Role of mitochondrial electron transport complex I in coenzyme Q1 reduction by intact pulmonary arterial endothelial cells and the effect of hyperoxia

Abstract

The objective was to determine the impact of intact normoxic and hyperoxia-exposed (95% O2 for 48 h) bovine pulmonary arterial endothelial cells in culture on the redox status of the coenzyme Q10 homolog coenzyme Q1 (CoQ1). When CoQ1 (50 μM) was incubated with the cells for 30 min, its concentration in the medium decreased over time, reaching a lower level for normoxic than hyperoxia-exposed cells. The decreases in CoQ1 concentration were associated with generation of CoQ1 hydroquinone (CoQ1H2), wherein 3.4 times more CoQ1H2 was produced in the normoxic than hyperoxia-exposed cell medium (8.2 ± 0.3 and 2.4 ± 0.4 μM, means ± SE, respectively) after 30 min. The maximum CoQ1 reduction rate for the hyperoxia-exposed cells, measured using the cell membrane-impermeant redox indicator potassium ferricyanide, was about one-half that of normoxic cells (11.4 and 24.1 nmol·min−1·mg−1 cell protein, respectively). The mitochondrial electron transport complex I inhibitor rotenone decreased the CoQ1 reduction rate by 85% in the normoxic cells and 44% in the hyperoxia-exposed cells. There was little or no inhibitory effect of NAD(P)H:quinone oxidoreductase 1 (NQO1) inhibitors on CoQ1 reduction. Intact cell oxygen consumption rates and complex I activities in mitochondria-enriched fractions were also lower for hyperoxia-exposed than normoxic cells. The implication is that intact pulmonary endothelial cells influence the redox status of CoQ1 via complex I-mediated reduction to CoQ1H2, which appears in the extracellular medium, and that the hyperoxic exposure decreases the overall CoQ1 reduction capacity via a depression in complex I activity.