Metabolic activation of N-hydroxy-4-acetylaminobiphenyl (N-OH-AABP) and N-acetoxy-4-acetylaminobiphenyl (N-OAc-AABP), the proximate carcinogenic metabolites of the human bladder carcinogen 4-aminobiphenyl (ABP), was examined in human uroepithelial cells (HUC). Bioconversion was studied by incubating HUC cultures with [3H]N-OAc-AABP or [3H]N-OH-AABP. Three organo-soluble metabolites, N-OH-AABP, 4-acetylaminobiphenyl (AABP), and ABP were identified in ethyl acetate extracts from cultures exposed to N-OAc-AABP. Similarly, AABP and ABP were characterized as the major metabolites from cultures treated with N-OH-AABP. Incubation of N-OAc-AABP with HUC microsomes in vitro yielded primarily the O-deacetylation product N-OH-AABP. The HUC microsomes also catalyzed the N-deacetylation of N-OAc-[14C]AABP, N-OH-[14C]AABP, and [3H]AABP. The O- and N-deacetylase activities for N-OAc-AABP were 55.9 and 38.2 nmol/mg/min, respectively. These O- and N-deacetylase activities were both blocked by paraoxon. Incubation of [3H]N-OAc-AABP or [3H]N-OH-AABP with HUC microsomes and tRNA or DNA showed that 23.0 and 8.0 nmol of N-OAc-AABP and 74.5 and 25.2 pmol of N-OH-AABP were bound per mg protein/mg RNA or DNA, respectively. In comparison, the acetyl CoA-dependent HUC cytosol-mediated bindings of [3H]N-OH-ABP to RNA and DNA were 801 and 447 pmol/mg nucleic acid/mg protein. The HUC microsome-mediated bindings of N-OAc-AABP and N-OH-AABP to nucleic acids were inhibited by paraoxon, whereas the cytosol-mediated binding of N-OH-ABP was insensitive to paraoxon inhibition. Chromatography of the DNA hydrolysate obtained from the in vitro incubation of [3H]N-OAc-AABP or [3H]N-OH-AABP with HUC microsomes showed N-(deoxyguanosine-8-yl)-4-aminobiphenyl as the major adduct, based on comparison with authentic synthetic standard. These results show that human uroepithelia contain microsomal acetyl transferases that are capable of converting the proximate metabolites N-OAc-AABP and N-OH-AABP of the human bladder carcinogen ABP, to reactive electrophiles that bind to DNA. The occurrence of these acetyl transferases in the target organ of the human bladder carcinogen ABP suggests that metabolic activation of some proximate metabolites of ABP could occur directly in HUC and could play a pivotal role in susceptibility to aryl-amine/acetamide induced human bladder cancers.