Characterization of cerebral 7-ethoxycoumarin-O-deethylase: evidence for multiplicity of cytochrome P450 in brain.

Characterization of 7-ECOD regulated by a number of cytochrome P450 species in liver has been undertaken in brain to understand whether it can metabolize a wide variety of substrates. Maximum activity of the enzyme was recorded when 0.8 to 2.0 mg of brain microsomal protein was added to the assay mixture at pH 7.4 and incubated for 30 min. The enzyme activity was found to be localized in the microsomal fraction with the maximum activity being in the olfactory lobes and the minimum in the pons medulla region. Kinetic studies revealed that rat brain microsomes exhibited a monophasic pattern of enzyme activity with a Km value of 2 mM. Rats pretreated with phenobarbital exhibited no change in enzyme activity whereas 3-methylcholanthrene preferentially stimulated enzyme activity (two- to threefold). The differential induction of enzyme activity after MC pretreatment and the differential response of alpha-naphthoflavone and metyrapone in vitro on 7-ECOD activity in control and MC-induced microsomes of rats, mice, guinea pigs, and rabbits do suggest the possibility of the presence of multiple forms of cytochrome P450 in the brain. Significant age, sex, and species-related differences observed in the activity of ECOD have also indicated that different forms of cytochrome P450 may be regulating the enzyme activity in the brain.