The histochemical distribution of aniline hydroxylase in rat tissues

Xenobiotica-metabolizing enzymes in the lung of experimental animals, man and in human lung models

The xenobiotic metabolism in the lung, an organ of first entry of xenobiotics into the organism, is crucial for inhaled compounds entering this organ intentionally (e.g. drugs) and unintentionally (e.g. work place and environmental compounds). Additionally, local metabolism by enzymes preferentially or exclusively occurring in the lung is important for favorable or toxic effects of xenobiotics entering the organism also by routes other than by inhalation. The data collected in this review show that generally activities of cytochromes P450 are low in the lung of all investigated species and in vitro models. Other oxidoreductases may turn out to be more important, but are largely not investigated. Phase II enzymes are generally much higher with the exception of UGT glucuronosyltransferases which are generally very low. Insofar as data are available the xenobiotic metabolism in the lung of monkeys comes closed to that in the human lung; however, very few data are available for this comparison. Second best rate the mouse and rat lung, followed by the rabbit. Of the human in vitro model primary cells in culture, such as alveolar macrophages and alveolar type II cells as well as the A549 cell line appear quite acceptable. However, (1) this generalization represents a temporary oversimplification born from the lack of more comparable data; (2) the relative suitability of individual species/models is different for different enzymes; (3) when more data become available, the conclusions derived from these comparisons quite possibly may change.

Localization, distribution, and induction of xenobiotic-metabolizing enzymes and aryl hydrocarbon hydroxylase activity within lung

The metabolism of xenobiotics within lung often leads to toxicity, although certain pulmonary cells are more readily damaged than others. This differential susceptibility can result from cell-specific differences in xenobiotic activation and detoxication. The localization and distribution of xenobiotic-metabolizing enzymes (cytochromes P-450, NADPH-cytochrome P-450 reductase, epoxide hydrolase, glutathione S-transferases, UDP-glucuronosyltransferases, and a sulfotransferase) and of aryl hydrocarbon (benzo[a]pyrene) hydroxylase activity determined immunohistochemically and histochemically, respectively, within lung are discussed. Findings reveal that xenobiotics can be metabolized in situ, albeit to different extents, by bronchial epithelial cells, Clara and ciliated bronchiolar epithelial cells, and type II pneumocytes and other alveolar wall cells and that enzymes and activities are not necessarily induced uniformly among these cells.

Identification of intratissue sites for xenobiotic activation and detoxication

Results of immunohistochemical and histochemical investigations on xenobiotic-metabolizing enzymes and aryl hydrocarbon hydroxylase activity have demonstrated that xenobiotic activation and detoxication do not occur uniformly throughout the liver, skin, respiratory tract, and pancreas, four tissues that are targets for the toxic actions of xenobiotics that are biotransformed into reactive metabolites. It has been shown that there can be significant differences in the levels and activities of xenobiotic-metabolizing enzymes among even morphologically similar cells, that an inducer can affect a specific xenobiotic-metabolizing enzyme to significantly different extents within different cells in a tissue, and that inducers of xenobiotic-metabolizing enzymes can alter differentially the extents to which different cells within a tissue participate in xenobiotic metabolism. These studies also have revealed that the route of administration of an inducer can affect significantly the induction of xenobiotic-metabolizing enzymes and aryl hydrocarbon hydroxylase activity within an organ such as the pancreas. Some of the immunohistochemical findings reported for the cellular localizations of xenobiotic-metabolizing enzymes within specific tissues, e.g., the nasal mucosa, may not appear to be entirely consistent with the intratissue distribution of benzo[a]pyrene hydroxylase activity, especially after induction. However, it must be appreciated that other cytochrome P-450 isozymes undoubtedly are present within these tissues which, although not studied, also are capable of catalyzing aryl hydrocarbon hydroxylase activity.

Modification by phenobarbital of chlorphentermine-induced changes in lung morphology and drug-metabolizing enzymes in newborn rats

Treatment of newborn rat pups with 60 mg/kg.d chlorphentermine for 7 d produced on accumulation of alveolar foam cells accompanied by an increase in relative pulmonary tissue weight. In contrast, administration of 20 mg/kg.d for 1 wk did not markedly alter lung ultrastructure or weight in newborns. Both doses of chlorphentermine elevated the activity of pulmonary aminopyrine N-demethylase but not that of aniline hydroxylase. The increase in relative liver weight was associated with stimulation of the activities of aniline hydroxylase and aminopyrine N-demethylase in newborns administered either chlorphentermine dose. Phenobarbital treatment produced an increase in relative liver weight accompanied by elevated activities of pulmonary aminopyrine N-demethylase and hepatic aniline hydroxylase and aminopyrine N-demethylase. Simultaneous barbiturate and chlorphentermine administration produced stimulation in liver enzymes to the same extent as phenobarbital alone. In contrast, phenobarbital potentiated the chlorphentermine-induced rise in pulmonary aminopyrine N-demethylase. In the case of 60 mg/kg chlorphentermine and barbiturate, the observed potentiation of lung enzyme activity was associated with a reduction in the number of alveolar foam cells. The results suggest that chlorphentermine and phenobarbital stimulate drug-metabolizing enzyme in lung and liver of newborn rats and that phenobarbital may provide protection against phospholipidosis through stimulation of pulmonary, drug-metabolizing enzymes.