Annonaceous acetogenins: Naturally occurring inhibitors of ATP synthesis and photosystem II in spinach chloroplasts

The effects of squamocin (1), bullatacin (2) and motrilin (3), 3 bis-tetrah ydrofuran Annonaceous acetogenins, isolated from Annona purpurea (Annonacea e), were investigated on several photosynthetic activities in spinach thyla koids. The results indicated that compounds 1-3 significantly inhibited bot h ATP synthesis and uncoupled electron transport. In addition, they enhance d light-activated Mg2+-ATPase, and basal electron flow. Therefore, acetogen ins 1-3 behave as uncouplers and Hill reaction inhibitors. Natural products 1-3 did not affect photosystem I (PSI) activity but they inhibited photosy stem II (PSII) electron flow. The study of the partial PSII reactions from H2O to DCPIPox, H2O to SiMo and diphenylcarbazide to DCPIP established that the site of inhibition was at the oxygen-evolving complex (OEC), Chlorophy ll a fluorescence measurements confirmed the behavior of the Annonaceous ac etogenins as water-splitting enzyme inhibitors.