PARTICIPATION OF CYTOCHROMES P4502B AND P4503A IN COCAINE TOXICITY INRAT HEPATOCYTES

The contributions of cytochromes P4502B (P4502B) and cytochrome P4503A (P4503A) to the bioactivation of cocaine in hepatocytes isolated from Sprague-Dawley rats were assessed using a number of approaches. Hepat ocytes were isolated from rats pre-treated with either phenobarbital o r dexamethasone. Exposure to from 50 to 500 mu M of either cocaine or norcocaine resulted in toxicity in hepatocytes from phenobarbital-indu ced rats. Hepatocytes from dexamethasone-induced rats displayed greate r resistance to toxicity mediated by either compound. Although microso mes from dexamethasone- and phenobarbital-induced rats catalyzed cocai ne N-demethylation at the same rate, only inhibition of P4502B activit y by chloramphenicol and not inhibition of P4503A activity by troleand omycin was associated with protection against cocaine or norcocaine-me diated toxicity, Further, inhibition of P4502B was only effective in p rotecting against toxicity in hepatocytes isolated from phenobarbital- induced rats. The effects of phenobarbital induction in rats, dogs, an d guinea pigs, and the abilities of purified P4502B proteins from rats , dogs, and rabbits to N-demethylate cocaine were investigated. Cytoch romes P4502B from different species exhibited different rates of cocai ne N-demethylation; microsomes from the guinea pig were able to N-deme thylate cocaine at the fastest rate, followed by the dog and the rat, Expressed human P4502B6 exhibited no ability to either N-demethylate c ocaine or produce cocaine- or norcocaine-mediated toxicity in lymphobl astoid cells, These results suggest that, although P4502B and P4503A b oth catalyze the initial oxidation of cocaine in rats, only P4502Bs ar e involved in further oxidations leading to toxicity, The importance o f P4502Bs toward cocaine bioactivation will depend on species-specific isoform activities.