Effect of selected coumarins on spinach chloroplast photosynthesis

Xanthyletin (1), 3-(1',1'-dimethylallyl)xanthyletin (2), and chalepensin (3 ), the major coumarins isolated from Stauranthus perforatus, inhibit ATP sy nthesis from water to methylviologen in spinach thylakoids in a concentrati on-dependent manner. At low concentration chalepensin (3) inhibits basal an d phosphorylating electron flow from water to K-3[Fe(CN)(6)] without affect ing uncoupled electron flow but accelerating Mg2+-ATPase activity. Thus, at low concentration the compound behaves as an energy transfer inhibitor. Ho wever, at higher concentrations this coumarin acts as an uncoupler because it enhances basal and phosphorylating electron transfer. On the other hand, coumarins 1 and 2 act as Hill reaction inhibitors, although 2 exhibited al so uncoupler properties because it induces stimulation of basal and phospho rylating electron flow from water to ferricyanide. The site of interference of xanthyletin was located at the b(6)f-PC level of the electron transport chain.