Studies on the mechanism of toxicity and of development of tolerance to the pulmonary toxin, alpha-naphthylthiourea (ANTU).

The in vivo administration of the radiolabeled lung toxin alpha-naphthylthiourea (ANTU) to rats leads to the covalent binding of radioactivity to the macromolecules of the lung and liver. In contrast, very little radioactivity is bound in these organs when an equal amount of the 14C-labeled oxygen analog of ANTU, 14C-alpha-naphthylurea (ANU), is administered. In addition, ANU is essentially nontoxic to rats. ANTU is metabolized in vitro by lung and liver microsomes to an intermediate which covalently binds to the macromolecules of the microsomes. This covalent binding, which requires NADPH, leads to a decrease in mixed-function oxidase activity and to a decrease in the level of cytochrome P-450 detectable as its carbon monoxide complex. Incubation of microsomes with ANTU in the absence of NADPH or with ANU in the presence of NADPH, has no effect on these parameters. Pretreatment of rats with small nonlethal doses of ANTU daily for 5 days brings about a decrease in the activity of the mixed-function oxidase enzyme system in the lung which metabolizes parathion. In addition, this pretreatment decreases the toxicity of ANTU and leads to a decrease in the amount of radioactivity bound to the macromolecules of the lung when the animals are given a lethal dose of 35S-ANTU. These data suggest that the lung toxicity of ANTU is brought about by its metabolic activation and covalent binding to lung macromolecules.