Participation of the microsomal electron transport system in mutagenic activation of 4-dimethylaminoazobenzene, 4-methylaminoazobenzene and their 3'-methyl-derivatives

Metabolism of azo dyes: implication for detoxication and activation

Azo dyes are consumed and otherwise utilized in varying quantities in many parts of the world. Such widely used chemicals are of great concern with regard to their potential toxicity and carcinogenic properties. Their metabolism has been studied extensively and is significant for detoxication and metabolic activation. Both oxidative and reductive pathways are involved in these processes. The majority of azo dyes undergo reduction catalyzed by enzymes of the intestinal microorganisms and/or hepatic enzymes including microsomal and soluble enzymes. The selectivity of substrate and enzyme may to a large extent be determined by the oxygen sensitivity of reduction since a normal liver is mainly aerobic in all areas, whereas the microorganisms of the lower bowel exist in an anaerobic environment. However, it should be pointed out that the pO2 of centrilobular cells within the liver is only a fraction that of air, where pO2 = 150 torr. Therefore, an azo dye reduction experiment performed aerobically may not be an accurate predictor of reductive metabolism in all areas of the liver. Many of the azo dyes in common use today have highly charged substituents such as sulfonate. These resist enzymic attack and for the most part are poorly absorbed from the intestinal tract, providing poor access to the liver, the major site of the mixed-function oxidase system. Lipophilic dyes, such as DAB, which are often carcinogenic, readily access oxidative enzymes and are activated by both mixed-function oxidase and conjugating systems. Reduction of the carcinogenic dyes usually leads to loss of carcinogenic activity. By contrast, most of the highly charged water-soluble dyes become mutagenic only after reduction. Even then, most of the fully reduced amines required oxidative metabolic activation. An outstanding example is the potent human bladder carcinogen benzidine, which derives from the reduction of several azo dyes. Many problems regarding mutagenic and carcinogenic activation remain to be solved. At the present time, it is apparent that both oxidative and reductive pathways yield toxic products. Toxicologic assessment of azo dyes must consider all pathways and particularly the oxygen sensitivity of azoreduction. This is critical in the treatment of waste from chemical plants where there is a great need for soil bacteria which catalyze reduction aerobically. Consideration of secondary pathways are also of great concern. For example, azoreduction of carcinogenic dyes such as DAB removes carcinogenic activity although oxidative metabolism of the primary amines yield mutagenic products. Such apparent dilemmas must be dealt with when considering metabolism/toxicity relationships for azo dyes.

Activation mechanisms to chemical toxicity

The pathobiology of chemical toxicity may involve "acute lethal injury" (necrosis), "autoxidative injury" (oxygen toxicity), "immunological injury" (neoantigen formation), and malignancy. Toxic chemicals may be activated by reduction, conjugation, radical formation, or oxidation. Oxidative activation may be effected by cytochromes P-450/P-448, flavoprotein monooxygenases, or hydroxyl radicals. The alternative pathways of oxidative metabolism of toxic chemicals, namely, detoxication and activation, are catalysed by the phenobarbital-induced cytochromes P-450 and by the 3-methylcholanthrene-induced cytochromes P-448 respectively. Oxidative metabolism by cytochromes P-450 is followed by conjugation and detoxication, whereas oxidative metabolism by cytochromes P-448 yields reactive intermediates which are not readily conjugated, and thus react with vital intracellular macromolecules, resulting in necrosis, redox cycling and oxygen radical formation, neoantigen production, and mutations. The molecular dimensions of specific substrates, inhibitors and inducers of the PB-cytochromes P-450 indicate that they are globular and are different from those of the cytochromes P-448 which are planar, suggesting that the active sites of the two families of enzymes are different. Oxidative metabolism of planar substrates of cytochromes P-448 results in conformationally-hindered oxygenations, which inhibits subsequent conjugations. Cytochrome P-448 activity may be quantified by the oxidative deethylation of 7-ethoxyresorufin which, unlike benzo(a)pyrene hydroxylation (AHH) is a specific reaction for this family of enzymes. Oxidative metabolism of chemicals varies inversely with the body weight of the animal species, so that chemical toxicity involving oxidative activation, redox cycling, and reactive oxygen is greater the smaller the animal species.

