EFFECT OF DIETARY CASEIN LEVELS ON ACTIVATION OF PROMUTAGENS IN THE SPIRAL SALMONELLA MUTAGENICITY ASSAY .2. STUDIES WITH INDUCED RAT-LIVERSG

In the previous study (Mutation Res., this issue), we showed that incr eased levels of dietary casein as the sole protein source for male F34 4 rats decreased the ability of the uninduced liver S9s to activate 2- aminoanthracene (2AN) to a mutagen in strain TA98 using the spiral Sal monella mutagenicity assay. No effects of dietary casein levels were n oted for the ability of uninduced liver S9s to activate the promutagen s aflatoxin B-1 (AFB) and benzo[a]pyrene (BAP). In the present study, we have extended this study to include liver S9s induced with either A roclor 1254, phenobarbital or 3-methylcholanthrene (3MC). S9s were der ived from individual male F344 rats fed for 6 weeks on semisynthetic d iets containing 8%, 12% or 22% methionine-supplemented casein as the s ole source of protein (diets were made isocaloric by adjusting the cor n starch content). Rats were housed in large, raised-bed cages by grou ps of three/diet/inducing agent. S9 activation mixtures were prepared at 5 mg of S9 protein/ml of S9 mix. Slopes from the linear portions of the mutagenicity dose-response curves were analyzed by ANOVA comparis ons, Assays used to elucidate the phase I activities of microsomal pre parations were cytochrome P-450 content, cytochrome-c reductase activi ty, flavin-containing monooxygenase activity, 7-ethoxyresorufin O-deet hylation (EROD) activity, N-demethylation of benzphetamine, and para-n itrophenol O-deethylation. Phase II activities were assayed by estimat ing glutathione (GSH) content and measuring the metabolism of 1-chloro -2,4-dinitrobenzene (CDNB) by glutathione S-transferase in cytosolic p reparations. None of the phase I or phase II endpoints were significan tly affected by dietary casein levels. In general, increasing levels o f dietary casein resulted in increased body and liver wet weight and a mount of S9 protein. Aroclor-induced S9s from rats fed the 22% or 12% casein diet were most effective at activating AFB, depending on the lo t of Aroclor used for induction; these divergent results were replicat ed with two groups of rats for each lot of Aroclor, The observed diffe rences between Aroclor lots are assumed to arise from variation in the mix of PCB isomers, The Aroclor-induced S9s did not exhibit any casei n-related effects for the activation of BAP or 2AN. For 3MC-induced S9 s, the 12% casein diets produced S9s with the highest ability to activ ate AFB and BAP when standardized for protein content. Phenobarbital-i nduced S9s did not demonstrate any dietary casein-related effects on t he activation of the three model promutagens. These results illustrate the complex interaction between dietary levels of casein, enzyme indu cing agent and promutagen.