Cytosol is required for the modulation by dietary casein of the hepatic microsomal activation of aflatoxin B1 to mutagenic metabolites detectable in Salmonella

We have shown previously that dietary protein (casein) levels can affect th e 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 metab olized 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 prote in source, compared with S9s from rats fed diets that contained nominally d eficient (8%) or high (22%) levels of casein. Here we have extended this ob servation by performing (i) mutagenicity studies with microsomes, cytosols and reconstituted S9s (recombinations of microsomes and cytosols across die tary groups), and (ii) in vitro incubations followed by analysis of metabol ites by fluorescence high-pressure liquid chromatography. Microsomes, but n ot 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 res ults, the greatest metabolism of the AFB parent compound and the highest le vel of the glutathione conjugate of the presumptively identified AFB-exo-8, 9-epoxide (the ultimate mutagenic form of AFB) were produced by sos from th e rats fed the 12% casein diet. The levels of these metabolites and the mut agenicity of AFB changed in parallel with changes in dietary casein levels. In summary, cytosolic elements, which are not affected by dietary casein l evels, interact with microsomal enzymes, which are modulated by dietary cas ein levels, to influence the ability of hepatic S9 to activate AFB to a mut agen.