Methyl ethyl ketone peroxide damage to cytochrome P-450 peroxidase activities

Some new aspects of damage to cytochrome P-450 peroxidase activities by a toxic peroxide were investigated. Methyl ethyl ketone peroxide (MEKP) damage to cytochrome P-450 and inhibition of cytochrome P-450-mediated peroxidase activities were studied. In vivo, hepatic microsomal NADH- and NADPH-peroxidase activities were induced in rats by phenobarbital and 3-methylcholanthrene. Phenobarbital treatment induced more NADH- and NADPH-peroxidase activity than did 3-methylcholanthrene treatment. In vitro, microsomal cytochrome P-450 from rat liver was more sensitive to damage by MEKP than was this protein from rat kidney. Destruction of cytochrome P-450 hemoprotein and inhibition of its associated peroxidase activities by MEKP increased as a function of time of exposure to the peroxide. The addition of MEKP to microsomes resulted in a progressive increase in spectral absorbance at 433 nm. MEKP was an irreversible inhibitor of NADH- and NADPH-peroxidase, and NADH-peroxidase was more sensitive to damage than was NADPH-peroxidase. At 0 time and low concentration, MEKP was immediately bound to microsomal cytochrome P-450 and it exhibited competitive inhibition of NADH- and NADPH-peroxidase. At high concentration, MEKP was a mixed inhibitor of NADH- and NADPH-peroxidase. This study also showed that MEKP is a substrate for microsomal P-450 peroxidase activities and suggests that these peroxidase activities may play an important role in metabolism of toxic lipophilic organic peroxides that enter the endoplasmic reticulum.