1-Nitropyrene incubations

Compared to the extensive data that have been obtained on the mutagenicity of nitro PAHs in S. typhimurium, relatively little is known about the metabolism of these compounds in this organism. Messier et al. (67) reported that incubation of 1-nitropyrene with S. typhimurium TA98 yielded 1-aminopyrene and 1-acetylaminopyrene as major and minor metabolites, respectively. The reduction of 1-nitropyrene was slow and was accompanied by a slow formation of DNA adducts. When incubations were conducted with the nitroreductase-deficient strain, TA100 F50, both the extent of 1-amino-pyrene formation and DNA binding decreased. Howard ej al. (71,115) also found reduction of 1-nitropyrene to 1-aminopyrene in strains TA98, TA1538 and ATCC 14028. 

Although 1-aminopyrene is a reduced metabolite of 1-nitropyrene, this arylamine will not covalently bind to DNA in vitro (72). In contrast, when incubations were conducted with the intermediate reduction product, N-hydroxy-l-aminopyrene, extensive covalent binding to DNA was detected (72). This observation is consistent with the previous report that several N-hydroxy arylamines formed DNA adducts and induced mutations in S. typhimurium (116), and suggests that, at least for 1-nitropyrene, reduction to N-hydroxy-l-aminopyrene is a critical step in mutation induction. 

Other bacteria. Intestinal bacteria may play a critical role in the metabolic activation of certain nitroaromatic compounds in animals (119) and several reports have appeared on the metabolism of nitro PAHs by rat and human intestinal contents and microflora (120-123). Kinouchi et al. (120) found that 1-nitropyrene was reduced to 1-aminopyrene when incubated with human feces or anaerobic bacteria. More recently, Kinouchi and Ohnishi (121) isolated four nitroreductases from one of these anaerobic bacteria (Bacteroides fragilis). Each nitroreductase was capable of converting 1-nitropyrene into 1-aminopyrene, and one form catalyzed the formation of a reactive intermediate capable of binding DNA. Howard ej al. (116) confirmed the reduction of 1-nitropyrene to 1-aminopyrene by both mixed and purified cultures of intestinal bacteria. Two additional metabolites were also detected, one of which appeared to be 1-hydroxypyrene. Recently, similar experiments have demonstrated the rapid reduction of 6-nitro-BaP to 6-amino-BaP (123). 

Figure 7. Accumulation of a mutagenic metabolite in an incubation mixture containing 1-nitropyrene (2 Ag/mL) and S9 enzymes (10 iL/mL). The activity is attributable to the extracted material from 0.1 mL of the incubation mixture with TA98 and TA98NR or from 0.4 mL with TA98/1,8-DNP. ...

Figure 8. The location of the mutagenic products extracted from the incubation of 1-nitropyrene with S9 enzymes after TLC separation on silica gel using the solvent mixture toluene/dichloromethane/methanol (9 1 0.4).

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