Hispidulin: antioxidant properties and effect on mitochondrial energy metabolism

Hispidulin (6-methoxy-5,7,4'-trihydroxyflavone) and eupafolin (6-methoxy-5,7,3',4'-tetrahydroxyflavone), are flavonoids found in the leaves of Eupatorium litoralle. They have recognized antioxidant and antineoplastic properties, although their action mechanisms have not been previously described. We now report the effects of hispidulin on the oxidative metabolism of isolated rat liver mitochondria (Mit) and have also investigated the prooxidant and antioxidant capacity of both flavonoids. Hispidulin (0.05-0.2 mM) decreased the respiratory rate in state III and stimulated it in state IV, when glutamate or succinate was used as oxidizable substrate. Hispidulin inhibited enzymatic activities between complexes I and III of the respiratory chain. In broken Mit hispidulin (0.2 mM) slightly inhibited ATPase activity (25%). However, when intact Mit were used, the flavonoid stimulated this activity by 100%. Substrate energized mitochondrial swelling was markedly inhibited by hispidulin. Both hispidulin and eupafolin were able to promote iron release from ferritin, this effect being more accentuated with eupafolin with the suggestion of a possible involvement of H₂O₂ in the process. Hispidulin was incapable of donating electrons to the stable free radical DPPH, while eupafolin reacted with it in a similar way to ascorbic acid. The results indicate that hispidulin as an uncoupler of oxidative phosphorylation, is able to release iron from ferritin, but has distinct prooxidant and antioxidant properties when compared to eupafolin.