5-O-?-d-galactopyranosyl-7-methoxy-3?,4?-dihydroxy-4-phenylcoumarin, an inhibitor of photophosphorylation in spinach chloroplasts

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Abstract
5-O-β-d-galactopyranosyl-7-methoxy-3′,4′-dihydroxy-4-phenylcoumarin isolated from Exostema caribaeum (Rubiaceae) has been found to act as an energy-transfer inhibitor in spinach chloroplasts. ATP synthesis and phosphorylating (coupled) electron flow were inhibited by 89 and 72%, respectively, at a concentration of 400 μM. H+-uptake, basal and uncoupled electron transport were not affected by the coumarin. The light-activated Mg+2-ATPase activity from bound membrane thylakoid chloroplasts was slightly inhibited by the coumarin. Also, the heat-activated Ca+2-ATPase activity of the isolated coupling factor protein was insensitive to this compound. In chloroplasts partially stripped of coupling factor 1 by an EDTA treatment, the coumarin showed a restoration of the proton uptake process. These results suggest that the 4-phenylcoumarin under investigation inhibited phosphorylation in chloroplasts by specifically blocking the transport of protons through a membrane-bound component or a carrier channel (CFO) located in a hydrophobic region at or near the functional binding site for the coupling factor 1.