Aflatoxin metabolic activation

Figure 5.14 Routes of oxidative metabolism of aflatoxin showing activation to an epoxide catalyzed by CYP1A2 and CYP3A4 and conjugation with glutathione catalyzed by a-GST. The 8,9-exoepoxide is the enantiomer, which binds extensively to DNA and is carcinogenic. The endoepoxide binds less readily. Abbreviation. a-GST, a-glutathione transferase.

Examples of initiators include alkylating agents, dime thy lsu If ate and ji-propiolactone, procarcinogens (requiring metabolic activation) such as benzo(a)pyrene, aflatoxin, and cyclophosphamide (Fig. 6.52) as follows ... 

Metabolic activation systems—such as microsome-, cell-, and host-mediated assays—have been included in mammalian cell mutagenesis systems. Microsome-mediated assays have been used to detect many chemicals, including nitrosamines, polycyclic hydrocarbons, aflatoxins, and vinyl chloride. Cell-mediated assays seem to be a better indicator of in vivo metabolic pathways. Microsome-mediated assays seem suitable for general screening of chemicals, and cell-mediated assays are more valuable in the assessment of data. 

Langenbach, R., H.J. Freed, D. Raveh, and E. Huberman. Cell specificity in metabolic activation of aflatoxin Bj and benzo(a)pyrene to mutagens for mammalian cells. Nature 276 277-280, 1978. 

Billings PC, Heidelberger C, and Landolph JR (1985) S-9 metabolic activation enhances aflatoxin-mediated transformation of C3H/10T1/2 cells. Toxicology and Applied Pharmacology 77 58-65. 

A number of chemicals with demonstrable suppression of immune function produce this action via indirect effects. By and large, the approach that has been most frequently used to support an indirect mechanism of action is to show immune suppression after in vivo exposure but no immune suppression after in vitro exposure to relevant concentrations. One of the most often cited mechanisms for an indirect action is centered around the limited metabolic capabilities of immunocompetent cells and tissues. A number of chemicals have caused immune suppression when administered to animals but were essentially devoid of any potency when added directly to suspensions of lymphocytes and macrophages. Many of these chemicals are capable of being metabolized to reactive metabolites, including dime-thylnitrosamine, aflatoxin Bi, and carbon tetrachloride. Interestingly, a similar profile of activity (i.e., suppression after in vivo exposure but no activity after in vitro exposure) has been demonstrated with the potent immunosuppressive drug cyclophosphamide. With the exception of the PAHs, few chemicals have been demonstrated to be metabolized when added directly to immunocompetent cells in culture. A primary role for a reactive intermediate in the immune suppression by dimethylnitrosamine, aflatoxin Bi, carbon tetrachloride, and cyclophosphamide has been confirmed in studies in which these xenobiotics were incubated with suspensions of immunocompetent cells in the presence of metabolic activation systems (MASs). Examples of MASs include primary hepatocytes, liver microsomes, and liver homogenates. In most cases, confirmation of a primary role for a reactive metabolite has been provided by in vivo studies in which the metabolic capability was either enhanced or suppressed by the administration of an enzyme inducer or a metabolic inhibitor, respectively. 

Buening MK, Fortner JG, Kappas A, Conney AH. 1978. 7,8-Benzoflavone stimulates the metabolic activation of aflatoxin B to mutagens by human liver. Biochem. Biophys. Res. Commun. 82 348-55... 

Metabolic activation of aflatoxin Bh belonging to the class of my cotoxins, is catalyzed by cytochrome P450. The metabolic conversion of this compound can follow many pathways however, only the epoxidation in position 8/9 produces the ultimate carcinogen ... 

Huh N, Nemoto N, Utakoji T. 1982. Metabolic activation of benzo[a]pyrene, aflatoxin Bl, and dimethyinitrosamine by a human hepatoma cell line. Mutat Res 94 339-348. 

The concept of metabolic activation developed with AAF was then apphed to PAHs, aflatoxins, nitrosamines, nitrosoureas, hydrazines, urethane, and vinyl chloride. Several metabolic activation schemes are presented in Figure 6.2. In each case a highly reactive electrophilic carbocation is formed. We now know that the concept of metabolic activation applies to many genotoxic carcinogens and helps to explain... 

Purpose. DNA adducts are nucleotide bases (i.e., purines and pyrimidines) that have been covalently modified by reactive electrophilic chemical intermediates or free radicals. The chemical structures of DNA adducts are diverse and vary from simple alkyl adducts induced by alkylating agents to complex bulky adducts such as those resulting from metabolic activation of polycyclic aromatic hydrocarbons, aromatic amines, and aflatoxins (Dipple 1995 Chiarelli and Jackson 1992 Rundle 2006 Xue and Warshawsky 2005). The purpose of measuring DNA adducts is to determine whether a DNA-reactive compound or a metabolically activated... 

The carcinogenicity of aflatoxin is reduced by protein deficiency, presumably because of reduced metabolic activation to the epoxide intermediate which may be the ultimate carcinogen which binds to DNA (figure 5,26). A deficiency in dietary fatty acids also decreases the activity of the microsomal enzymes. Thus, ethylmorphine, hexobarbital and aniline metabolism are decreased, possibly because lipid is required for cytochromes P-450. Thus, a deficiency of essential fatty acids leads to a decline in both cytochromes P-450 levels and activity in vivo. 

If a test compound significantly increases the number of observed colonies relative to a control for spontaneous reversion, which always occurs to some extent, that compound must have caused mutation that resulted in reversion. Each type of strain, of which five are normally used for the full test and two for more limited versions, detects a specific type of mutation TA98 detects frameshift mutations while TAIOO picks up base pair substitutions. Since the mutagenic abilities of compounds often depend on metabolic activation, as it does for aflatoxin Bl, the test can be run in the presence of S9 liver fraction from rats whose CYPs have been induced by the polychlorinated biphenyl Aroclor 1254. An increase in reversion only in the presence of S9 shows that the mutagen must be a metabolite. 

Compared with some of the other mycotoxins such as aflatoxin, the trichothecenes do not appear to require metabolic activation to exert their biological activity.50 After direct dermal application or oral ingestion, the trichothecene mycotoxins can cause rapid irritation to the skin or intestinal mucosa. In cell-free systems or single cells in culture, these mycotoxins cause a rapid inhibition of protein synthesis and polyribosomal disaggregation.35 47 50 Thus, we can postulate that the trichothecene mycotoxins have molecular capability of direct reaction with cellular components. Despite this direct effect, it is possible to measure the toxicokinetics and the metabolism of the trichothecene mycotoxins. 

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