1. The activities of several commercial polychlorinated naphthalenes including Halowaxes 100, 1001, 1099, 1013, 1014 and 1051 as hepatic microsomal enzyme inducers in the immature male rat were determined by measuring their effects on microsomal dimethylaminoantipyrine N-demethylase and benzo[a]pyrene (B[a]P) hydroxylase (or aryl hydrocarbon hydroxylase, AHH) activities, the cytochrome b5 content and the relative peak intensities and spectral shifts of the reduced microsomal cytochrome P-450:CO and ethylisocyanide binding difference spectra. 2. The results obtained for the Halowaxes were compared with the effects of animal pretreatment with phenobarbitone, 3-methylcholanthrene, phenobarbitone plus 3-methylcholanthrene (coadministered) and Aroclor 1254, a commercial polychlorinated biphenyl. 3. At the higher dose level (600 μmol.kg-1) all the Halowaxes enhanced microsomal benzo[a]pyrene hydroxylase activity (2- to 15-fold) and the hepatic microsomal cytochrome P-450 content (1.2- to 2.5-fold) whereas the dimethylaminoantipyrine N-demethylase activities were increased <40% compared to noninduced animals. 4. Based on the enzymic and spectral properties of the microsomal enzymes from the Halowax pretreated animals, Halowaxes 1000, 1001 and 1099 were phenobarbitone-type inducers and Halowaxes 1013, 1014 and 1051 were mixed-type inducers similar to Aroclor 1254 or phenobarbitone plus 3-methylcholanthrene (coadministered).
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