Detoxification aromatic amines

The polymerization of phenols or aromatic amines is applied in resin manufacture and the removal of phenols from waste water. Polymers produced by HRP-catalyzed coupling of phenols in non-aqueous media are potential substitutes for phenol-formaldehyde resins [123,124], and the polymerized aromatic amines find applications as conductive polymers [112]. Phenols and their resins are pollutants in aqueous effluents derived from coal conversion, paper-making, production of semiconductor chips, and the manufacture of resins and plastics. Their transformation by peroxidase and hydrogen peroxide constitutes a convenient, mild and environmentally acceptable detoxification process [125-127]. 

Environmental applications of HRP include immunoassays for pesticide detection and the development of methods for waste water treatment and detoxification. Examples of the latter include removal of aromatic amines and phenols from waste water (280-282), and phenols from coal-conversion waters (283). A method for the removal of chlorinated phenols from waste water using immobilised HRP has been reported (284). Additives such as polyethylene glycol can increase the efficiency of peroxidase-catalyzed polymerization and precipitation of substituted phenols and amines in waste or drinking water (285). The enzyme can also be used in biobleaching reactions, for example, in the decolorization of bleach plant effluent (286). 

Simula, T.P, Glancey, M.J. Wolf, C.R. (1993) Human glutathione S -transferase-expressing Salmonella typhimurium tester strains to study the activation/detoxification of mutagenic compounds studies with halogenated compounds, aromatic amines and aflatoxin Bp Carcinogenesis, 14, 1371-1376... 

Husain Q, Jan U (2000) Detoxification of phenols and aromatic amines from polluted wastewater by using phenol oxidases. J Sci Ind Res 59 286—293... 

Figure 13.9. NAT-mediated detoxification and metabolic activation of aromatic amines.

Sulfotransferases (SULTs) are cytosolic phase II detoxification enzymes involved in sulfonation of various xenobiotics and endobiotics. There are also membrane-bound SULTs that are not involved in phase II metabolism but are involved in the sulfonation of proteins and polysaccharides. Substrates of cytosolic SULTs include alcohols (ethanol, 2-butanol, cholesterol, bile acids), phenols (phenol, naphthol, acetaminophen), aromatic amines and hydroxyamines (2-naphthylamine, A-hydroxy 2-naphthylamine). SULTs transfer sulfonate (S03) to a hydroxy or amino group of a substrates from the cofactor 3 -phosphoadenosine-5 -phosphosulfate (PAPS), generating highly water-soluble metabolites for elimination through the kidney and liver. 

Figure 43-1 I Schematic view of the role of NAT enzymes in the metabolism of aromatic amines. N-acetylation might be a detoxification reaction in a number of cases however, after N-hydroxylation of aromatic amines (e.g., by CYP enzymes), NAT enzymes can bioactivate these intermediates by either 0-acetylation or intramolecular N,0-acety transfer, leading to the formation of nitrenium ions, which might react with DNA or alternatively be detoxified by, for example, GST enzymes. Importantly, it is shown that a number of other biotransformation enzymes are also involved in the metabolism of aromatic amines as well. (Redrawn from Wormhoudt LW, Commandeur jNM, Vermeuien NPE. Genetic polymorphisms of human N-acetyitransferase, cytochrome P450, glutathione-S-transferase, and epoxide hydrolase enzymes relevance to xenobiotic metabolism and toxicity. Crit Rev Toxicol 1999 29 59-124. Reproduced by permission from Taylor and Francis, Inc.)...

Tissue-specific NAT expression can ajfect toxicity of environmental pollutants. NATl is ubiquitously expressed in human tissues, whereas NATl is found in liver and the GI tract. Both enzymes have a capacity to form N-hydroxy-acetylated metabolites from bicyclic aromatic hydrocarbons, a reaction that leads to the nonenzymatic release of the acetyl group and the generation of highly reactive nitrenium ions. Thus, N-hydroxy acetylation is thought to activate certain environmental toxicants. In contrast, direct N-acetylation of the environmentally generated bicyclic aromatic amines is stable and leads to detoxification. NATl fast acetylators efficiently metabolize and detoxify bicyclic aromatic amine through liver-dependent acetylation. Slow acetylators (NATl deficient) accumulate bicyclic aromatic amines, which are metabolized by CYPs to N-OH metabolites that are eliminated in the urine. In bladder epithelium, NATl efficiently catalyzes the N-hydroxy acetylation of bicyclic aromatic amines, a process that leads to deacetylation and the formation of the mutagenic nitrenium ion. Slow acetylators due to NATl deficiency are predisposed to bladder cancer if exposed to environmental bicyclic aromatic amines. 

