Metabolism of N-acetylbenzidine and initiation of bladder cancer

A 100-fold increased incidence of bladder cancer is observed with workers exposed to high levels of benzidine (BZ). This review evaluates the overall metabolism of BZ to determine pathways involved in initiation of carcinogenesis. Enzymatic and liver slice incubations demonstrated N-acetylation and N-glucuronidation of BZ and N-acetylbenzidine (ABZ). With rat, N,N′-diacetylbenzidine (DABZ) is the major slice metabolite. With human, ABZ is the major metabolite along with N-glucuronides. Differences between rat and human are attributed to preferential acetylation of BZ and deacetylation of DABZ, resulting in N-glucuronide formation by human liver. Glucuronidation of BZ and its analogues exhibited the following relative ranking of UDP-glucuronosyltransferase (UGT) metabolism: UGT1A9>UGT1A4⪢UGT2B7>UGT1A6≈UGT1A1. N-Glucuronides of BZ, ABZ, and N′-hydroxy-N-acetylbenzidine (N′HA) are acid labile with the latter having a much longer t1/2 than the former two glucuronides. O-Glucuronides are not acid labile. In urine from BZ-exposed workers, an inverse relationship between urine pH and levels of free (unconjugated) BZ and ABZ is observed. This is consistent with the presence of labile urinary N-glucuronides. Cytochrome P-450 oxidizes BZ to an inactive product (3-OHBZ) and ABZ to N′HA and N-hydroxy-N-acetylbenzidine (NHA). Cytochrome P-450, PHS, and horseradish peroxidase activate ABZ to bind DNA forming N′-(3′-monophospho-deoxyguanosin-8-yl)-N-acetylbenzidine (dGp-ABZ). This is the major adduct detected in bladder cells from workers exposed to BZ. An inverse relationship exists between urine pH and levels of bladder cell dGp-ABZ. Bladder epithelium contains relatively high levels of prostaglandin H synthase (PHS) and low levels of cytochrome P-450, suggesting activation by PHS. Activation by PHS involves a peroxygenase oxidation of ABZ to N′HA, while horseradish peroxidase activates ABZ to a diimine monocation. Reactive nitrogen oxygen species (RNOS) offer a new pathway for metabolism and potential activation. Results suggest BZ initiation of bladder cancer is complex, involving multiple organs (i.e. liver, kidney, and bladder) and metabolic pathways (i.e. N-acetylation, N-glucuronidation, peroxidation, and RNOS).