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

Tricolorin A, (11S)-11-hydroxyhexadecanoic acid 11-O-α-L-rhamnopyranosyl-(1 → 3)-O-α-L-{2-O-(2S-methylbutanoyl)4-O-(2S-methylhutanoyl)}-rhamnopyranosil- (1 → 2)-O-β-D-glucopyranosil-(l → 2)-β-fucopyranoside-(1,3'-lactone), the major phytogrowth inhibitor isolated from Ipomoea tricolor Cav. (Convolvulaceae) was found to be a potent uncoupler (U⁵⁰ = 0.33 μM) 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 ΔpH 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 μM) 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.