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UVA light-induced DNA single-strand cleavage by 1-hydroxy, 3-hydroxy, 7-hydroxy, and 9-hydroxybenzo[ a ]pyrenes (OH-B a Ps) and 6-acetoxybenzo[ a ]pyrene (6-OAc-B a P) was studied. Under experimental conditions, the concentrations of 1-OH, 3-OH, 7-OH, and 9-OH-B a Ps and 6-OAc-B a P needed to cause 25% of the supercoiled form I plasmid DNA to become relaxed form II DNA were found to be 0.6, 2.5, 1.0, 1.3, and 1.1 μM, respectively. These concentrations are all smaller than that of B a P, which was 6 μM. These results indicate that on photoirradiation, OH-B a Ps are more cytotoxic and/or genotoxic than their parent compound, B a P. Mechanistic studies reveal that singlet oxygen and superoxide free radicals are involved in causing DNA cleavage. [ABSTRACT FROM AUTHOR]

The combination of UVA light and 1-aminopyrene, 1-hydroxypyrene, 1-hydroxybenzo[ a ]pyrene, 3-aminofluoranthene, 6-aminochrysene, or 5-, 6-, and 7-methylbenz[ a ]anthracenes causes DNA single-strand cleavage. 1-Hydroxypyrene, 1-hydroxybenzo[ a ]pyrene, and 5-methylbenz[ a ]anthracene have been shown to cause DNA cleavage, at least partially, by generating singlet oxygen. Therefore, the presence of histidine, a singlet oxygen quencher, should inhibit the DNA photocleavage. However, the presence of 50 mM histidine greatly enhances the DNA photocleavage caused by these compounds. This effect is due to the inhibition of the photodegradation of the PAH compounds. Therefore, care must be taken when interpreting the singlet oxygen quenching data by histidine. Histidine may coexist with PAHs that have entered the body. The presence of histidine can alter the photochemical reaction and, possibly, the phototoxicity mechanism of the PAHs. [ABSTRACT FROM AUTHOR]

Two sources of humic acid (HA) were added to a phosphate-buffered solution to determine the effect of HA on 1-aminopyrene (1-AP) photolysis rate under UV-A irradiation and natural solar irradiation. Microbial assemblages in a river water sample were used to determine the ecotoxicity of 1-AP and its photoproducts. The microbial bioassay includes spread plate counting and microbial mineralization of 14 C-D-glucose, succinate, and citrate. At 1-AP concentration of 8 μM, bacterial viability was not affected and heterotrophic activities were inhibited by 41% to 63% after exposure for 24 hr. Exposure to 1-AP photoproducts inhibited bacterial viability and heterotrophic activities by 58% and 77% to 84%, respectively. Therefore, photoinduced toxicity of 1-AP to microbial assemblages was observed in this study. With additions of HA at concentrations of 10-80 ppm, the photolysis rate constants of 1-AP (10 μM) in phosphate-buffered (pH 7.0; 1 mM) solutions were enhanced by up to fivefold. The magnitude of the enhancement depends on the type and concentration of HA. Under irradiation from both sources, photoysis rate of 1-AP was four to five times higher in the river water than that in phosphate-buffered solution. The enhancement is assumed to be the result of sensitization by HA present in the river water. [ABSTRACT FROM AUTHOR]