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-tetrahydrofuran Annonaceous acetogenins, isolated from Annona purpurea (Annonaceae), were investigated on several photosynthetic activities in spinach thylakoids. The results indicated that compounds 1-3 significantly inhibited both ATP synthesis and uncoupled electron transport. In addition, they enhanced light-activated Mg²⁺-ATPase, and basal electron flow. Therefore, acetogenins 1-3 behave as uncouplers and Hill reaction inhibitors. Natural products 1-3 did not affect photosystem I (PSI) activity but they inhibited photosystem II (PSII) electron flow. The study of the partial PSII reactions from H₂O to DCPIP_ox, H₂O to SiMo and diphenylcarbazide to DCPIP established that the site of inhibition was at the oxygen-evolving complex (OEC). Chlorophyll a fluorescence measurements confirmed the behavior of the Annonaceous acetogenins as water-splitting enzyme inhibitors.