HUMAN LIVER GLUCURONIDATION OF BENZIDINE

Although glucuronidation is considered an important pathway in aromati c amine-induced bladder cancer, benzidine glucuronidation has not been assessed in humans. Glucuronidation of benzidine was assessed with hu man liver microsomes and slices. Emulgen 911-treated microsomes exhibi ted a K, for benzidine of 0.8+/-0.06 mM and a V-max of 4.2+/-0.7 nmol/ mg protein/min. A variety of agents were tested for their ability to i nhibit benzidine N-glucuronide formation. At 0.25 mM, estriol, 17-epie striol, bilirubin, hyodeoxycholic acid and cyproheptadine were good in hibitors (<50% of control). Dose-dependent inhibition studies with est riol, testosterone and 4-aminobiphenyl demonstrated that each agent re ached a plateau as its concentration was increased. When these agents were combined at maximal inhibitory concentrations, additive inhibitio n was observed. These results suggest that more than one UDP-glucurono syltransferase metabolizes benzidine. The cDNA clones pUDPGT(h)-1 and -2 encode transferases which metabolize hyodeoxycholic acid and estrog en derivatives, but neither transferase catalyzed benzidine glucuronid ation. Slices were used to assess metabolism by intact tissue and conv erted [H-3]benzidine (0.09 mM) to N-acetylbenzidine. N-Glucuronides of both benzidine and N-acetylbenzidine were observed and represented 14 -37% of the total recovered radioactivity. The amount of N-acetylbenzi dine N'-glucuronide observed was proportional to the amount of N-acety lbenzidine produced. Thus, N-glucuronidation appears to represent a ma jor pathway for metabolism of benzidine in humans. The extent of N-ace tylation affects the proportion of benzidine and N-acetylbenzidine glu curonidated by human liver slices.