title: pH effects on the N-demethylation and formation of the cytochrome P-450 iron II nitrosoalkane complex for erythromycin derivatives.
abstract: 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.
authors.affiliation: Université R. Descartes, URA 400 CNRS, Paris, France.
authors.forename: M
authors.initials: M
authors.lastname: Delaforge
authors.forename: P
authors.initials: P
authors.lastname: Ladam
authors.forename: G
authors.initials: G
authors.lastname: Bouillé
authors.forename: J G
authors.initials: JG
authors.lastname: Benarous
authors.forename: M
authors.initials: M
authors.lastname: Jaouen
authors.forename: J P
authors.initials: JP
authors.lastname: Girault
mesh_terms: D000818:Animals; D000900:Anti-Bacterial Agents / Q000378:metabolism*; D001665:Binding Sites; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D003907:Dexamethasone / Q000494:pharmacology; D004917:Erythromycin / Q000031:analogs & derivatives / Q000378:metabolism*; D005296:Ferrous Compounds / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008099:Liver / Q000201:enzymology; D008297:Male; D008745:Methylation; D009603:Nitroso Compounds / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D015575:Roxithromycin / Q000378:metabolism; D013329:Structure-Activity Relationship
chemical_list: D000900:Anti-Bacterial Agents; D005296:Ferrous Compounds; D009603:Nitroso Compounds; D015575:Roxithromycin; C045512:erythralosamine; D004917:Erythromycin; D003907:Dexamethasone; D003577:Cytochrome P-450 Enzyme System