Kinetics of the hydroperoxide-dependent dealkylation reactions catalyzed by rabbit liver microsomal cytochrome P-450.

Dennis Koop, P. F. Hollenberg

Research output: Contribution to journalArticle

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Abstract

Rabbit liver microsomal cytochrome P-450 catalyzes the dealkylation of a variety of substrates when organic hydroperoxides, peracids, or peroxyesters are substituted for NADPH and O2. The peroxide-supported demethylation of p-nitroanisole by rabbit liver microsomes exhibited normal Michaelis-Menten kinetics with respect to both substrates. The Vmax values for the hydroperoxide-dependent reactions were dependent on the identity of the hydroperoxide and with several of the peroxides they were significantly greater than those obtained with NADPH and O2. The pH profiles for the hydroperoxide-supported demethylations were similar to the profile for the NADPH-supported reaction and exhibited optima from 7.0 to 7.6, depending on the identity of the organic oxidant. The kinetic mechanism of the t-butyl hydroperoxide-supported demethylation of p-nitroanisole was determined. Plots of reciprocal velocity versus the reciprocal concentration of either substrate at several different fixed concentrations of the other substrate converged to common points of intersection on the negative side of the ordinate and above the abscissa, suggesting a sequential mechanism involving the formation of a ternary complex between cytochrome P-450, p-nitroanisole, and t-butyl hydroperoxide followed by one or more reactions and the subsequent release of the products. Potassium cyanide was a competitive inhibitor with respect to t-butyl hydroperoxide and uncompetitive with respect to p-nitroanisole. t-Butyl alcohol, a product of the t-butyl hydroperoxide-supported reaction, was a noncompetitive inhibitor with respect to both substrates. These results, which indicate that the reaction proceeds via an Ordered Bi Bi mechanism in which p-nitroanisole binds to the enzyme prior to the binding of t-butyl hydroperoxide, are discussed in relationship to the peroxidase-type mechanism which has been suggested for the action of cytochrome P-450.

Original languageEnglish (US)
Pages (from-to)9685-9692
Number of pages8
JournalJournal of Biological Chemistry
Volume255
Issue number20
StatePublished - Oct 25 1980
Externally publishedYes

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Dealkylation
tert-Butylhydroperoxide
Liver
Cytochrome P-450 Enzyme System
Hydrogen Peroxide
Rabbits
Kinetics
NADP
Substrates
Peroxides
Potassium Cyanide
1-Butanol
Liver Microsomes
Oxidants
Peroxidase
4-nitroanisole
Enzymes

ASJC Scopus subject areas

  • Biochemistry

Cite this

Kinetics of the hydroperoxide-dependent dealkylation reactions catalyzed by rabbit liver microsomal cytochrome P-450. / Koop, Dennis; Hollenberg, P. F.

In: Journal of Biological Chemistry, Vol. 255, No. 20, 25.10.1980, p. 9685-9692.

Research output: Contribution to journalArticle

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abstract = "Rabbit liver microsomal cytochrome P-450 catalyzes the dealkylation of a variety of substrates when organic hydroperoxides, peracids, or peroxyesters are substituted for NADPH and O2. The peroxide-supported demethylation of p-nitroanisole by rabbit liver microsomes exhibited normal Michaelis-Menten kinetics with respect to both substrates. The Vmax values for the hydroperoxide-dependent reactions were dependent on the identity of the hydroperoxide and with several of the peroxides they were significantly greater than those obtained with NADPH and O2. The pH profiles for the hydroperoxide-supported demethylations were similar to the profile for the NADPH-supported reaction and exhibited optima from 7.0 to 7.6, depending on the identity of the organic oxidant. The kinetic mechanism of the t-butyl hydroperoxide-supported demethylation of p-nitroanisole was determined. Plots of reciprocal velocity versus the reciprocal concentration of either substrate at several different fixed concentrations of the other substrate converged to common points of intersection on the negative side of the ordinate and above the abscissa, suggesting a sequential mechanism involving the formation of a ternary complex between cytochrome P-450, p-nitroanisole, and t-butyl hydroperoxide followed by one or more reactions and the subsequent release of the products. Potassium cyanide was a competitive inhibitor with respect to t-butyl hydroperoxide and uncompetitive with respect to p-nitroanisole. t-Butyl alcohol, a product of the t-butyl hydroperoxide-supported reaction, was a noncompetitive inhibitor with respect to both substrates. These results, which indicate that the reaction proceeds via an Ordered Bi Bi mechanism in which p-nitroanisole binds to the enzyme prior to the binding of t-butyl hydroperoxide, are discussed in relationship to the peroxidase-type mechanism which has been suggested for the action of cytochrome P-450.",
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