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Inducibility of hepatic CYP1A enzymes by 3-methylcholanthrene and isosafrole differs in male rats fed diets containing casein, soy protein isolate or whey from conception to adulthood.

Hepatic cytochrome P450 (CYP)1A1 and 1A2 enzymes were studied in male Sprague-Dawley rats derived from 5-7 litters fed diets in which the protein source was casein, soy protein isolate or whey. At age 65 d, rats were gavaged with corn oil (vehicle), 40 mg/kg 3-methylcholanthrene (3-MC) or 75 mg/kg isosafrole (ISO). Hepatic expression of CYP1A1 and CYP1A2 mRNA, apoprotein and associated monooxygenase activities were measured 17 h later. No significant dietary effects were observed on basal expression of either enzyme. However, interactions between diet and the two inducers (3-MC and ISO) were observed in soy-fed rats for ethoxy- and methoxyresorufin O-dealkylase activity, CYP1A1 and CYP1A2 apoprotein and mRNA (P < 0.05). The level of induction of CYP1A1 mRNA and apoprotein was lower in rats fed soy diets than in rats fed casein diets (P < 0.05), and the level of induced CYP1A2 mRNA was lower in rats fed soy or whey (P < 0.05) after treatment with the aryl hydrocarbon (Ah) receptor-dependent inducer 3-MC. This was accompanied by a 50% reduction in constitutive levels of the Ah receptor in liver cytosol of soy-fed, relative to casein-fed rats, and a slightly smaller reduction in whey-fed rats. Expression of the Ah receptor correlated with 3-MC-inducibility of CYP1A1 mRNA in rats fed the three diets. In contrast, in rats induced with ISO, which does not bind to the Ah receptor and induces CYP1As via different mechanisms than 3-MC, ethoxyresorufin O-deethylase activity and levels of CYP1A1 apoprotein and mRNA were elevated to a greater degree in soy-fed than in casein- or whey-fed rats (P < 0.05). Moreover, after ISO treatment, induction of methoxyresorufin O-demethylase activity, CYP1A2 apoprotein and mRNA levels was observed only in rats fed soy (P < 0.05). These data suggest potential effects of dietary protein source on metabolism of a wide variety of CYP1A substrates, including environmental and dietary carcinogens, many of which induce their own metabolism.