, and with their liver fractions prepared in parallel. Maximal phenobarbital induction was achieved by introducing 1-2g/l into the drinking water for over 3 weeks before the experiment, or by injecting 100mg/kg intraperitoneally daily for 3 days and assaying on the fifth day. In control rat livers the purified nuclear fractions contained less than 10% nonnuclear membrane and showed few non-membranous contaminants. The purified nuclear fraction contained 19.6% of the total homogenate DNA, but its maximal glucuronyltransferase activity was only 0.5% of the total. The true nuclear glucuronyltransferase activity therefore appears to be approx. 2.5% of the homogenate activity, a value substantially less than indicated by crude cell fractionation. The nuclear envelopes retained over 80% of the total nuclear activity, indicating that the envelope is the primary, if not the sole, location of the nuclear enzyme. Maximal nuclear, nuclear-envelope and microsoma1 glucuronyltransferase activities per mg of phospholipid were comparable (230nmol of o-aminophenyl glucuronide/h per mg of phospholipid). Similar results were obtained when 5-hydroxytryptamine or bilirubin was used as substrate instead of o-aminophenol. With o-aminophenol as substrate, the maximal activity obtained after digitonin treatment of ‘treated microsome’ preparations was nearly twice that obtained with the standard microsomal preparations, and it seems that the envelopisolation procedure used (low ionic concentration, pH8.5, deoxyribonuclease) has an activating