Stereoselectivity and regioselectivity of uridine 5'-diphosphoglucuronosyltransferase toward vicinal dihydrodiols of polycyclic aromatic hydrocarbons

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Abstract
The ability of a purified rat liver microsomal UDP-glucuronosyltransferase to catalyze the glucuronidation of stereoisomeric trans- and cis-9,10-dihydroxy-9,10-dihydrophenanthrenes and 4,5-dihydroxy-4,5-dihydrobenzo[a]pyrenes was examined. The enzyme showed 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 and kc/Kmapp values of 4.4 .times. 103 and 1.1 .times. 103 M-1 s-1 for the 9R,10R and 9S,10S stereoisomers. Glucuronidation of the nondissymmetric cis-9,10-dihydroxy-9,10-dihydrophenanthrene proceeded with a turnover number of 0.037/s and kc/Kmapp of 18 .times. 103 M-1 s-1 to give a 60/40 mixture of the 2 possible diastereomeric products. Of 4 stereoisomers of 4,5-dihydroxy-4,5-dihydrobenzo[a]pyrene, 3 were regioselectively glucuronidated by the enzyme with a high degree of kinetic discrimination. Turnover numbers for the 4S,5S, 4R,5R and 4S,5R stereoisomers were 4.1, 0.37 and 0.23/s with kc/Kmapp values of 23.8 .times. 103, 0.23 .times. 103 and 3.15 .times. 103 M-1 s-1, respectively. The 4R,5S cis isomer was not a substrate. Enzyme-catalyzed reactions of the 4S,5S and 4S,5R isomers gave exclusively (.gtoreq. 95%) the 4-glucuronide with the 4R,5R isomer giving the 5-glucuronide. The kinetic and regiochemical results indicated that the enzyme recognized OH groups on the .beta.-face or bottom face of the 4,5-dihydroxy-4,5-dihydrobenzo[a]pyrenes. The reaction was more dependent on the facial specificity of the enzyme than on the relative configuration of the vicinal OH groups. The stereochemical results and the stereoselectivities of cytochrome P-450c and epoxide hydrolase predicted that if glucuronidation participated effectively in the metabolism of benzo[a]pyrene 4,5-dihydrodiols, the major isomeric glucuronide formed was (4R,5R)-5-.beta.-D-glucuronosyl-4-hydroxy-4,5-dihydrobenzo[a]pyrene.

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