INHIBITION OF PHOTOPHOSPHORYLATION BY PHLORIZIN AND CLOSELY RELATED COMPOUNDS

In isolated chloroplasts, adenosine triphosphate formation and the concomitant stimulation of electron transport are reversibly inhibited by phlorizin (4,4′,6′-trihydroxy-2′-glucosidodihydrochalcone). Phlorizin does not inhibit that part of the electron transport which normally continues in the absence of phosphorylation nor does phlorizin inhibit the rapid nonphosphorylating electron transport induced by uncouplers such as methylamine, atebrin, and carbonyl cyanide 4-trifluoromethoxyphenylhydrazone. Chloroplasts uncoupled by treatment with ethylenediaminetetraacetic acid are also insensitive to phlorizin. Chloroplasts uncoupled by arsenate and adenosine diphosphate are sensitive to the inhibitor. Similar concentrations of phlorizin inhibit the light-triggered adenosine triphosphatase of chloroplasts. Phlorizin inhibition is weaker when the phosphorylation process is limited by low concentrations of adenosine diphosphate or low light intensity but much stronger when phosphorylation is limited by low phosphate concentrations. It therefore seems probably that phlorizin interferes with a reaction close to phosphate utilization. However, the interactions of phlorizin and phosphate are only semicompetitive. 4,6′-Dihydroxy-2′-glucosidodihydrochalcone (II), which has one less hydroxyl but is otherwise structurally identical with phlorizin, was prepared and found to be equally specific and about ten times more potent than phlorizin. A number of compounds with structural relationships to II were prepared and tested. These resorcyl analogs of II were much easier to synthesize than the corresponding phloroglucyl analogs of phlorizin and shared with II the increased potency. The position of the sugar on the “B” ring proved to be critical for specificity, since 2′,4-dihydroxy-4′-glucosidodihydrochalcone (with the glucoside in a position para to the carbonyl rather than ortho as in II) inhibits electron transport whether the electron transport is associated with phosphorylation or not. In contrast considerable modification of the “A” ring is possible without affecting the specificity of inhibition. The hydroxyl group can be moved from the 4 position to the 2 position, methylated or removed entirely with no apparent change in the specificity and little change in activity relative to that shown by II. When the “A” ring is absent, however, as it is in the corresponding acetophenone glucosides, the compounds have much reduced activity. If the double bonds of the two ring systems of the molecule are conjugated, as in the chale ones, the molecule shows little if any inhibitory action. There seems to be no requirement for a specific sugar, since the galactoside corresponding to the glucoside, II, is equally active. However the presence of some sugar at the 2′ position is required to confer specificity since the aglycones of phlorizin and its resorcyl analogs are equally inhibitory to phosphorylating and nonphosphorylating electron transport. © 1969, American Chemical Society. All rights reserved.