Effect of aromatic nitro compounds on oxidative metabolism by cytochrome P-450 dependent enzymes.

The effect of aromatic nitro compounds on the oxidative metabolism of representative type I (hexobarbital and aminopyrene) and type II (aniline and zoxazolamine) substrates by cytochrome P-450 dependent liver enzymes was studied. Nitro compounds (nitrobenzene, p-nitrobenzoate, 2-nitrofluorene, and 2-nitronaphthalene) inhibited the oxidation of type II substrates by rabbit liver microsomal enzymes; however, they had no effect on the metabolism of the type I compounds. Inhibition of type II metabolism was characterized graphically as S,I-hyperbolic non-competitive. The influence of aromatic nitro compounds on the interaction of type I and type II substrates with oxidized and reduced cytochrome P-450 was studied by difference spectroscopy. From Lineweaver-Burke plots, nitro compounds were shown to competitively interact with type II compounds for cytochrome p-450 binding sites. Nitro compounds completely prevented the appearance of a type I binding spectrum with either hexobarbital or aminopyrene even when the modifier was present at concentrations less than 10(-8)M. Aromatic nitro compounds appear to therefore inhibit the metabolism of the type II substrates through a mixed mechanism of interaction with the microsomal drug-metabolizing enzymes.