Tricolorin A, a potent natural uncoupler and inhibitor of photosystem II acceptor side of spinach chloroplasts

Tricolorin A, (11S)-11-hydroxyhexadecanoic acid 11-O-alpha-L-rhamnopyranosy l-(1 --> 3)-O-alpha-L-{2-O-(2S-methylbutanoyl)-4-O-(2S-methylbutanol)}-rham nopyranosil-(1 --> 2)-O-beta-D-glucopyranosil-(1--> 2)-beta-fucopyranoside- (1,3 "-lactone), the major photogrowth inhibitor isolated from Ipomoea tric olor Cav, (Convolvulaceae) was found to be a potent uncoupler (U-50 = 0.33 mu M) of photophosphorylation in spinach chloroplasts, Tricolorin A inhibit ed H+-uptake and adenosine 5'-triphosphate (ATP) synthesis, and stimulated basal and phosphorylating electron flows. Using a combination of two well-k nown fluorescent Delta pH probes, 9-aminoacridine and 9-amino-6-chloro-2-me thoxyacridine, the uncoupling behavior of tricolorin A was also demonstrate d for submitochondrial particles. Polarographic data showed that high conce ntrations (20 mu M) of tricolorin A inhibited photosystem II (PSII) electro n flow at the level of plastoquinone B (Q(B)). Chlorophyll (Chl) a fluoresc ence analysis showed that tricolorin A induced accumulation of Q(A)(-) and strongly decreased the electron transport capacity, suggesting that the tar get of this molecule was located at the Q(B) level. The macrocyclic lactone -type structure of this allelopathic agent proved to be an important struct ural requirement for uncoupling activity since its hydrolysis caused loss o f the inhibitory potential.