The metabolism of MBOCA was also investigated in vitro by incubating human and rat liver microsomes with C-MBOCA. The formation of metabolites was quantified using appropriate chemically synthesized standards (Morton et al. 1988). The rate of N-hydroxylation of MBOCA, an obligatory step in metabolic activation of aromatic amines, was higher in rat than in human microsomes (Morton et al. 1988). Rat liver microsomes were also found to be more efficient in o-hydroxy-MBOCA formation when compared with human microsomes (see Figure 2-2). The same in vitro microsomal system was used to elucidate the role of hepatic cytochrome P-450 monooxygenases in metabolic oxidation and detoxification of MBOCA (Butler et al. 1989). The... 

In this article we will outline in vitro metabolic studies which have been performed on representatives of several classes of carcinogens. These studies have been selected from the literature through 1979. Metabolic studies which employed various cellular subfractions, cell culture, or organ culture systems will be compared. The effect of species variations on in vitro metabolism will also be considered. The relationship of these in vitro studies to current concepts on metabolic activation and detoxification will be discussed for aromatic amines, polycyclic aromatic hydrocarbons, nitrosamines, hydrazines, azoxy compounds, halogenated hydrocarbons, and carcinogenic natural products. 

Ring hydroxylation of aromatic amines is the major route of detoxification. These compounds are generally excreted in the urine or feces in the form of glucuronides or sulfates. In the case of AAF, the major detoxification products which were found in the urine of rats were glucuronic acids of individual hydroxylated metabolites. The major C-hydroxylated metabolites formed... 

In addition to the reversibility of the N-oxidation of certain aromatic amines, as demonstrated by in vitro metabolism studies, several irreversible detoxification mechanisms also occur. The formation of C-hydroxylated metabolites is the major route for detoxification of aromatic amines. The influence of enzyme inducers and inhibitors on the extent of C-hydroxylation of aromatic amines has been explored by several investigators (Table VII). In the case of AAF, the 1-, 3-, 5-, and 7-hydroxy derivatives were found to be formed in vivo. It was also shown in vitro that 1- and 3-hydroxy-AAF could be formed by an isomerization of V-OH-AAF. The conversion of V-OH-AAF to these hydroxy derivatives as catalyzed by various liver preparations is illustrated in Fig. 3. The effects of different species and substrates on this transformation are listed in Table VIII. Further oxidation of these o-aminophenols in vitro by mitochondria to quinonimines, which have been shown to bind to cellular protein, has been studied (Table IX). 

The formation of N-oxides of tertiary amines and demethylation of secondary amines are biochemical processes which may also be involved in detoxification. In the case of A, iV-dimethylaniline, the N-oxide is considered to be the possible intermediate in its NADPH- and oxygen-dependent oxidative demethylation. The formation of this A -oxide appears to be catalyzed via a flavoprotein oxidase (509). Tertiary amine A -oxides, thus formed, are often excreted in urine. In the case of A -methyl-4-aminoazobenzene (MAB), demethylation to 4-aminoazoben-zene (AB) appears to occur via a cytochrome P-450 mediated oxidation. Since an A -methyl substituent is regarded as essential for carcinogenic activity among derivatives of AB, this oxidative demethylation would be a specific mode of detoxifying this carcinogenic aromatic amine. 

PAHs and aromatic amines are carcinogenic and are rapidly converted in the liver into a suite of metabolites. Considering that the point of metabolism is to alter the solubility of the compound such that it can be excreted, it stands to reason that detoxification would not only decrease the lipid solubility of the PAH but also decrease its reactivity, or in other words, its propensity to form DNA adducts. However sensible that may seem, the converse is actually true. Highly reactive metabolites, such as epoxides and quinones, are often formed during Phase I metabolism. These chemicals actually increase, rather than decrease, chemical reactivity. Consequendy, as many PAHs are metabolized and prepared for excretion, their carcinogenicity may actually increase. 

Over the years, hundreds of new local anesthetics have been synthesized and tested. For one reason or another, most have not come into general use. The search for the perfect local anesthetic is still under way. All the drugs found to be active have certain structural features in common. At one end of the molecule is an aromatic ring. At the other is a secondary or tertiary amine. These two essential features are separated by a central chain of atoms usually one to four units long. The aromatic part is usually an ester of an aromatic acid. The ester group is important to the bodily detoxification of these compounds. The first step in deactivating them is a hydrolysis of this ester linkage, a process that occurs in the bloodstream. 

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