The biological activity of floccosin, which is characterized by a semiquinone structure with epoxide rings, was compared with that of xanthomegnin which had been demonstrated to exhibit strong uncoupling effect and to produce an electron transport shunt to the respiratory system in mitochondria. Floccosin was found to exhibit a similarily strong uncoupling effect on isolated mitochondria to that of xanthomegnin. Xanthomegnin markedly enhanced the letent ATPase activity of mitochondria, whereas floccosin showed biphasic effect on the ATPase activity : a weak stimulation at concentrations lower than about 80 μM, producing a maximum activity at around 40 μM, and a progressive inhibition at the higher concentrations than that. Xanthomegnin did not influence the ATPase activity of submitochondrial particles, whereas floccosin strongly inhibited the ATPase activity. Xanthomegnin induced mitochodrial swelling in KCl isotonic solution, whereas floccosin showed only faint effect on it. These results indicate that floccosin has different biological activities from those of xan-thomegnin.
The biochemical property of a xanthone derivative, secalonic acid D (SA-D) was compared with that of an anthraquinone derivative, skyrin which had been demonstrated to be an uncoupler on the oxidative phosphorylation in mitochondria. Both compounds were revealed to have pKa values at very close pH to each other (SA-D at pH 6.4, skyrin at pH 6.1). SA-D was found to emit a strong fluorescence (Ext. 380 nm, Em. 535 nm) when dissolved in organic solvents or bound by liposomes, mitochondria, or BSA, whereas skyrin did not show any fluorescence even when bound by these materials. SA-D was demonstrated to uncouple the oxidative phorphorylation in mitochondria at very close concentrations to those of skyrin, but not to induce mitochondrial swelling, though skyrin displayed both abilities of uncoupling mitochondrial respiration and of inducing mitochondrial swelling.