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Metabolism of Benzo(a)pyrene by Subcellular Fractions of Rat Liver: Evidence for Similar Patterns of Cytochrome P-450 in Rough and Smooth Endoplasmic Reticulum but not in Nuclei and Plasma Membrane

Institute of Toxicology, University of Mainz, Obere Zahlbacher Strasse 67, D-6500 Mainz, Federal Republic of Germany

Since our earlier work (P. Stasiecki, F. Oesch, G. Bruder, E. D. Jarasch, and W. W. Franke, Eur. J. Cell Biol., 21: 79–92, 1980) had shown that carcinogen-metabolizing monooxygenase activity was present in almost all investigated cellular membranes, the possibility of differential control of the various metabolic pathways in the individual cellular membranes arose. Using high pressure liquid chromatography we have now studied the benzo(a)pyrene metabolites formed by rough and smooth endoplasmic reticulum, nuclei, and plasma membrane as well as mitochondrial fractions and investigated the metabolic cooperation between the monooxygenases and epoxide hydrolase in these fractions. Since various cytochrome P-450 isozymes catalyze the oxidative attack on the benzo(a)pyrene molecule at defined preferential sites, this analysis also provides an indirect trace of potential differences in the pattern of cytochrome P-450 isozymes present in the individual membranes.

The metabolic profiles produced by the two most active fractions, smooth and rough endoplasmic reticulum, were very similar to each other but different from those produced by the other three preparations. The metabolite pattern produced by incubations containing nuclear fractions differed slightly from that produced by the fractions of endoplasmic reticulum, but plasma membrane and mitochondria produced markedly different patterns.

Since the similarity of the benzo(a)pyrene metabolite pattern produced by the smooth and rough endoplasmic reticulum suggested similar cytochrome P-450 isozyme patterns in these two subfractions, they were further investigated by the use of selective inducers as well as a broad spectrum substrate, 7-ethoxycoumarin, in the absence and presence of selective inhibitors. Treatment of animals with trans-stilbene oxide or phenobarbital (a) increased the total amount of metabolites per protein mass and time, (b) changed the pattern of metabolites, but (c) induced a pattern of metabolites which was again very similar in rough and smooth endoplasmic reticulum. Even more distinct changes were found following treatment with 3-methylcholanthrene or ß-naphthoflavone. Both of these compounds (a) preferentially induced the activity of rough endoplasmic reticulum, (b) changed the profile of metabolites, but (c) again did not disturb the similarities of the benzo(a)pyrene metabolite pattern between both fractions. Selective inhibitors (methyrapone, -naphthoflavone, tetrahydrofuran) had characteristically different effects on 7-ethoxycoumarin O-deethylase activity in preparations from untreated versus trans-stilbene oxide (or phenobarbital)-treated versus ß-naphthoflavone (or 3-methylcholanthrene)-treated rats, but in any of these situations the effects of the selective inhibitors were similar in the smooth and rough endoplasmic reticulum fraction.

These findings indicate that the composition of cytochrome P-450 isozymes is different in nuclei, plasma membrane, mitochondrial fractions, and the endoplasmic reticulum but that the isoenzyme pattern of smooth and rough endoplasmic reticulum is similar even after enzyme induction.

The total amount of benzo(a)pyrene metabolites produced by smooth and rough endoplasmic reticulum fractions from controls and from the animals treated with the various inducers correlated well with 7-ethoxycoumarin O-deethylase activities in absence of inhibitors and in presence of metyrapone or tetrahydrofuran (r = +0.983, +0.995, +0.990) but not in the presence of -naphthoflavone (r = -0.225), indicating that the monooxygenase forms inhibited by -naphthoflavone were major contributors to benzo(a)pyrene metabolism in all these fractions. No good correlation was found between dihydrodiol formation and epoxide hydrolase activity. In line with the relatively high ratio of epoxide hydrolase to monooxygenase in rat liver microsomes, this indicates that in all these fractions epoxide hydration is not the ratedetermining step in the formation of the dihydrodiols studied.

Received 6/28/83. Revised 12/ 5/84. Accepted 6/ 3/85.

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