NAT1 and NAT2 have different but overlapping substrate specificities, although no substrate is exclusively acetylat-ed by one isozyme or the other, and no clear structural motif that determines substrate specificity for the different isoforms has been identified. Substrates preferentially N-acetylated by human NAT1 are p-aminobenzoic acid, p-aminosalicylic acid, sulfamethoxazole, and the folate catabolite p-aminobenzoyl glutamate. Substrates primarily N-acetylated by human NAT2 include a number of sul-fonamides (sulfamethazine, sulfapyridine, sulfadiazine, sulfameridine, and sulfadoxine), isoniazid, aminoglutethi-mide, amonafide, procainamide, dapsone, dipyrone, endra-lazine, hydralazine, prizidilol, batracylin, and metabolites of clonazepam and caffeine. Some compounds, such as the carcinogenic aromatic amines 2-aminofluorene, ben-zidine, 4-aminobiphenyl, 4,4-dichloroaniline and 2-naph-thylamine, and the food-derived heterocyclic amines 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine and 2-amino-3,4-dimethyl-imidazo[4,5-f]quinoxaline, are N-acetylated to varying degrees by both human NAT1 and NAT2. Generally, O-acetylation of the N-hydroxy metabolites of carbocyclic arylamines is catalyzed selectively by NAT1, whereas NAT2 O-acetylates N-hydroxy metabolites of the dietary heterocyclic amine carcinogens.
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