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1Department of Biomedical Engineering, Marquette University, Milwaukee 53201; Departments of 2Pulmonary and Critical Care Medicine, 3Physiology, 6Anesthesiology, and 7Pharmacology/Toxicology, Medical College of Wisconsin, Milwaukee 53226; 4Zablocki Veterans Administration Medical Center, Milwaukee, Wisconsin 53295; and 5School of Pharmacy, University of Colorado Health Sciences Center, Denver, Colorado 80262
Submitted 12 June 2003 ; accepted in final form 21 July 2003
The lungs can substantially influence the redox status of redox-active plasma constituents. Our objective was to examine aspects of the kinetics and mechanisms that determine pulmonary disposition of redox-active compounds during passage through the pulmonary circulation. Experiments were carried out on rat and mouse lungs with 2,3,5,6-tetramethyl-1,4-benzoquinone [duroquinone (DQ)] as a model amphipathic quinone reductase substrate. We measured DQ and durohydroquinone (DQH2) concentrations in the lung venous effluent after injecting, or while infusing, DQ or DQH2 into the pulmonary arterial inflow. The maximum net rates of DQ reduction to DQH2 in the rat and mouse lungs were 
4.9 and 2.5 µmol · min-1 · g dry lung wt-1, respectively. The net rate was apparently the result of freely permeating access of DQ and DQH2 to tissue sites of redox reactions, dominated by dicumarol-sensitive DQ reduction to DQH2 and cyanide-sensitive DQH2 reoxidation back to DQ. The dicumarol sensitivity along with immunodetectable expression of NAD(P)H-quinone oxidoreductase 1 (NQO1) in the rat lung tissue suggest cytoplasmic NQO1 as the dominant site of DQ reduction. The effect of cyanide on DQH2 oxidation suggests that the dominant site of oxidation is complex III of the mitochondrial electron transport chain. If one envisions DQ as a model compound for examining the disposition of amphipathic NQO1 substrates in the lungs, the results are consistent with a role for lung NQO1 in determining the redox status of such compounds in the circulation. For DQ, the effect is conversion of a redox-cycling, oxygen-activating quinone into a stable hydroquinone.
lung oxygen consumption; indicator dilution; reaction kinetics; mathematical modeling; NAD(P)H-quinone oxidoreductase; mitochondrial electron transport
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