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-rhamnopyranosyl-(1 --> 3)-O-alpha-L-{2-O-(2S-methylbutanoyl)-4-O-(2S-methylbutanol)}-rhamnopyranosil-(1 --> 2)-O-beta-D-glucopyranosil-(1--> 2)-beta-fucopyranoside-(1,3 "-lactone), the major photogrowth inhibitor isolated from Ipomoea tricolor Cav, (Convolvulaceae) was found to be a potent uncoupler (U-50 = 0.33 mu M) of photophosphorylation in spinach chloroplasts, Tricolorin A inhibited H+-uptake and adenosine 5'-triphosphate (ATP) synthesis, and stimulated basal and phosphorylating electron flows. Using a combination of two well-known fluorescent Delta pH probes, 9-aminoacridine and 9-amino-6-chloro-2-methoxyacridine, the uncoupling behavior of tricolorin A was also demonstrated for submitochondrial particles. Polarographic data showed that high concentrations (20 mu M) of tricolorin A inhibited photosystem II (PSII) electron flow at the level of plastoquinone B (Q(B)). Chlorophyll (Chl) a fluorescence analysis showed that tricolorin A induced accumulation of Q(A)(-) and strongly decreased the electron transport capacity, suggesting that the target of this molecule was located at the Q(B) level. The macrocyclic lactone-type structure of this allelopathic agent proved to be an important structural requirement for uncoupling activity since its hydrolysis caused loss of the inhibitory potential.