CYTOCHROME-P450 METABOLIC DEALKYLATION OF 9 N-NITROSODIALKYLAMINES BYHUMAN LIVER-MICROSOMES

The metabolic dealkylation of nine nitrosodialkylamines, including fiv e symmetrical (nitrosodimethylamine, nitrosodiethylamine, nitrosodipro pylamine, nitrosodibutylamine and nitrosodiamylamine) and four asymmet rical nitrosodialkylamines (nitrosomethylethylamine, nitrosomethylprop ylamine, nitrosomethylbutylamine and nitrosomethylamylamine), was inve stigated in 14 samples of human liver microsomes, All these nitrosodia lkylamines were dealkylated to aldehydes that were separated by revers ed phase HPLC and UV detected as dinitrophenylhydrazones. As the lengt h of the alkyl chain increased from methyl to pentyl, dealkylation of symmetrical nitrosodialkylamines became less efficiently catalyzed by cytochrome P450, Conversely, oxidation of the methyl moiety of asymmet rical nitrosomethylalkylamines increased with the size of the alkyl mo iety, while dealkylation of the longer alkyl group decreased, N-Dealky lase activities were significantly correlated with P450 activities mea sured in human liver microsomes, These catalytic activities involve CY P2A6 (coumarin 7-hydroxylation), CYP2C (mephenytoin 4-hydroxylation an d tolbutamide hydroxylation), CYP2D6 (dextromethorphan O-demethylation ), CYP2E1 (chlorzoxazone and p-nitrophenol hydroxylation) and CYP3A4 ( nifedipine oxidation), By using 10 heterologously expressed P450s, it was shown that nitrosodimethylamine was mainly demethylated by CYP2E1. However, such enzyme specificity was lost with increasing size of the alkyl group, Therefore, the chain length of the alkyl group of nitros odialkylamines determined the P450 involved in its oxidation. All thes e results emphasize that the catalytic site of P450 2E1 has a geometri c configuration such that only small molecules like nitrosodimethylami ne fit favorably within the putative active site of the enzyme, Furthe rmore, there is good evidence that P450s other than P450 2E1, such as P450 2A6, 2C8/2C9/2C19 and 3A4, are involved in the metabolism of nitr osodialkylamines bearing bulky alkyl chains.