Carcinogen benzidine, metabolism

It is of more than a little interest to note that the sites of tumor formation do not always match across species. Benzidine, a substance once widely used in dye manufacture, was shown many years ago to be a carcinogenic risk for the bladder in workers exposed to excessive levels. The rat bladder is not responsive to this substance, but its liver is. It wasn t until Wilhelm Hueper turned to the dog that bladder cancer could be reproduced in a laboratory animal. It is now understood that benzidine metabolism is similar in dogs and people, and that metabolism in the rat takes a different course. It is also understood that certain benzidine metabolites, and not benzidine itself, are the proximate causes of tumors. Knowledge of metabolic differences helps explain the species similarities and differences in tumor response. If we had available the rat data and no human data, we would be in error to conclude that benzidine was a cause of human liver cancer. 

M. Boeniger, The Carcinogenicity and Metabolism of Azo Dyes, Especially those derived from Benzidine, DHHS (NIOSH) Publication No. 80 119, US Department of Health and Human Services, Ohio 1980. 

Figure 4.69 Metabolism of the carcinogen benzidine showing oxidation and acetylation. Both routes of acetylation can give rise to a reactive nitrenium ion and DNA adducts. Abbreviation. NAT, N-acetyltransferase.

A number of arylamines are potent bladder carcinogens such as 4-aminobiphenyl, 1-naphthylamine, and benzidine. Metabolic activation of these carcinogens requires the action of UDP-glucuronosyl transferase on N-hydroxyarylamines to form N-glucuronides. 

Fig. 16B) [56]. Another class of azo-containing compounds that readily undergo reduction either by mammals or intestinal bacteria are the azo dyes, such as Congo red, which is metabolized to the carcinogen benzidine (Fig. 16B) [57]. 

Benzidine Dyes. The well-documented metabolic breakdown of benzidine dyes to the starting material and human carcinogen, benzidine, is today recognized by the dyestuff industries in both Europe and the United States. Because of this, a voluntary withdrawal of this type of dyestuff was enacted several years ago in the US, and more recently, in Europe. Today, however, it is still possible to import benzidine dyes into the US from Mexico and India, and regulatory action appears to be necessary even by such a simple stratagem as deleting the dyes in question from the TSCA Inventory. 

The metabolic formation of N-sulfonyloxy-N-acetyl-2-aminofluorene (N-sulfonyloxy-AAF) and its observed electrophilic reactivity, provided the first evidence for the importance of enzymatic conjugation reactions in chemical carcinogenesis (23,24). This reaction was shown to be catalyzed by PAPS-dependent sulfotrans-ferases that are located predominantly in liver cytosol and has been subsequently demonstrated for N-hydroxy arylamide metabolites of several other carcinogens, including N-acetyl-4-aminobiphenyl (AABP), benzidine, N-acetyl-2-aminophenanthrene and phenacetin. 

Limited information is available on the metabolism of 1,2-diphenylhydrazine. Two of the known metabolites, aniline and benzidine, may contribute to the toxicity and/or carcinogenicity of the substance. 

H.B. (1980) Metabolism ofhisazohiphenyl dyes derived from benzidine, 3, 3 -methylbenzidine and 3, 3 -dimethoxylbenzidine to carcinogenic aromatic amines in the dog and rat. Toxicol. Appl. Pharmacol. 56, 248—258. 

Benzidine is metabolized to highly toxic, reactive metabolites, such as N-hydroxyarylamides and N-hydroxyarylamines, which act as procarcinogens and are more mutagenic than parent compounds. The metabolites act as DNA adducts and bind to cell receptors. The metabolites on conjugation with sulfuric, acetic, and glucuronic acids form ultimate carcinogens. Acetylated benzidine metabolites such as N-acetoxyarylamines are known to cause bladder cancer in dye industry workers. 

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. 

The mechanism of carcinogenicity of benzidine is thought to involve its metabolic transformations forming reactive intermediates binding to DNA. Such DNA adducts have been identified in rodent liver. It tested positive in most genotoxic tests. Its carcinogenicity may possibly be related to the slow rate of liver detoxification by acetylation allowing activation of benzidine or its metabolites in urine (Whysner et al. 1996). 

Moreover, certain materials that apparently exist at subtoxic levels in the workplace can substantially appreciate in toxicity by undergoing chemical reactions within the body. For example, a metabolic reaction occurs in the case of benzidine dyes. The carcinogenic nature of benzidine has been recognized for many years, but benzidine-based dyes have been considered relatively safe on the assumption that they contain very little free or unreacted benzidine. Studies of analyses of urine samples collected from workers exposed to these dyes, however, indicate that metabolization of benzidine dye results in significantly elevated benzidine levels within the body. 

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