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.
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