Detoxi®cation of carcinogenic aromatic and heterocyclic amines by enzymatic reduction of the N-hydroxy derivative Roberta S. King a,1 , Candee H. Teitel a , Joseph G. Shaddock b , Daniel A. Casciano b , Fred F. Kadlubar a, * a Division of Molecular Epidemiology, National Center for Toxicological Research, Jefferson, AR 72079-9501, USA b Division of Genetic Toxicology, National Center for Toxicological Research, Jefferson, AR 72079-9501, USA Received 24 November 1998; received in revised form 27 January 1999; accepted 1 February 1999 Abstract The metabolic activation pathways associated with carcinogenic aromatic and heterocyclic amines have long been known to involve N-oxidation, catalyzed primarily by cytochrome P4501A2, and subsequent O-esteri®cation, often catalyzed by acet- yltransferases (NATs) and sulfotransferases (SULTs). We have found a new enzymatic mechanism of carcinogen detoxi®ca- tion: a microsomal NADH-dependent reductase that rapidly converts the N-hydroxy arylamine back to the parent amine. The following N-OH-arylamines and N-OH-heterocyclic amines were rapidly reduced by both human and rat liver microsomes: N- OH-4-aminoazobenzene, N-OH-4-aminobiphenyl (N-OH-ABP), N-OH-aniline, N-OH-2-naphthylamine, N-OH-2-amino- ¯uorene, N-OH-4,4 0 -methylenebis(2-chloroaniline) (N-OH-MOCA), N-OH-1-naphthyamine, N-OH-2-amino-1-methyl-6- phenylimidazo[4,5-b]pyridine (N-OH-PhIP), N-OH-2-amino-a -carboline (N-OH-Aa C), N-OH-2-amino-3,8-dimethylimi- dazo[4,5-f]quinoxaline (N-OH-MeIQx), and N-OH-2-amino-3-methylimidazo[4,5-f]quinoline (N-OH-IQ). In addition, primary rat hepatocytes and human HepG2 cells ef®ciently reduced N-OH-PhIP to PhIP. This previously unrecognized detoxi®cation pathway may limit the bioavailability of carcinogenic N-OH heterocyclic and aromatic amines for further activation, DNA adduct formation, and carcinogenesis. q 1999 Published by Elsevier Science Ireland Ltd. Keywords: Heterocyclic amine; Aromatic amine; Detoxi®cation; N-hydroxy; PhIP; Aa C; IQ; MeIQx 1. Introduction An N-hydroxy (N-OH) amine reductase activity was ®rst reported in pig liver microsomes in 1973, and this activity was not altered by the presence of oxygen, carbon monoxide, EDTA, cyanide, nor sodium azide [1]. It required the presence of NADH or NADPH, although NADH was the preferred cofactor. This enzyme system had a pH optimum of 6.3 and its puri®cation showed that it consisted of cytochrome b 5 , cytochrome b 5 reductase, and a third partially characterized protein [2]. More recently, in human liver microsomes, this enzyme system has been studied for its ability to catalyze the reduction of N-hydroxy-sulfamethoxazole [3] and of several N- hydroxy-aminoguanidines (amidoximes) that are used Cancer Letters 143 (1999) 167±171 0304-3835/99/$ - see front matter q 1999 Published by Elsevier Science Ireland Ltd. PII: S0304-3835(99)00119-6 * Corresponding author. Tel: 11-870-543-7204; fax: 11-870- 543-7773. E-mail address: fkadlubar@nctr.fda.gov (F.F. Kadlubar) 1 Present address: Department of Biomedical Sciences, College of Pharmacy, University of Rhode Island, Kingston, RI 02281, USA Abbreviations:Aa C, 2-amino-a -carboline; ABP, 4-aminobi- phenyl; IQ, 2-amino-3-methylimidazo[4,5-f]quinoline; MeIQx, 2- amino-3,8-dimethylimidazo[4,5-f]quinoxoline; 3-MC, 3-methyl- cholanthrene; N-OH, N-hydroxy; N-OH-MBA, N-hydroxy-N- methylbenzylamine; MOCA, 4,4 0 -methylenebis(2-chloroaniline); PhIP, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine