nitrophenols and 4-nitrophenetol

Although most cytochrome P450 (P450) reactions demonstrate saturation kinetics that fit to the standard Michaelis-Menten equation, there are important exceptions where sigmoidal or nonhyperbolic behavior is observed and have been fit instead to kinetic models involving two binding sites. To assess these models, we demonstrate the consistency of a two binding site model to interpret both steady-state kinetics and binding events. Rates of 4-nitrophenol and formaldehyde production from the O-demethylation of 1-methoxy-4-nitrobenzene by P450 1A2 isolated from rabbit liver produced biphasic plots, when plotted against substrate concentration. Experiments confirmed the absence of the further oxidation of the products. Recombinant rabbit P450 1A2 yielded the same maximal velocity and more marked biphasicity. Overall, these steady-state data fit well to kinetic models involving two binding sites. Steady-state studies of substrates with bulkier O-ethyl or O-isopropoxy groups indicated decreased affinity for the second site. Based on binding studies, the affinity of P450 1A2 for these substrates increased 200-fold with the larger alkyl groups. To analyze the single binding site model, competition studies were conducted with 1,4-phenyldiisocyanide and the alkyl 4-nitrophenyl ethers. Although the observed dissociation constants and the competing titrant demonstrated a linear dependence, the affinity for the competing titrant depended on the presence of the other titrant, which violates the single binding site model. Alternatively, we applied a two binding site model to these data to obtain dissociation constants for the binary and ternary complexes. The agreement between the dissociation constants for the heterogeneous complexes supports the appropriateness of the two binding site model. This novel finding for P450 1A2 may be more common than originally perceived for P450s.

Topics: Animals; Binding, Competitive; Cytochrome P-450 CYP1A2; Kinetics; Ligands; Microsomes, Liver; Models, Chemical; Nitrobenzenes; Nitrophenols; Oxidation-Reduction; Protein Binding; Rabbits; Substrate Specificity; Titrimetry; Vinyl Compounds

The rate of conversion of p-nitrophenetole to p-nitrophenol by rat liver microsomes was studied. Inhibition of the reaction by CO and by SKF 525A and the absolute dependence on NADPH and oxygen indicate that cytochrome P-450 catalyzes the reaction. The apparent Km for oxygen was 0.07 microM. Furthermore, cytochrome b5 seemed to be involved in the formation of p-nitrophenol. The effect on p-nitrophenol formation of drugs known to be involved in drug interaction in clinical practice was studied. There was a competitive inhibition by phenytoin (inhibitor constant, Ki, 30 microM), disulfiram (Ki, 2 microM) and chloramphenicol (Ki, 20 microM), whereas a mixed-type inhibition by isoniazid was observed (Ki, 1,3 mM and Kii, 10,6 mM).

Topics: Animals; Biotransformation; Carbon Monoxide; Chloramphenicol; Cytochrome P-450 Enzyme System; Disulfiram; Drug Interactions; Isoniazid; Kinetics; Male; Microsomes, Liver; Nitrobenzenes; Nitrophenols; Oxidation-Reduction; Phenytoin; Proadifen; Rats; Rats, Inbred Strains