Bacterial nitroreductase

Fig. 8.17. Mechanism of bioactivation of self-immolative carbamate prodrugs of cytotoxic amines. A bacterial nitroreductase coupled to a tumor-directed antibody reduces the prodrug to a hydroxylamine (Reaction a), which breaks down spontaneously to liberate the antitumor...

An important microbiological clue to the chemical nature of genotoxic species in primary emissions and ambient POM has been provided by strain TA98NR (Mermelstein et al., 1981 Rosenkranz and Mermel-stein, 1983, 1985a, 1985b). This isolate of TA98 is deficient in the classical bacterial nitroreductase that catalyzes the bioactivation of most mononitro-PAHs to... 

Einisto P, Role of bacterial nitroreductase and O-acetyltransferase in mine mutagenicity assay of rats exposed to 2,4,6-trinitrotoluene (TNT), Mut. Res., 262,167,1991. 

The nitroaromatic compounds such as the nitrofurans are known to be activated by a bacterial nitroreductase system in susceptible microorganisms. Intermediate, highly reactive species such as free radicals produced during the reduction process are likely responsible for damage to DNA strands that lead to bacterial and protozoal cell death. Thus a reduced nitroaryl anion radical could be oxidized by 02 to produce superoxide anions (Eq. 7.3). Under the influence of superoxide dismutase (SOD) (Chapter 4) hydrogen peroxide can be produced (Eq. 7.4), which, in turn, interacts with additional superoxide anion radical, producing ionizing toxic hydroxyl radicals (Eq. 7.5). 

A comparison of mutagenic activity using normal and nitroreductase-deficient bacteria provides a method for detection and identification of nitroaromatic mutagens. But this idea is complicated by the existence of several bacterial nitroreductases with differing specificities (11). Three different nitroreductase-deficient derivatives of the normal tester strain TA98 were obtained from Dr H. S. Rosenkranz at Case Western University who has isolated and characterized these strains. Strain TA98NR was selected for its resistance to the nitrocompound niridazole and subsequently found to be resistant to the mutagenicity of niridazole, and also to that of the nitrofurans, nitronaphthalenes, and nitrofluorene (3). It is also resistant to 1-nitropyrene and a number of other nitro-PAH... 

The contrasting specificities of bacterial nitroreductases is very evident in the activity of the nitropyrenes. The nitroreductase-deficiency in strain TA98NR effects a 90% reduction in the mutagenicity of 1-nitropyrene. The compounds 1,3-dinltro- and tetranitropyrene are also less mutagenic in this strain, but 1,6-dinltro-, 1,8-dinitro-, and trinitropyrene retain their activity. In the experiment recorded in Table 2, the activities of the latter three compounds were, in fact. 

Those carcinogens that are proximate mutagens in Salmonella are chemicals that S-9 can metabolize to reactive species like alkylating and arylat-ing agents. Proximate mutagens generally activated by S-9 include triazenes, azoxy compounds, A -nitrosamines, aromatic amines, polyaromatics, heteroaromatics, and some proximate mustards. The nitroaromatics, though active without activation per se, are metabolized by bacterial nitroreductases to their reactive forms. 

Many nitro-PAHs are mutagenic to bacteria and mammalian cells and are a potential carcinogenic risk to humans (40). The genotoxic activity of many nitro-PAHs is dependent upon the reduction of the nitro group to A-arylhydroxylamines by either mammalian or bacterial nitroreductases (3, 54, 76). Nitro reduction can occur as a series of two-electron transfers which yield the nitroso, hydroxyl amino, and amino derivatives of the parent... 

Early observations of bacterial nitroreductase activity were made with cells capable of reducing chloramphenicol and/or / -nitrobenzoic acid to the corresponding aryl amines (78, 78, 80, 81, 82). From this body of work there emerged an early image of a bacterial nitroreductase as a soluble metalloflavoprotein that uses reduced pyridine nucleotide as an electron donor and catalyzes at least one two-electron reduction (Fig. 4). 

Yamashina and co-workers (94, 95) showed that azoxy-, azo- and hydrazo-com-pounds were not reduced by bacterial nitroreductase. Their results supported the observations of Saz and coworkers (78, 78, 80, 81) on the occurrence of individual nitro andnitroso reductase activities, and inhibitor studies indicated the involvement of metals in enzymatic activity. Furthermore, these studies contradicted the earlier work of Egami et al. (24) which indicated that nitroreductase activity could be attributed to nitrite reductase. 

The late 1980 s and early 1990 s were an active time in the study of bacterial nitroreductase enzymes, and particularly in the study of the classical nitroreductase of... 

McCoy, E. C., H. S. Rosenkranz, and R. Mermelstein. 1981. Evidence for the existence of a family of bacterial nitroreductases capable of activating nitrated polycyclics to mutagens. Environ. Mutagen. 3 421-437. 

Reduction of nitro compounds occurs via the action of cyji450 in conjunction with NADPH in higher organisms, e.g. the inactivation of nitrazepam (Fig. 8.34) but an important pathway which is exploited in antibacterial compounds is the reduction of antibacterial compounds such as metronidazole (Fig. 8.34) and nitrofurantoin to their active forms by bacterial nitroreductases. 

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