title: The role of metabolic activation by cytochrome P-450 in covalent binding of VP 16-213 to rat liver and HeLa cell microsomal proteins.
abstract: Covalent binding of 3H-labeled VP 16-213 to rat liver and HeLa cell microsomal proteins was studied in vitro. Metabolic activation by cytochrome P-450 was found to play a role in the covalent binding of VP 16-213 to rat liver microsomal proteins, as shown by the need of NADPH cofactor, the increased binding after phenobarbital pretreatment and the inhibition by SFK-525A. Addition of ascorbic acid or alpha-phenyl-N-tert. butylnitrone to the incubation mixture depressed covalent binding by about 85%, suggesting that formation of a reactive metabolite from the phenolic structure may be involved in the binding process. VP 16-213 did not inhibit aminopyrine N-demethylase at the concentration used in the binding experiments (17 microM), indicating that metabolism of its methylenedioxy group does not play a role in binding to microsomal proteins. HeLa cell microsomes were found to possess aminopyrine N-demethylase activity. Covalent binding of radiolabeled VP 16-213 to HeLa cell microsomes decreased by about 64% if NADPH was omitted.
mesh_terms: D000818:Animals; D001205:Ascorbic Acid / Q000494:pharmacology; D001711:Biotransformation / Q000187:drug effects; D003577:Cytochrome P-450 Enzyme System / Q000494:pharmacology*; D005047:Etoposide / Q000378:metabolism*; D006367:HeLa Cells / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D008861:Microsomes / Q000378:metabolism; D008862:Microsomes, Liver / Q000378:metabolism*; D009249:NADP; D009363:Neoplasm Proteins / Q000378:metabolism*; D009589:Nitrogen Oxides / Q000494:pharmacology; D010634:Phenobarbital / Q000494:pharmacology; D011034:Podophyllotoxin / Q000031:analogs & derivatives*; D011335:Proadifen / Q000494:pharmacology; D011485:Protein Binding; D011725:Pyridines; D051381:Rats; D011919:Rats, Inbred Strains
chemical_list: D009363:Neoplasm Proteins; D009589:Nitrogen Oxides; D011725:Pyridines; C043438:alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone; D009249:NADP; D005047:Etoposide; D003577:Cytochrome P-450 Enzyme System; D011335:Proadifen; D011034:Podophyllotoxin; D001205:Ascorbic Acid; D010634:Phenobarbital