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Isozyme-selective metabolic intermediate complex formation of guinea pig hepatic cytochrome P450 by N-aralkylated derivatives of 1-aminobenzotriazole. 1995

C J Sinal, and J R Bend
Department of Pharmacology and Toxicology, University of Western Ontario, London, Canada.

The capacity for metabolic intermediate (MI) complex formation as a mechanism of action for the isozyme-selective cytochrome P450 (P450) inhibitors N-benzyl-1-aminobenzotriazole (BBT), N-(alpha-methylbenzyl)-1-aminobenzotriazole (alpha MB), and N-(alpha-ethylbenzyl)-1-aminobenzotriazole (alpha EB) was investigated in hepatic microsomes from untreated, phenobarbital-induced, and beta-naphthoflavone-induced guinea pigs. Similar to other complex forming amines, MI complex formation was observed as an absorbance maximum at approximately 455 nm by optical-difference spectroscopy, was dependent upon incubation with NADP(H), and was dissociable by the addition of 50 microM potassium ferricyanide. MI complexes formed by BBT, alpha MB, and alpha EB were also dissociable by sedimentation and resuspension, as well as in the presence of limiting concentrations of NADP(H). Maximal complexation with the three compounds was observed in microsomes from phenobarbital-induced guinea pigs where the initial rate of complex formation was dependent upon inhibitor concentration and apparent Km values of 108 +/- 44, 338 +/- 96, and 84 +/- 15 microM for BBT, alpha MB, and alpha EB, respectively, were found. Inclusion of 1 mM glutathione in the incubation mixtures had a significant attenuating effect upon complex formation, suggesting the involvement of an electrophilic, reactive intermediate. Complex formation was not observed with the three inhibitors in pulmonary microsomes from either guinea pigs or rabbits. MI complexation is not likely to contribute to the mechanism-based inactivation of guinea pig hepatic P450 2Bx, the homologue of rabbit P450 2B4, due to the irreversible inactivation of this isoform at very low inhibitor concentrations, the lack of glutathione attenuation of this destruction, the instability of formed MI complexes, and the absence of MI complex formation with guinea pig or rabbit pulmonary P450.

UI MeSH Term Description Entries
D007527 Isoenzymes Structurally related forms of an enzyme. Each isoenzyme has the same mechanism and classification, but differs in its chemical, physical, or immunological characteristics. Alloenzyme,Allozyme,Isoenzyme,Isozyme,Isozymes,Alloenzymes,Allozymes
D007700 Kinetics The rate dynamics in chemical or physical systems.
D008168 Lung Either of the pair of organs occupying the cavity of the thorax that effect the aeration of the blood. Lungs
D008297 Male Males
D008861 Microsomes Artifactual vesicles formed from the endoplasmic reticulum when cells are disrupted. They are isolated by differential centrifugation and are composed of three structural features: rough vesicles, smooth vesicles, and ribosomes. Numerous enzyme activities are associated with the microsomal fraction. (Glick, Glossary of Biochemistry and Molecular Biology, 1990; from Rieger et al., Glossary of Genetics: Classical and Molecular, 5th ed) Microsome
D009249 NADP Nicotinamide adenine dinucleotide phosphate. A coenzyme composed of ribosylnicotinamide 5'-phosphate (NMN) coupled by pyrophosphate linkage to the 5'-phosphate adenosine 2',5'-bisphosphate. It serves as an electron carrier in a number of reactions, being alternately oxidized (NADP+) and reduced (NADPH). (Dorland, 27th ed) Coenzyme II,Nicotinamide-Adenine Dinucleotide Phosphate,Triphosphopyridine Nucleotide,NADPH,Dinucleotide Phosphate, Nicotinamide-Adenine,Nicotinamide Adenine Dinucleotide Phosphate,Nucleotide, Triphosphopyridine,Phosphate, Nicotinamide-Adenine Dinucleotide
D003577 Cytochrome P-450 Enzyme System A superfamily of hundreds of closely related HEMEPROTEINS found throughout the phylogenetic spectrum, from animals, plants, fungi, to bacteria. They include numerous complex monooxygenases (MIXED FUNCTION OXYGENASES). In animals, these P-450 enzymes serve two major functions: (1) biosynthesis of steroids, fatty acids, and bile acids; (2) metabolism of endogenous and a wide variety of exogenous substrates, such as toxins and drugs (BIOTRANSFORMATION). They are classified, according to their sequence similarities rather than functions, into CYP gene families (>40% homology) and subfamilies (>59% homology). For example, enzymes from the CYP1, CYP2, and CYP3 gene families are responsible for most drug metabolism. Cytochrome P-450,Cytochrome P-450 Enzyme,Cytochrome P-450-Dependent Monooxygenase,P-450 Enzyme,P450 Enzyme,CYP450 Family,CYP450 Superfamily,Cytochrome P-450 Enzymes,Cytochrome P-450 Families,Cytochrome P-450 Monooxygenase,Cytochrome P-450 Oxygenase,Cytochrome P-450 Superfamily,Cytochrome P450,Cytochrome P450 Superfamily,Cytochrome p450 Families,P-450 Enzymes,P450 Enzymes,Cytochrome P 450,Cytochrome P 450 Dependent Monooxygenase,Cytochrome P 450 Enzyme,Cytochrome P 450 Enzyme System,Cytochrome P 450 Enzymes,Cytochrome P 450 Families,Cytochrome P 450 Monooxygenase,Cytochrome P 450 Oxygenase,Cytochrome P 450 Superfamily,Enzyme, Cytochrome P-450,Enzyme, P-450,Enzyme, P450,Enzymes, Cytochrome P-450,Enzymes, P-450,Enzymes, P450,Monooxygenase, Cytochrome P-450,Monooxygenase, Cytochrome P-450-Dependent,P 450 Enzyme,P 450 Enzymes,P-450 Enzyme, Cytochrome,P-450 Enzymes, Cytochrome,Superfamily, CYP450,Superfamily, Cytochrome P-450,Superfamily, Cytochrome P450
D005978 Glutathione A tripeptide with many roles in cells. It conjugates to drugs to make them more soluble for excretion, is a cofactor for some enzymes, is involved in protein disulfide bond rearrangement and reduces peroxides. Reduced Glutathione,gamma-L-Glu-L-Cys-Gly,gamma-L-Glutamyl-L-Cysteinylglycine,Glutathione, Reduced,gamma L Glu L Cys Gly,gamma L Glutamyl L Cysteinylglycine
D006168 Guinea Pigs A common name used for the genus Cavia. The most common species is Cavia porcellus which is the domesticated guinea pig used for pets and biomedical research. Cavia,Cavia porcellus,Guinea Pig,Pig, Guinea,Pigs, Guinea
D000818 Animals Unicellular or multicellular, heterotrophic organisms, that have sensation and the power of voluntary movement. Under the older five kingdom paradigm, Animalia was one of the kingdoms. Under the modern three domain model, Animalia represents one of the many groups in the domain EUKARYOTA. Animal,Metazoa,Animalia

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