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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-esterification, often catalyzed by acetyltransferases (NATs) and sulfotransferases (SULTs). We have found a new enzymatic mechanism of carcinogen detoxification: 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: NOH-4-aminoazobenzene, N-OH-4-aminobiphenyl (N-OH-ABP), N-OH-aniline, N-OH-2-naphthylamine, N-OH-2-aminofluorene, N-OH-4,4'-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-alpha-carboline (N-OH-AalphaC), N-OH-2-amino-3,8-dimethylimidazo[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 efficiently reduced N-OH-PhIP to PhIP. This previously unrecognized detoxification pathway may limit the bioavailability of carcinogenic N-OH heterocyclic and aromatic amines for further activation, DNA adduct formation, and carcinogenesis.

Fourteen weeks-old male F344 rats maintained on a reduced caloric diet (60% of ad libitum (AL) food consumption) for 6 weeks or for 14 months did not affect the hepatic cell proliferation in terms of % S phase population, determined by evaluation of DNA synthesis in hepatocytes isolated from either young (5 months) or aged (18 months) rats. However, hepatic basal cellular DNA synthesis estimated by [3H]thymidine incorporation was reduced through acute dietary restriction (DR) in young rats, but increased in aged animals after 14 months restriction. Partial hepatectomy (PH) on aged rats stimulated hepatocyte regeneration and restored some aging-associated biochemical functions, such as drug metabolizing enzyme-dependent xenobiotic metabolic activation which was determined by measuring the formation of carcinogen-DNA adducts. Forty-eight hours after partial hepatectomy, the % of S phase population and the basal nuclear DNA synthesis of hepatocytes isolated from the partial hepatectomized DR-rats were 4- and 2.8-fold, respectively, greater than those of hepatocytes from AL-animals. DR reduced aflatoxin B1 (AFB1) metabolizing enzyme activity and decreased the AFB1-DNA adduct formation in young rats treated with AFB1. In aged AL-rats, the formation of AFB1-DNA adducts diminished to the same level as that of DR-groups and probably was due to the faster decline of drug metabolizing enzymes in aging AL-rats. However, 48 h after PH, the metabolic activation of AFB1 was restored in AL- and DR-groups which resulted in the increase of AFB1-DNA binding by 4.2 and 1.9-fold, respectively. During the liver regeneration of old PH-rats, DR inhibited the AFB1-DNA adduct formation after the PH-rats received a single dose of AFB1. DR increased benzo[a]pyrene (BaP) metabolic activation in both young and aged rats. Aging also decreased BaP-DNA adduct formation in both DR and AL-rats. The increase of BaP-DNA adduct formation in PH-groups was attributed to the restoration of BaP-metabolizing enzyme activity during liver regeneration. The PH-stimulated BaP-DNA adduct formation in AL- and DR-rats was 3.4- and 2.0-fold greater than control aged rats. Our results indicated that the stimulation of PH-induced liver regeneration by DR in aged animals may be attributed to the retardation of aging by DR and the retention of more active biochemical and enzymological functions in old DR-animals.