pH effects on the N-demethylation and formation of the cytochrome P-450 iron II nitrosoalkane complex for erythromycin derivatives

• Published in: Chemico-biological interactions Vol. 85; no. 2-3; p. 215
• Main Authors: Delaforge, M, Ladam, P, Bouillé, G, Benarous, J G, Jaouen, M, Girault, J P
• Format: Journal Article
• Language: English
• Published: Ireland 01.12.1992
• Subjects: Animals; Anti-Bacterial Agents - metabolism; Binding Sites; Cytochrome P-450 Enzyme System - metabolism; Dexamethasone - pharmacology; Erythromycin - analogs & derivatives; Erythromycin - metabolism; Ferrous Compounds - metabolism; Hydrogen-Ion Concentration; Kinetics; Liver - enzymology; Male; Methylation; Nitroso Compounds - metabolism; Rats; Rats, Sprague-Dawley; Roxithromycin - metabolism; Structure-Activity Relationship
• ISSN: 0009-2797; 1872-7786
• DOI: 10.1016/0009-2797(92)90063-Q

The effects of pH on access to the cytochrome P-450 active site, N-demethylation and formation of the cytochrome P-450 Fe(II)-RNO metabolite complex for a series of erythromycin derivatives were examined. Studies were performed with dexamethasone-treated rat liver microsomes containing large amounts of cytochrome P-450 3A isozymes. In addition to factors such as hydrophobicity or hindrance around the dimethyl-amino function, the ionisation state of the N(CH3)2 group played an important role in the recognition and metabolism of the substrate by cytochrome P-450. Esterification of the desosamine in the beta position of the N(CH3)2 group leads to lower pKa values for the R--N+ H(CH3)2 <--> [R--N (CH3)2] + H+ equilibrium. At physiological pH, the amine group is mainly in the unprotonated form. Consequently, easier access to the protein active site and significant formation of cytochrome P-450 Fe(II)-RNO metabolite complex are observed for these derivatives. These results led us to interpret the formation of cytochrome P-450 Fe(II)-RNO metabolite complex as a series of multiple steps equilibria depending on the ionisation state of the N(CH3)2 group, the partition coefficient of the substrate between the microsomal layer and the aqueous media and a series of metabolic reactions leading partially to the final inhibitory nitrosoalkane-cytochrome P-450 Fe(II) complex.