Journal of Applied Physiology, Vol 54, Issue 4 1089-1093, Copyright © 1983 by American Physiological Society
Paraquat inhibits mixed-function oxidation by rat lung
T. K. Aldrich, A. B. Fisher and H. J. Forman
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.
