Interference of the Natural Product 7-Oxo-7-deacetoxygedunin with CF0of H+-ATPase of Spinach Chloroplasts

7-Oxo-7-deacetoxygedunin, a limonoid isolated from Guarea grandiflora (Meliaceae) has been found to act as an inhibitor of photophosphorylation in spinach thylakoids. ATP synthesis and phosphorylating electron flows were inhibited by 88 and 83%, respectively, at 300 µM of 7-oxo-7deacetoxygedunin without affecting proton uptake, basal and uncoupled electron transports. The ketone group of 7-oxo-7-deacetoxygedunin is an important structural requirement for the displayed inhibitory activity on chloroplast H+-ATPase, since gedunin possessing a 7-acetoxy group was a less potent energy transfer inhibitor. The light-activated membrane bound Mg2+-ATPase and the heat-activated Ca2+-ATPase of the isolated coupling factor were insensitive to 7-oxo-7deacetoxygedunin. 7-oxo-7-deacetoxygedunin shows a competitive kinetic behavior with 5-O--Dgalactopyranosyl-7-methoxy-3ʹ,4ʹ-dihydroxy-4-phenylcoumarin when they inhibit H+-ATPase activity, but a noncompetitive kinetic with DCCD. These results suggest that 7-oxo-7deacetoxygedunin and the 4-phenylcoumarin have the same surrounding site of inhibition in C0. Ki for 7-oxo-7-deacetoxygedunin, 4-phenylcoumarin, and DCCD were 146, 220, and 10 µM, respectively.