The formation of 2,3-dihydroxy-2,3-dihydro-aflatoxin B1 by the metabolism of aflatoxin B1 by liver microsomes isolated from certain avian and mammalian species and the possible role of this metabolite in the acute toxicity of aflatoxin B1

Indirect immunoperoxidase localization of aflatoxin B1 in rat liver

Aflatoxin B2a (AFB2a) antibody was used as a histochemical probe in the indirect immunoperoxidase localization of aflatoxin B1 (AFB1) bound to rat liver. The efficacy of the indirect method was initially demonstrated by detecting AFB1 covalently bound to DNA in an enzyme-linked immunosorbent assay. AFB1-modified DNA was attached to a polystyrene microtissue culture plate (solid phase) and then subjected to sequential incubation with AFB2a antiserum followed by goat anti-rabbit peroxidase conjugate. Assays for bound peroxidase revealed that the AFB2a antiserum could be diluted 200,000-fold and still yield a signal-to-noise ratio of 10 when compared to an unmodified DNA control. When the same indirect immunoperoxidase protocol was applied to the light-microscopic localization of AFB1 in liver sections of rats treated in vivo with the mycotoxin, bound toxin could be identified in excellent detail in tissues fixed with periodate-lysine-paraformaldehyde and embedded in glycol methacrylate, but was detectable with only poor resolution in unfixed cryostat sections. Peroxidase-positive reactions in hepatocytes typically exhibited strong nuclear and relatively lighter cytoplasmic staining. Greater concentrations of peroxidase-positive hepatocytes were detected in the periportal area than in the area of the central vein, suggesting a circulatory pattern for AFB1 binding in the liver.

Some mass-spectral and n.m.r. analytical studies of a glutathione conjugate of aflatoxin B1

A system for the formation of an aflatoxin B1-reduced glutathione conjugate in vitro was developed, capable of yielding 80% conversion of aflatoxin B1 into the conjugate. A reverse-phase high-pressure-liquid-chromatography system was also devised that not only facilitates improved resolution of the compound but that, by manipulation of the pH, is also capable of an extensive purification of the compound from other aflatoxin B1 metabolites in a single step. Material produced by these techniques, after further purification, has been used in 1H-n.m.r. and mass-spectroscopic studies. Results were obtained that support the proposed linkage of the aflatoxin B1 to reduced glutathione in a 1:1 molar ratio via a thioether linkage. Amino acid analyses were also consistent with this structure. The absence of a Schiff-base linkage of aflatoxin B1 8,9-dihydrodiol to glutamate was further demonstrated by the presence of a gamma-glutamyltransferase-catalysed-transferable glutamate moiety. These data are consistent with the structure 8,9-dihydro-8-(S-glutathionyl)-9-hydroxy-aflatoxin B1.

Reactivity of aflatoxin B2a antibody with aflatoxin B1-modified DNA and related metabolites

Aflatoxin B2a (AFB2a) antiserum has been previously used in an enzyme-linked immunosorbent assay (ELISA) for the quantitation of AFB1 and AFB2a. The present investigation examined the reactivity of the antiserum toward those adducts and metabolites of AFB1 believed to play a major role in aflatoxicosis and carcinogenesis. 2,3-Dihydro-2-(N7-guanyl)-3-hydroxyaflatoxin B1 (AFB1-N7-Gua), the putative 2,3-(N5-formyl-2-2', 5',6'-triamino-4-oxo-N5-pyrimidyl)-3-hydroxyaflatoxin B1 (AFB1-FAPyr), 2,3-dihydro-2,3-dihydroxyaflatoxin B1 (AFB1-diol), AFB1-N7-Gua-modified DNA, and AFB1-FAPyr-modified DNA were prepared by in vitro incubation or chemical methods and subjected to competitive AFB2a ELISA. The antiserum showed significant reactivity with all five compounds, indicating that it had a high degree of specificity for both the cyclopentenone and the methoxy group of the parent aflatoxin molecule. Sensitivity for AFB-N7-Gua-modified DNA, AFB1-FAPyr-modified DNA, and AFB1-diol by the ELISA method was 0.1 pmol per assay. To test the applicability of immunological detection of covalent binding of AFB1 to DNA, the ELISA was compared with a conventional radioisotopic assay in two in vitro studies. The results showed that estimates of the kinetics and substrate dependence of covalent binding to calf thymus DNA in rat microsomal incubation mixtures by both methods were comparable. The broad specificity AFB2a antibody might be of considerable value in the detection of AFB1 macromolecular adducts and related metabolites in epidemiological investigations or in the diagnosis of aflatoxicosis.

Toxicological significance of dihydrodiol metabolites

Dihydrodiols are often found as the major organic-extractable metabolites of various olefinic or aromatic xenobiotics in many biological samples. Studies on the chemistry of dihydrodiol metabolites have provided insight into the pharmacokinetic behavior and the mode of action of the parent compound. The toxicology of dihydrodiol is more complex than what can be deduced solely on the basis of diminished bioavailability of the epoxide precursor, and the increased hydrophilicity associated with the dihydrodiol moiety. Dihydrodiols can be intrinsically toxic and may even represent metabolically activated species. Some of the dihydrodiol metabolites may still retain sufficient lipophilic character to serve again as substrates for microsomal oxygenases. Because of the tremendous chemical and biological diversity that existed among the various dihydrodiols, more mechanistic studies are needed to examine the toxicological properties of these compounds. It may be premature to conclude dihydrodiol formation as purely a detoxification route for xenobioties.