Coal tar is a complex mixture that exhibits high carcinogenic potency in lungs of animals when administered in the diet. Studies have noted that lung tumor induction does not correlate with the benzo[a]pyrene content of coal tar, suggesting that other hydrocarbons may be involved in the observed tumorigenicity. Our previous studies have demonstrated that a major 'unknown' chemical-DNA adduct is formed in the lung of mice exposed to coal tar. We have used an in vitro rat microsomal activation system to generate the 'unknown' adduct with neat coal tar and fractions of coal tar obtained by chemical fractionation and HPLC. Chemical-DNA adduct formation was evaluated by (32)P-postlabeling using both multi-dimensional TLC and HPLC. GC-MS analysis of the coal tar fractions obtained from HPLC, which produced the 'unknown' adduct in vitro, demonstrated that the adducting hydrocarbon had a mass of 216. A careful evaluation of candidate hydrocarbons led to the conclusion that a benzofluorene derivative may be responsible for forming the 'unknown' chemical-DNA adduct. Comparative in vitro and in vivo studies on the adducting properties of all three isomers of benzofluorene indicated that 7H-benzo[c]fluorene is responsible for producing the 'unknown' adduct observed in the lung of mice ingesting coal tar. Animal feeding studies also demonstrated that 7H-benzo[c]fluorene formed considerably more lung DNA adducts than 11H-benzo[a]fluorene and 11H-benzo[b]fluorene. These data indicate that the four-ring polycyclic aromatic hydrocarbon 7H-benzo[c]fluorene, a hydrocarbon not previously shown to form DNA adducts in lung, is in fact a potent lung DNA adductor and is a candidate PAH for causing lung tumors in animals treated with coal tar.

Both syn and anti dihydrodiol epoxides from 5-methylchrysene (5-MCDE) and 5,6-dimethylchrysene (5,6-DMCDE) were reacted under the same conditions with native DNA, denatured DNA, and purine deoxyribonucleotides, and the products were quantified. The extents of reaction with the deoxyribonucleotides were consistently greater for 5,6-DMCDE than for 5-MCDE. The yield of adducts in the reaction with DNA ranged from being a few-fold to 50-fold greater than those found in the corresponding deoxyribonucleotide reactions for both 5-MCDE and 5,6-DMCDE. The DNA-dependent enhancement of product yield was greater for 5-MCDE than for 5,6-DMCDE with a few exceptions among cis and trans deoxyadenosine adducts. The most substantial differences in DNA-dependent enhancement were found for deoxyguanosine adducts; thus, steric hindrance between the 6-methyl group in the 5,6-DMCDE and the minor groove in the DNA double helix may account for the greater DNA-dependent enhancement found in the 5-MCDE reactions.