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Abstract:
Benzo[a]pyrene (BP) and sulfur dioxide (SO2) are ubiquitous air pollutants and are also components of tobacco smoke. Although SO2 itself is not carcinogenic, concurrent administration with BP results in enhancement of respiratory tract tumorigenesis. In biological systems, SO2 exists as its hydrated form, sulfite (SO3(2-) ). Sulfite readily undergoes autoxidation, generating potent oxidant species. When 7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (BP-7,8-diol) is included in sulfite autoxidation mixtures it is converted to more polar products, most notably 7,8,9,10-tetrahydroxy-7,8,9,10-tetrahydrobenzo[a]pyrenes (BP tetraols). This implies the intermediacy of 7,8-dihydroxy-9,10-epoxy- 7,8,9,10-tetrahydro-benzo[a]pyrenes (BPDE). We report here the sulfite-dependent conversion of BP-7,8-diol to forms highly mutagenic to Salmonella typhimurium strain TA 98. This activation is observed at BP-7,8-diol concentrations of from 2 to 40 microM and at sulfite concentrations of from 0.5 to 10 mM. In the presence of 10 microM BP-7,8-diol, half-maximal activation is observed at 1.6 mM sulfite. Sulfite itself is neither toxic nor mutagenic to the bacteria under these conditions. The time course of the activation of BP-7,8-diol and its sensitivity to inhibition by antioxidants indicate a requirement for sulfite autoxidation. These data further support the sulfite-dependent epoxidation of BP-7,8-diol. Not only does sulfite convert this promutagen to its active mutagenic form, sulfite also enhances the mutagenic activity of BP diolepoxides toward the tester strain. The reversion frequency in response to 0.1-0.5 microM anti-BPDE is increased by up to 33% in the presence of 1 mM sulfite, and by up to 270% with 10 mM sulfite. The mechanism of this enhancement of anti-BPDE activity is not known, but could be related to inhibition of the glutathione-S-transferase system which has been previously reported for sulfite. These results are discussed in regard to the noted cocarcinogenicity of sulfur dioxide for BP.