Regio‐Selectivity of Purified Forms of Rabbit Liver Microsomal Cytochrome P‐450 in the Metabolism of Benzo(a)pyrene, n‐Hexane and 7‐Ethoxyresorufin

The specificity of electrophoretically homogeneous preparations of rabbit liver microsomal cytochrome P-450LM_2-4 towards oxygenation of n-hexane, 7-ethoxyresorufin and benzo(a)pyrene was examined using a reconstituted system consisting of cytochrome P-450. NADPH-cytochrome P-450 reductase and dilauroylphatidylcholine. Epoxide hydrase was included when benzo(a)pyrene was used as substrate. Cytochrome P-450LM₂ was most active in n-hexane and benzo(a)pyrene oxygenation especially with regard to the formation of 2-hexanol, B(a)P-4,5-dihydrodiol and B(a)P-phenol metabolites. 7-Ethoxyresorufin was, however, a very poor substrate for cytochrome P-450LM₂. Cytochrome P-450LM₃ had less activity towards the investigated substrates while cytochrome P-450LM₄ preferentially formed 2- and 3-hexanol, resorufin and B(a)P-9,10-dihydrodiol. Cytochrome P-450LM₄ isolated after pretreatment with 3-methylcholanthrene or phenobarbital showed roughly the same characteristics except in the formation of 1-hexanol where cytochrome P-450LM₄ isolated after phenobarbital treatment was the most effective. The formation of B(a)P4,5- and -9,10-dihydrodiols was greatly increased by incorporation of epoxide hydrase. Our results indicate a certain specificity of the different forms of cytochrome P-450 in the liver microsomes although some overlap in activities was observed.