Nitro-aromatic compounds enzymes

There are numerous variations on the general mechanism outlined in Figure 7.10. Glutathione forms conjugates with a wide variety of xenobiotic species, including alkenes, alkyl epoxides (1,2-epoxyethylbenzene), arylepoxides (1,2-epoxynaphthalene), aromatic hydrocarbons, aromatic halides, alkyl halides (methyl iodide), and aromatic nitro compounds. The glutathione transferase enzymes required for the initial conjugation are widespread in the body. 

Another approach for removing reactive dye hydrolysates from the fibre and from the wash water (decolourised waste water) is the use of peroxidases (oxidative active enzymes such as Baylase RP). This multipurpose enzymatic rinse process saves time, energy and water but it is restricted mainly to jet applications. The question of the potential toxicity of the resulting aromatic nitro-compounds (cleavage products of the reactive azoic dyes) has to be resolved. 

N-oxidation can occur in a number of ways to give either hydroxylamines from primary and secondary amines [Eqs. (11) and (12)], hydroxamic acids from amides, or N-oxides from tertiary amines [Eq. (13)]. The enzyme systems involved are either cytochrome P450 or a flavoprotein oxygenase. Hydroxylamines may be further oxidized to a nitro compound via a nitroso intermediate [Eq. (11)]. Oximes can be formed by rearrangement of the nitroso intermediate or N-hydroxylation of an imine, that could in turn be derived by dehydration of a hydroxylamine [Eq. (11)]. N-Oxides may be formed from both tertiary arylamines and alkylamines and from nitrogen in heterocyclic aromatic systems, such as a pyridine ring. 

The full structure of glutathione (p. 1356) is a tripeptide but the reactive group is a thiol (SH) on a cysteine in the middle. We shall represent glutathione as GSH. The first compound reacts by nucleophilic aromatic substitution (pp. 590-5) aided by the nitro groups. After the reaction, the dangerously electrophilic dinitrochlorobenzene cannot react with enzymes or DNA but is carried away attached to a short water-soluble peptide. 

Metabolic processes in the body include reactions that have electron transfer (ET) associated with them. Most xenobiotics or their Phase I enzyme metabolites contain ET moieties. The principal groups include phenols, quinones, aromatic nitro compounds, amines, imines, and metal complexes or complexors. 

In addition to the oxidative systems, liver microsomes also contain enzyme systems that catalyze the reduction of azo and nitro compounds to primary amines. A number of azo compounds, such as Prontosil ahd sulfasalazine (Fig. 10.16), are converted to aromatic primary amines by azoreductase, an NADPFI-dependent enzyme system in the liver microsomes. Evidence exists for the participation of CYP450 in some reductions. Nitro compounds (e.g., chloramphenicol and nitrobenzene) are reduced to aromatic primary amines by a nitroreductase, presumably through hitrosamine and hydroxylamine intermediates. These reductases are not solely responsible for the reduction of azo ahd hitro compouhds reductioh by the bacterial flora ih the anaerobic environment of the intestine also may occur. 

A reduction of nitro substituent, under both aerobic and anaerobic conditions, seems to be a common enzymatic mechanism in the environment [5,29]. Such reduction has been demonstrated in various organisms which utilize the nitro compotind as an electron acceptor. The activity of nitroreductases, many of which have a broad substrate specificity, has been demonstrated in cell-free systems, and some enzymes have been purified and characterized [5,19,29]. The resulting aromatic amines are often further transformed into persistent azo compounds or polymers by biotic or abiotic processes [1,30,31]. In the second pathway, the nitro substituent is directly removed as nitrite [24,32,33], with the formation of catechol. 

Aromatic nitro and azo compounds are reduced by hepatic enzymes of mammals, birds, reptiles, fish, and bacteria. Azo-reductase and nitro-reductase are similar in that they have low affinities for their substrates in addition, they are both flavoproteins having FAD as their prosthetic group. Fouxs et al. [57]... 

PETN reductase appears to be an industrially viable enzyme due to its robustness [10,44]. It catalyzes the reduction not only of aromatic nitro compounds, but also of activated alkenes such as unsaturated aldehydes and ketones [10,44]. Of particular synthetic interest is its ability to catalyze the reduction of prochiral E- and Z-a,(3-unsaturated nitroolefins leading to chiral nitro products. The mechanism is shown in Figure 5.7. 

The enzymatic reduction of the nitro group involves the stepwise addition of six reducing equivalents potentially derived from reduced pyridine nucleotides (Fig. 8). The first reaction yields a nitroso derivative which is subsequently reduced to a hydroxylamine the hydroxylamino compound is then reduced to the amine. In most systems studied to date (Cemiglia and Somerville, this volume) a single nitroreductase enzyme is responsible for all three reactions and there is little or no accumulation of the intermediates. However, reduction of nitro compounds does not seem to be the physiological function of the enzymes that have been reported to carry out these reactions. Diaphorases (23), ferredoxin-NADPH reductase (33), and a variety of other enzymes from procaryotes and eucaryotes have been shown to catalyze the fortuitous reduction of aromatic nitro groups. 

Angermaier, L., F. Hein, and H. Simon. 1981. Investigations on the reduction of aliphatic and aromatic nitro compounds by Clostridium species and enzyme systems, p. 266-275. In Bothe, H., and A. Trebst, (ed.) Biology of inorganic nitrogen and sulfur. Springer, Berlin. 

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