Inhibition of mitochondrial respiration and cyanide-stimulated generation of reactive oxygen species by selected flavonoids.

A continuation of our structure-activity study on flavonoids possessing varied hydroxyl ring configurations was conducted. We tested six additional flavonoids for their ability to inhibit beef heart mitochondrial succinoxidase and NADH-oxidase activities. In every case, the IC50 observed for the NADH-oxidase enzyme system was lower than for succinoxidase activity, demonstrating a primary site of inhibition in the complex I (NADH-coenzyme Q reductase) portion of the respiratory chain. The order of potency for inhibition of NADH-oxidase activity was robinetin, rhamnetin, eupatorin, baicalein, 7,8-dihydroxyflavone, and norwogonin with IC50 values of 19, 42, 43, 77, 277 and 340 nmol/mg protein, respectively. Flavonoids with adjacent tri-hydroxyl or para-dihydroxyl groups exhibited a substantial rate of auto-oxidation which was accelerated by the addition of cyanide (CN-). Flavonoids possessing a catechol configuration exhibited a slow rate of auto-oxidation in buffer that was stimulated by the addition of CN-. The addition of superoxide dismutase (SOD) and catalase in the auto-oxidation experiments each decreased the rate of oxygen consumption, indicating that O2- and H2O2 are generated during auto-oxidation. In the CN(-)-stimulated oxidation experiments, the addition of SOD also slowed the rate of oxygen consumption. These findings demonstrate that the CN-/flavonoid interaction generated O2- non-enzymatically, which could have biological implications.