EFFECT OF DIETARY CASEIN LEVELS ON ACTIVATION OF PROMUTAGENS IN THE SPIRAL SALMONELLA MUTAGENICITY ASSAY .1. STUDIES WITH NONINDUCED RAT-LIVER S9

Xenobiotic metabolism can be influenced by various nutritional factors , including protein. In the present study, we have examined the effect of dietary protein (casein) levels on the ability of rat liver S9 to activate the promutagens aflatoxin B-1 (AFB), 2-aminoanthracene (2AN) and benzo[a]pyrene (BAP) in strain TA98 using the spiral Salmonella mu tagenicity assay. S9s were derived from individual male F344 rats fed for 6 weeks on semisynthetic diets containing 8%, 12% or 22% methionin e-supplemented casein as the sole source of protein (diets were made i socaloric by adjusting the corn starch content). Rats were housed in l arge, raised-bed cages by groups of three per diet. S9 activation mixt ures were prepared at 5 mg of S9 protein/ml of S9 mix. Slopes from the linear portions of the mutagenicity dose-response curves were analyze d by ANOVA comparisons. Assays used to elucidate the phase I activitie s of microsomal preparations were cytochrome P450 content, cytochrome- e reductase activity, flavin-containing monooxygenase activity, 7-etho xyresorufin O-deethylation (EROD) activity, N-demethylation of benzphe tamine and para-nitrophenol O-deethylation. Phase II activities in cyt osolic preparations were assayed by estimation of glutathione (GSH) co ntent and glutathione S-transferase activity through metabolism of 1-c hloro-2,4-dinitrobenzene (CDNB). Increased levels of dietary casein in creased liver wet weights and decreased the ability of the S9 to activ ate 2AN. Dietary casein levels did not influence the S9-mediated activ ation of BAP; and consistent but nonsignificant increases in activatio n of AFB were produced by S9 from animals fed the 22% casein diet. The phase I and phase II activities measured here were not altered signif icantly by dietary casein levels; thus, other, more specific enzymatic activities may account for the mutagenesis data. These results illust rate the complex interaction between dietary levels of casein and prom utagen activation mechanisms, which prevents drawing broad generalizat ions regarding the influence of dietary casein levels on the capacity of hepatic S9s to activate promutagens.