Glucuronidation of N-hydroxy metabolites of N-acetylbenzidine

Glucuronidation of N-hydroxy arylamines is thought to be a necessary step in their initiation of bladder cancer. This was evaluated for the N-hydroxy metabolites of N-acetylbenzidine (ABZ). N'-Hydroxy-N-acetylbenzidine (N-HA), N-hydroxy-N-acetylbenzidine (N-HA) and N-hydroxy-N,N'-diacetylbenzidine (N-HDA) were synthesized. Except for N'-HA, these compounds were quite stable. Ascorbic acid and/or acidic pH increased the stability of N'-HA. When each N-hydroxy compound was added to reaction mixtures containing [¹⁴C]UDP-glucuronic acid, 3 mM ascorbic acid and human liver microsomes a new product was detected by HPLC. Emulgen 911 was a better detergent than Triton X-100 for expressing microsomal activity, with maximal glucuronidation observed with 0.3% Emulgen 911. At 0.125 mM amine the rate of glucuronidation was N-HDA >> N'-HA = benzidine > ABZ > N-HA. In contrast, at 0.5 mM amine the rate of glucuronidation of N-HA was only exceeded by N-HDA. At pH 5.5 and 37°C the t_1/2 for the enzymatically prepared glucuronide conjugates of ABZ, N'-HA and N-HA were 7.5 min and 3.5 and 1.8 h respectively. For N-HDA >90% of this glucuronide remained after 24 h. At pH 7.4 and 37°C the t_1/2 for the glucuronide conjugates of ABZ and N-HA were 2.3 and 2 h respectively, with the amounts remaining after 24 h for N'-HA and N-HDA being 75 and 90% respectively. At pH 6.5 the t_1/2 for N'-HA was 14 h. Thus only glucuron-ides of ABZ and N'-HA exhibit pH-dependent changes in t_1/2. Compared with ABZ, glucuronides the N-hydroxy metabolites are more stable at acidic pH. Acidic urine would be more likely to hydrolyze the glucuronide conjugate of ABZ than those of its N-hydroxy metabolites. Because these results are different from that hypothesized for arylmonoamines, a new model was developed to explain the role of N-oxidation, N-glucuronidation and N-acetylation in the carcinogenesis of benzidine, an aryldiamine.