Binding and inhibitory effect of 2-heptyl-4-hydroxyquinoline-N-oxide in the presence of ubiquinone-3 in Saccharomyces cerevisiae.

1. 2-Heptyl-4-hydroxyquinoline-N-oxide (HpHOQnO) binds to oxidized mitochondria of Saccharomyces cerevisiae with a dissociation constant of 5 x 10(-8) M and with a ratio of 1 mol per mol cytochrome b. 2. After addition of 171 micro M ubiquinone-3 to oxidized mitochondria, 0.3 mol HpHOQnO bind to 1 mol cytochrome b. The binding strength of the inhibitor is not essentially affected. 3. Reduction of mitochondria with succinate results in a strong decrease of the HpHOQnO binding. The dissociation constant could not be calculated on the basis of the applied method. 4. On the basis of the inhibitory effect of HpHOQnO on reduced mitochondria, a dissociation constant of 5 x 10(-6) M is calculated. 5. The respiratory electron flow to oxygen is about 100 times as sensitive to HpHOQnO as the electron flow to cytochrome c. 6. Ubiquinone-3 reverses the inhibition by HpHOQnO and stimulates the electron flow. 7. The different influence of ubiquinone-3 in binding and in inhibition experiments is discussed as a result of redox conditions.