Mutagenesis, Vol. 14, No. 4, 365-373,
July 1999
© 1999 UK Environmental Mutagen Society/Oxford University Press
Cytosol is required for the modulation by dietary casein of the hepatic microsomal activation of aflatoxin B1 to mutagenic metabolites detectable in Salmonella
Department of Toxicology and 1 Department of Poultry Science, Box 7608, North Carolina State University, Raleigh, NC 27695 and 2 Environmental Carcinogenesis Division, US Environmental Protection Agency, Research Triangle Park, NC 27711, USA
We have shown previously that dietary protein (casein) levels can affect the ability of rat liver S9 to metabolize aflatoxin B1 (AFB) as well as other promutagens detectable in Salmonella strain TA98 [Mutat. Res. (1997), 360, 115–126 and 127–143]. The mutagenic potency of AFB was greatest when metabolized by the Aroclor 1254-induced hepatic S9 prepared from F344 male rats that consumed an isocaloric, semisynthetic diet for 6 weeks that contained an adequate (12%) level of methionine-supplemented casein as the sole protein source, compared with S9s from rats fed diets that contained nominally deficient (8%) or high (22%) levels of casein. Here we have extended this observation by performing (i) mutagenicity studies with microsomes, cytosols and reconstituted S9s (recombinations of microsomes and cytosols across dietary groups), and (ii) in vitro incubations followed by analysis of metabolites by fluorescence high-pressure liquid chromatography. Microsomes, but not cytosols, activated AFB; however, activation to the level observed with S9 occurred only when microsomes from the rats fed 12% casein were combined with cytosols from any dietary group. Consistent with the mutagenicity results, the greatest metabolism of the AFB parent compound and the highest level of the glutathione conjugate of the presumptively identified AFB-exo-8,9-epoxide (the ultimate mutagenic form of AFB) were produced by S9s from the rats fed the 12% casein diet. The levels of these metabolites and the mutagenicity of AFB changed in parallel with changes in dietary casein levels. In summary, cytosolic elements, which are not affected by dietary casein levels, interact with microsomal enzymes, which are modulated by dietary casein levels, to influence the ability of hepatic S9 to activate AFB to a mutagen.
4 To whom correspondence should be addressed. Tel: +1 919 541 1510; Fax: +1 919 541 0694; Email: demarini.david{at}epa.gov
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