Liver, azoreductase

The enzyme involved in the degradation of the dyes has been shown to be azoreductase. The enzymes were first isolated from the intestinal microflora and was later found to be produced by the cytosolic and microsomal fractions of the liver [47]. The enzyme was sensitive to oxygen and was inactivated by oxygen. In experiments involving intestinal anaerobic bacteria, Rafii et al. found the requirement of... 

Reduction. The activity of azo- and nitroreductase varies between different species, as shown by the in vitro data in Table 5.11. Thus, azoreductase activity is particularly high in the guinea pig, relative to the other species studied, whereas nitroreductase activity is greatest in the mouse liver. 

Species Azoreductase (jimol sulfanilamide formed in liver g-1 hr-1) Nitroreductase (j.imol p-aminobenzoic acid formed in liver g-1 hr-1)... 

Reductive reactions, like oxidation, are carried out at different rates by enzyme preparations from different species. Microsomes from mammalian liver are 18 times or more higher in azoreductase activity and more than 20 times higher in nitroreductase activity than those from fish liver. Although relatively inactive in nitroreductase, fish can reduce the nitro group of parathion, suggesting multiple forms of reductase enzymes. 

By far the most predominant metabolic pathway for water-soluble azo dyes is cleavage of the azo linkage by azoreductase of the liver and extrahepatic tissue or by intestinal microflora in the body [25,26], Oxidative metabolism occurs for lipid-soluble dyes, e.g., solvent dyes. Three oxidation pathways are known for such dyes (1) C-(ring-)hydroxylation, (2) A-hydroxylation at a primary or secondary amino group or (3) by stepwise oxidation of the methyl groups of dimethylamino compounds (demethylation). All three oxidative degradation ways leave the azo bond intact. For further details of the mechanisms, see [27,28],... 

Azoreductase activity is present in both the endoplasmic reticulum and the cytosol of the liver. It is thought to produce the hydrazo intermediate that is cleaved to give two primary amines. The best known example is that of the azo dye Prontosil that is cleaved to form sulfanilamide, an active antibacterial compound, a reaction that involves a four-electron transfer. 

Azo Compounds Azo dyes are widely used in the food, pharmaceutical, cosmetic, textile, and leather industry. They are synthetic compounds characterized by one (monoazo) or several intramolecular N = N bonds. Azo dyes, if they are systemically absorbed, can be metabolized by the way of azoreductases of intestinal microflora by liver cells and skin surface bacteria. This metabolism leads to aromatic amines that can be hazardous. In the 1930s, some azo derivatives like 4-dimethyl aminoazoben-zene (Butter Yellow, Cl Solvent Yellow 2, Cl 11020) and o-aminoazotoluene were experimentally found to be directly carcinogenic to liver and bladder after feeding. Other complex azo dyes like Direct Black 38 or Direct Blue 6 (Figure 28) release the aromatic amine benzidine. Some examples of azo dyes metabolized in benzidine and benzidine-congeners are listed in Table 3. 

Reductions are less commonly encountered than are oxidations, yet they constitute a very important group of biotransformations. An example is the reduction of nitro and azo groups. Nitro- and azoreductase enzymes effect these reactions in the liver. Ketones and aldehydes are reducible to secondary and primary alcohols, respectively. On occasion a carbon-carbon double bond reduction is encountered. Table 3-2 summarizes several of these reactions. 

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. 

Huang M-T, Miwa GT, Cronheim N, Lu AYH. Rat liver cytosolic azoreductase. Electron transport properties and the mechanism of dicoumarol inhibition of the purified enzyme. J Biol Chem 1979 254 11223-11227. 

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