title: Paraquat inhibits mixed-function oxidation by rat lung.
abstract: The effect of paraquat on 7-ethoxycoumarin (7-EC)deethylation, an NADPH-dependent cytochrome P-450-linked reaction, was studied in isolated rat lungs. Control lungs oxidatively deethylated 7-EC at a constant rate, metabolizing 178 +/- 12 nmol/g dry wt in 50 min (n=3, mean +/- SE). When glucose was omitted from the perfusate, 7-EC deethylation was lower (85 +/- 17 nmol/g, P less than 0.05), and the rate progressively decreased, suggesting inadequate pentose shunt regeneration of NADPH. During perfusion with paraquat (10(-3) M), 7-EC metabolism was still lower (75 +/- 5 nmol/g in the presence of glucose and 18 +/- 4 nmol/g in the absence of glucose, P less than 0.05). Addition of lactate plus pyruvate partially restored 7-EC deethylase activity in control lungs but had no effect in lungs perfused with paraquat. Varying perfusate paraquat concentrations (10(-6) to 10(-3) M) inhibited 7-EC deethylation in a dose-dependent fashion (r = 0.79, P less than 0.02). Oxidative demethylation of p-nitroanisole, another cytochrome P-450-linked reaction, was similarly inhibited by paraquat. In contrast, microsomes isolated from paraquat-perfused lungs showed no inhibition of 7-EC metabolism (measured in the presence of an NADPH-regenerating system). We conclude that paraquat depletes NADPH in the isolated lung to a sufficient extent to impair mixed-function oxidation. Such NADPH depletion may play a role in the toxicity of paraquat.
mesh_terms: D000818:Animals; D000840:Anisoles / Q000378:metabolism; D003374:Coumarins / Q000378:metabolism; D003577:Cytochrome P-450 Enzyme System / Q000502:physiology; D008168:Lung / Q000378:metabolism*; D008297:Male; D008861:Microsomes / Q000378:metabolism; D010084:Oxidation-Reduction / Q000187:drug effects*; D010269:Paraquat / Q000494:pharmacology*; D010477:Perfusion; D051381:Rats; D011919:Rats, Inbred Strains; D013997:Time Factors; D014468:Umbelliferones / Q000096:biosynthesis
chemical_list: D000840:Anisoles; D003374:Coumarins; D014468:Umbelliferones; C017299:7-ethoxycoumarin; D003577:Cytochrome P-450 Enzyme System; C005306:4-nitroanisole; D010269:Paraquat