Azo reduction

The ability of mammalian tissue to reduce azo bonds is rather poor. With p-[2,4-(diaminophenyl)azo]benzenesulfonamide (Prontosil), in vivo reduction forms sulfanilamide. However, pretreatment with antibiotics destroys the intestinal bacteria, which results in a decrease in the formation of the amino compound. Generally, it would appear that both nitro and azo reduction as a function of specific tissues is of minor importance, and intracellular bacteria as well as intestinal bacteria are actually responsible for these reductions. 

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Many azo dyes, such as tartrazine (Section 4.04.4.1.3), are susceptible to reduction by bacterial reductases in the intestinal flora. Azo reduction is believed to proceed through a hydrazo intermediate that undergoes subsequent reductive cleavage of the nitrogen-nitrogen bond to yield the arylamine derivatives (B-80MI40406). 

Ramalho PA, MH Cardoso, A Cavaco-Paulo, MT Ramalho (2004) Characterization of azo reduction activity in a novel ascomycete yeast strain. Appl Environ Microbiol 70 2279-2288. 

Gingell R, Walker R (1971) Mechanisms of azo reduction by Streptococcus faecalis II The role of soluble flavins. Xenobiotica 1 231... 

Kudlich M, Keck A, Klein J, Stolz A (1997) Localization of the enzyme system involved in the anaerobic degradation of azo dyes by Sphyngomonas sp. BN6 and effect of artificial redox mediators on the rate of azo reduction. Appl Environ Microbiol 63 3691-3694... 

Azo dyes made from 47, and also their cleavage products from azo reduction, are appreciably less genotoxic than the corresponding benzidine-based dyes. An example is the mutagenic (and carcinogenic) benzidine-based dye Direct Violet 43 (48) and its corresponding isosteric analog (49), in which the benzidine moiety is replaced with 47. Other examples are available [84]. 

Azo-based dyes, known to be carcinogenic, contain easily hydrolyzed azo bonds. In the GI tract, these bonds are cleaved to yield the free aromatic amine(s) [20]. Azo reduction may also take place in the liver of humans and other mammals by reductase enzymes, but it is likely that hydrolysis in the GI tract is predominant [21]. The resultant aromatic amines are easily absorbed in the intestines. It was found that inclusion of sulfonate moieties on the aromatic amine feedstocks mitigates the toxicity, as illustrated with the azo dye Brilliant Black BN (Cl Food Black 1) in Figure 13.6. The sulfonate moieties are highly ionized in the GI tract and at environmental pHs (5-9), and their reduction products cannot penetrate the GI endothelial membranes following oral exposure. Consequently, the chemicals are poorly absorbed, and any portion that is absorbed is rapidly excreted in the urine [22, 23]. 

Figure 13.6 Brilliant Black BN (5) and its environmental or intestinal azo reduction products. The sulfonate moieties and each of its three reduction products are highly ionized in the environment and in the Cl tract, and cannot penetrate the membranes of the cells lining the intestines.

Reduction of the azo dye piontosil to produce the antibacterial drug sulfanilamide (Fig. 4.38) is a well-known example of azo reduction. This reaction is catalyzed by cytochromes P-450 and is also carried out by the reductases in the gut bacteria. The reduction of azo groups in food coloring dyes such as amaranth is catalyzed by several enzymes, including cytochromes P-450, NADPH cytochrome P-450 reductase, and DT-diaphorase, a cytosolic enzyme. 

Major reduction reactions are azo reduction and nitro reduction. The enzymes (reductases) are found in gut flora but also mammalian tissues. Reduction catalyzed by cytochrome P-450 can occur (e.g., dehalogenation). DT diaphorase carries out two-electron reductions. 

Azo Reduction. Requirements for azo reduction are similar to those for nitroreduc-tion, namely anaerobic conditions and NADPH. They are also inhibited by CO, and presumably they involve CYP. The ability of mammalian cells to reduce azo bonds is rather poor, and intestinal microflora may play a role. 

Thus sorption, followed by intra-particle diffusion of the dyes, causes rapid initial loss followed by slow long term loss. Transformation pathways probably involve azo reduction. Diffusion may limit the dyes transformation rates because of the large size of the molecules. 

Aldehyde reduction Azo reduction Nitro reduction RCHO RCHjOH R,N = NR2 R,NH2 + R2NH2 o2nr H2NR Chloral hydrate Azo gantrisin Chloramphenicol... 

Epoxidation and hydroxylation A-Dealkylation O-Dealkylation -Dealkylation -Oxidation A-Oxidation P-Oxidation Desulfuration Dehalogenation Nitro reduction Azo reduction Cytochrome P450 (CYP) Aflatoxin, aldrin, benzo[a]pyrene, bromobenzene, naphthalene Ethylmorphine, atrazine, dimethylnitrocarbamate, dimethylaniline p-Nitroanisole, chlorfenvinphos, codeine Methylmercaptan Thiobenzamide, phorate, endosulfan, methiocarb, chlorpromazine 2-Acetylaminofluorene Diethylphenylphosphine Parathion, fonofos, carbon disulfide CCLt, CllCb Nitrobenzene O-Aminoazotoluene Flavin-Containing Monooxygenase (FMO)... 

Although insects contain reductases that catalyze the reduction of xenobiotics, reduction is less common than oxidation. Three types of reduction reactions, i.e., nitro reduction, azo reduction, and aldehyde or ketone reduction, are known to occur in insects. 

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