| Abstract: | The ability of a purified rat liver microsomal uridine-5'-diphosphoglucuronosyltransferase to catalyze the glucuronidation of stereoisomeric trans- and cis-9, 10-dihydroxy-9, 10-dihydrophenanthrenes and 4, 5-dihydroxy-4,5-dihydrobenzoalpha]pyrenes is examined. The enzyme shows the ability to discriminate kinetically between the antipodes of trans-9, 10-dihydroxy-9, 10-dihydrophenanthrene with turnover numbers of 0.070 and 1.4 s-1 and kc/Kmapp values of 4.4 X 10(3) and 1.1 X 10(3) M-1 s-1 for the 9R, 10R and 9S, 10S stereoisomers. Glucuronidation of the nondissymmetric cis-9, 10-dihydroxy-9, 10-dihydrophenanthrene proceeds with a turnover number of 0.037 s-1 and kc/Kmapp of 18 X 10(3) M-1 s-1 to give a 60/40 mixture of the two possible diastereomeric products. Three of the four stereoisomers of 4,5-dihydroxy-4,5-dihydrobenzoalpha] pyrene are regioselectively glucuronidated by the enzyme with a high degree of kinetic discrimination. Turnover numbers for the 4S,5S, 4R,5R, and 4S,5R stereoisomers are 4.1, 0.37, and 0.23 s-1 with kc/Kmapp values of 23.8 X 10(3), 0.23 X 10(3), and 3.15 X 10(3) M-1 s-1, respectively. The 4R,5S cis isomer is not a substrate. Enzyme-catalyzed reactions of the 4S,5S and 4S,5R isomers give exclusively (greater than or equal to 95%) the 4-glucuronide with the 4R,5R isomer giving the 5-glucuronide. The kinetic and regiochemical results indicate that the enzyme recognizes hydroxyl groups on the beta-face or bottom face of the 4,5-dihydroxy-4,5-dihydrobenzoalpha]pyrenes.(ABSTRACT TRUNCATED AT 250 WORDS) |
