Characteristics of benzo(a)pyrene metabolism and cytochrome P-450 heterogeneity in rat liver nuclear envelope and comparison to microsomal membrane.

The formation of benzo(a)pyrene metabolites by highly purified nuclear envelope from hepatocytes of control rats was extremely low. and unlike the microsomal system, was not induced by phenobarbital pretreatment. However. after 3-methylcholanthrene pretreatment. metabolism increased more in nuclear envelope (60 times) than in microsomes c.5.5 times) so that the specific activities (per nmol of cytochrome P-450) equalized. Product distribution was similar between nuclear and microsomal membranes, although a shift toward more phenols was seen concomitant with a decrease in dihydrodiols. This shift relates to the lower levels of epoxide hydratase in the nuclear envelope. The metabolism of benzo(a)pyrene (-)-truns-7.8-dihydrodiol by nuclear envelope was induced by 3-methylcholanthrene treatment (13fold) and favored formation of the anti dial-oxide stereoisomer ((~)-7~.8P-dihydroxy-9P.10P-epoxy-7,8,9,lO-tetrahydrobenzo(a)pyrene). Different control over phenobarbital induction in nuclear envelope and microsomes was shown by other enzymes. Epoxide hydratase activity in nuclear envelope was decreased in animals pretreated with inducers in comparison to the 3-fold induction of the enzyme by phenobarbital in microsomes. NADPH-cytochrome c reductase was also only induced in the microsomes. Four separate forms of cytochrome P-450 were identifiable in the nuclear envelope by their type I difference spectra. The total nuclear envelope cytochrome P-450 remained the same in control and phenobarbital-treated animals; however, phenobarbital administration changed the relative proportions of the forms of cytochrome P-450. In contrast, 3-methylcholanthrene treatment doubled the net nuclear envelope cytochrome P-450 content. shifting the absorbance maximum to 448 nm. Absence of a microsomal. camphor-binding cytochrome P4,50 species in nuclear envelope preparations allowed an estimate of less than 1% maximum microsomal contamination.