32P-postlabeling analysis of the DNA adducts of 6-fluorobenzo[a]pyrene and 6-methylbenzo[a]pyrene formed in vitro

Studies of benzo[a]pyrene (BP) and selected derivatives are part of the strategy to elucidate mechanisms of tumor initiation by polycyclic aromatic hydrocarbons. Substitution of BP at C-6 with fluorine to form 6-fluorobenzo[a]pyrene (6-FBP) or a methyl group to form 6-methylbenzo[a]pyrene (6-CH3BP) decreases tumorigenicity compared to BP. BP, 6-FBP, and 6-CH3BP formed adducts with DNA when (1) they were activated by 3-methylcholanthrene-induced rat liver microsomes, (2) they were activated by horseradish peroxidase (HRP), (3) their 7,8-dihydrodiols were activated by microsomes, or (4) the radical cation of BP, 6-FBP, or 6-CH3-BP was directly reacted with DNA. With microsomes, 6.5 mumol of [3H]6-FBP/mol of DNA-P and 10 mumol of [14C]6-CH3BP/mol of DNA-P were bound vs 15 mumol of [3H]BP. With microsomes, two major 6-FBP adducts and some minor adducts were obtained. One major adduct coincided with that from 6-FBP-7,8-dihydrodiol. With microsomes, the minor 6-FBP adducts coincided with the adducts obtained from 6-FBP radical cation plus DNA and the major adduct of HRP-activated 6-FBP. With microsomes, 6-CH3BP showed adducts similar to some formed with HRP and one from 6-CH3BP radical cation. 6-CH3BP-7,8-dihydrodiol produced a small amount of one adduct that did not coincide with any from 6-CH3BP. The adducts of 6-FBP appear to be formed mostly through the diolepoxide pathway, whereas those of 6-CH3BP appear to arise mostly via one-electron oxidation.