A comparison of the intercalative binding of non-reactive benzo[a]pyrene metabolites and metabolite model compounds to DNA.

The reversible DNA physical binding of a series of non-reactive metabolites and metabolite model compounds derived from benzo[a]pyrene (BP) has been examined in UV absorption and in fluorescence emission and fluorescence lifetime studies. Members of this series have steric and pi electronic properties similar to the highly carcinogenic metabolite trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) and the less potent metabolite 4,5-epoxy-4,5-dihydrobenzo(a)pyrene (4,5-BPE). The molecules examined are trans-7,8-dihydroxy-7,8-dihydrobenzo[a]-pyrene (7,8-di(OH)H2BP), 7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene (tetrol) 7,8,9,10-tetrahydrobenzo[a]pyrene (7,8,9,10-H4BP), pyrene, trans-4,5-dihydroxy-4,5-dihydrobenzo[a]pyrene (4,5-di(OH)H2BP) and 4,5-dihydrobenzo[a]pyrene (4,5-H2BP). In 15% methanol at 23 degrees C the intercalation binding constants of the molecules studied lie in the range 0.79-6.1 X 10(3) M-1. Of all the molecules examined the proximate carcinogen 7,8-di(OH)-H2BP is the best intercalating agent. The proximate carcinogen has a binding constant which in UV absorption studies is found to be 2.8-6.0 times greater than that of the other hydroxylated metabolites. Intercalation is the major mode of binding for 7,8-di(OH)H2BP and accounts for more than 95% of the total binding. Details concerning the specific role of physical bonding in BP carcinogenesis remain to be elucidated. However, the present studies demonstrate that the reversible binding constants for BP metabolites are of the same magnitude as reversible binding constants which arise from naturally occurring base-base hydrogen bonding and pi stacking interactions in DNA. Furthermore, previous autoradiographic studies indicate that in human skin fibroblasts incubated in BP, pooling of the unmetabolized hydrocarbons occurs at the nucleus. The high affinity of 7,8-di(OH)H2BP for DNA may play a role in similarly elevating in vivo nuclear concentrations of the non-reactive proximate carcinogen.