Annonaceous acetogenins (ACG) are natural products isolated from plants of the family Annonaceae and which have potent biological activities. Since Jolad isolated the first acetogenin (uvaricin) from Uvaria acuminata in 1982, 1) almost 400 acetogenins have been discovered, and new structural ACG will be isolated from other plants along as they are studied in depth. The reasons that ACG have drawn so much attention are not only their novel structural features but also their potent and broad activities which are seldom found in other natural compounds.The general structures of ACG are characterized by long alkyl chain compounds of 35-37 carbons containing terminal 2,4-disubstituted-g-lactone, zero to three tetrahydrofuran (THF) rings, and a number of oxygenated moieties (hydroxyls, acetoxyls, ketones, epoxides) or double bonds. According to the number and stereochemistry of THF, ACG have been classified into mono-THF, adjacent bis-THF, non-adjacent bis-THF, tri-THF, non-THF, and non-classical acetogenins.
2)Because of the wide use of ACG in pesticidal, antimalarial, antimicrobial, antiparasitic, antiprotozoal, cytotoxic activities and antitumor effects, studies on the isolation and identification of ACG as well as their action mechanism are a field presently focused on.ACG show potent activities by inhibiting NADHubiquinone oxidoreductase (complex I) in mitochondrial respiratory chain, and complex I is the largest, most complicated and important protein complex of the mitochondrial electron transport systems, which are related to ATP production.2,3) Additionally, ACG are also strong inhibitors of NADH oxidase of the plasma membrane in cancer cells, but not in rat liver plasma membrane. 4) Actually, complex I can be evaluated by NADH-ubiquinone oxidoreductase activity or by NADH oxidase activity in mitochondria, although they are not equivalent. NADH oxidase activity represents an integrated activity in which NADH is oxidized and the electrons are transferred along the respiratory chain to be finally accepted by molecular oxygen. Otherwise, the assay of NADHubiquinone oxidoreductase activity is performed with ubiquinone or its analogue as electron acceptor. As ACG only inhibit complex I, the decrease of NADH oxidase activity is directly attributed to the inhibition of complex I.
5)Most studies suggest that the terminal g-lactone is essential for the potency of ACG as complex I inhibitors, although some reports propose that the alkyl chain plays a more crucial role. However, for the roles of other ring moieties have not been sufficiently emphasized. In the present study, the roles of these structural factors are discussed by three acetogenins inhibiting NADH oxidase of chicken liver mitochondria with different ring moieties in addition to terminal g-lactone, especially the non-THF type compound exhibiting inhibitory activity, and in comparison with rotenone which is the classic inhibitor of complex I.
MATERIALS AND METHODS
Materials and ReagentsThree acetogenins were isolated and purified from seeds of Annona squamo...