RELATIONSHIP OF CYTOCHROME-P450 ACTIVITY TO CLARA CELL CYTOTOXICITY .4. METABOLISM OF NAPHTHALENE AND NAPHTHALENE OXIDE IN MICRODISSECTED AIRWAYS FROM MICE, RATS, AND HAMSTERS

Parenteral administration of naphthalene produces a dose-dependent and tissue-, species-, and cell-selective lesion of murine Clara cells. T he rate and stereoselectivity of naphthalene metabolism by microsomal preparations correlate with tissue and species differences in cytotoxi city. Because earlier studies used microsomes obtained from whole tiss ue, differences in susceptibility of proximal and distal airways could not be related to differences in the metabolic activation or detoxica tion of naphthalene. Specific subcompartments of the respiratory syste m, obtained by microdissection, have been used to study the cytochrome P450-dependent metabolism of naphthalene and the epoxide hydrolase/gl utathione transferase-dependent metabolism of naphthalene oxide. The r ates of naphthalene metabolism were substantially higher in mouse airw ays than in comparable airways of hamsters or rats. Rates of metabolis m were higher in distal airways than in the trachea of all species stu died. Metabolism in mouse airways was highly stereoselective, whereas that in hamster and rat tissues was not. Nonciliated cells at all airw ay levels in mice were heavily labeled with an antibody to cytochrome P450 2F2; little labeling was observed in any portion of rat and hamst er lungs. Postmitochondrial supernatants prepared from mouse and hamst er airways metabolized racemic naphthalene oxide to diet and glutathio ne adducts at substantially higher rates than did comparable preparati ons from rats. Although glutathione levels varied 2-4-fold at differen t airway levels in the three species studied, levels at the most susce ptible site (mouse distal bronchioles) were as high as or higher than those at other, less susceptible, sites. These studies support the vie w that the rate and stereoselectivity of naphthalene metabolism to nap hthalene 1R,2S-oxide catalyzed by cytochrome P450 2F2 are critical det erminants in the species-specific and region-selective cytotoxicity of naphthalene in mice. The lack of major differences in the catalytic a ctivity or enantioselectivity of putative detoxication enzymes (epoxid e hydrolase or glutathione transferases) between mouse and hamster tis sue, combined with data showing that the differences in the metabolic fate of naphthalene oxide in proximal versus distal airways are not dr amatic, suggests that the initial epoxidation of naphthalene is an imp ortant factor in site-selective toxicity. These studies support the ne ed to use tissue from defined airway levels for studies on the relatio nship of biochemical and metabolic factors important in cellular injur y by lung toxicants, such as naphthalene, where there are dramatic reg ional differences in susceptibility to injury within the respiratory s ystem.