Molecular dimensions of the substrate binding site of cytochrome P-448

The molecular geometries of specific substrates, inhibitors and inducers of cytochrome P-448 activity were determined using computer-graphic techniques for use in defining the molecular dimensions of the substrate binding site of this enzyme. Specific substrates of cytochrome P-448 are essentially planar molecules characterised by a small depth and a large area/depth ratio. In contrast, compounds that do not serve as substrates of cytochrome P-448 are bulky, non-planar molecules characterised by small area/depth ratios and greater flexibility in molecular conformation. Specific inhibitors of cytochrome P-448 whose effect is mediated through interaction with the haem still meet the dimensional criteria for substrates indicating that they must also interact with the substrate binding-site, which is probably located in proximity to the haem. Inducers of cytochrome P-448 activity exhibit similar molecular geometries to the substrates from which it may be inferred that the cytosolic receptor associated with the induction of cytochrome P-448 activity is structurally related to the active site of the cytochrome.

Cytochrome P-448 and the activation of toxic chemicals and carcinogens

The metabolic activation of carcinogens and some toxic chemicals appears to involve oxygenation in conformationally hindered positions in the chemical molecules. Oxygenation of xenobiotics in hindered positions is effected by cytochrome P-448 (LM4) but not by cytochrome P-450 (LM2). Substrate-interaction spectra show that cytochrome P-448 has an active site with a conformation different from that of cytochrome P-450. Induction of cytochrome P-448, as specifically measured by ethoxyresorufin O-deethylase activity, occurs in rat liver, kidney and lung after administration of the carcinogens, 3-methylcholanthrene, Aroclor 1254, 2-anthramine, safrole, 7,12-dimethylbenz[a]anthracene, MNNG and 2-acetamidofluorene. The doubtful carcinogens, saccharin, DDT and aldrin, resulted in no significant induction. The drugs paracetamol, antipyrine, imipramine and rifampicin resulted in diminished enzyme activity, indicating the absence of any induction of cytochrome P-448. In studies with the matched pairs of carcinogens and non-carcinogens, benzo[a]pyrene and benzo[e]pyrene, and 1,2,5,6-dibenzanthracene and anthracene, only the carcinogenic analogue resulted in induction of cytochrome P-448. With alpha- and beta-naphthylamine, both resulted in marked induction of cytochrome P-448 in liver, kidney and lung, indicating that both isomers might be carcinogenic.

Active metabolites in toxicology: the role of cytochrome P-448 and flavoprotein oxidases

The activation of toxic chemicals and carcinogens into reactive intermediates involves oxygenation in hindered positions of the molecule, by cytochrome P-448 (LM4), flavoprotein oxidoreductases, or transoxygenation during prostaglandin biosynthesis. Cytochrome P-450 (LM2) does not oxygenate in hindered positions and therefore generally detoxicates carcinogens and toxic chemicals. Cytochrome P-448 has a different active site from cytochrome P-450, which enables it to oxygenate substrates in conformationally-hindered positions.

Participation of cytochrome P450 in mutagenic activation of the carcinogen 3'-hydroxymethyl-N,N-dimethyl-4-aminoazobenzene and its N-demethylated compounds by rat liver

Mutagenicity of the hepatocarcinogen 3'-hydroxymethyl-N, N-dimethyl-4-aminoazobenzene (3'-CH2OH-DAB) and its N-demethylated compounds was examined. Rat-liver 9000 X g supernatant (S9) fraction was used together with Salmonella typhimurium TA98 or TA100 as a tester strain. The expression of mutagenicity of 3'-CH2OH-DAB, 3'-hydroxymethyl-N-methyl-4-aminoazobenzene (3'-CH2OH-MAB) and 3'-hydroxymethyl-4-aminoazobenzene (3'-CH2OH-AB) required the presence of both microsomes and cytosol as sources of enzymes as well as NADPH as a cofactor. 3'-CH2OH-AB showed positive mutagenicity on both strains in the presence of liver S9 from untreated rats whereas 3'-CH2OH-DAB and 3'-CH2OH-MAB were negative. The treatment of rats with polychlorinated biphenyls (PCB) or 3-methylcholanthrene (3-MC) resulted in a marked increase in the ability of S9 to activate these three compounds, whereas phenobarbital (PB) induction was not effective, except for the activation of 3'-CH2OH-AB. The mutagenic activities of the three compounds in strain TA98 were considerably decreased by adding cytochrome c to the S9 mixture, but the activation reactions were insensitive to 1-(1-naphthyl)-2-thiourea (NTU) and methimazole, high-affinity flavin-containing monooxygenase (FMO) substrates. Metyrapone and 2-diethylaminoethyl-2,2-diphenylvalerate hydrochloride (SKF-525A, potent inhibitors of cytochrome P450, had no inhibitory effect on the activation of these compounds by S9 from PCB-treated rat livers. In contrast, 7,8-benzoflavone (BF), a specific inhibitor of cytochrome P448, decreased the activities of 3'-CH2OH-DAB and 3'-CH2OH-MAB by 88 and 78%, respectively, but the inhibition was negligible for 3'-CH2OH-AB.