Species differences in the biochemical properties of liver microsomal arylamine and arylamide N-hydroxylases

Cytochrome P-448 dependent microsomal N-hydroxylases are key enzymes in the metabolic activation of both arylamides and arylamines.Using 2-acetylaminofluorene (2-AAF) and 2-aminofluorene (2-AF) as substrates, the present report compares the biochemical properties of rat, hamster and mouse liver N-hydroxylases. There are marked species differences both in terms of the affinity for the two substrates and in terms of maximum velocity of the enzymes. The rat and hamster liver arylamide N-hydroxylases are induced by pretreatment with 2-AAF which also significantly increases their affinity for the substrate. In mouse liver neither arylamide nor arylamine N-hydroxylases are modified or induced. With 2-AF as substrate, arylamide treatment never enhances N-hydroxylation but it reduces the K_m-value of the rat and hamster liver enzymes.Among the effectors tested in vitro, 3-methylcholanthrene (3-MC), 7,8-benzoflavone (BF), benzoα]pyrene (Bα]P) and miconazole (MN) inhibit hepatic arylamide N-hydroxylase in the submicromolar range. Harman (H) and paraoxon (PX) act in a dose-dependent manner in the micromolar range and metyrapone (MP) is not an inhibitor even at 50-μM concentration.Among the position isomers, 1- and 3-AAF are inhibitors of the N-hydroxylating enzymes whereas 4-AAF is not.These data are discussed in relationship to the toxic effects (mutagenicity and hepatocarcinogenicity) of arylamides and arylamines with respect to the role and the complexity of their microsomal metabolism.