Evidence for Two Sites of Inhibition of Photosynthetic Electron Transport by Dibromothymoquinone

Two sites in the photosynthetic electron transport chain of spinach chloroplasts are sensitive to inhibition by the plastoquinone antagonist dibromothymoquinone (2,5-dibromo-3-methyl-&isopropyl-p-benzoquinone). This compound imposes maximal inhibition on re-actions involving electron transport from water to a terminal acceptor such as ferricyanide at concentrations of about 1 PM. At concentrations of about 10 PM, dibromothymoquinone also inhibits electron transport reactions catalyzed by photosystem II in the presence of p-phenylenediimines or p-benzoquinones. This inhibition is observed in both untreated and KCN/Hg-inhibited chloroplast preparations. Thiol incubation of chloroplasts exposed to dibromothymoquinone relieves inhibition at both sites. This reversal of inhibition is, however, different for the two sites. Restoration of ferricyanide reduction, which is blocked by 1 pM dibromothymoquinone, required high thiol/inhibitor ratios and incubation times with thiol of up to 3 min. The reversal of inhibition of p-phenylenediimine reduction by photosystem II, on the other hand, requires a thiol/inhibitor ratio of 1, and incubation times as short as 5 s. Addition of bovine serum albumin to absorb dibromothymoquinone results in a partial restoration of photosystem II reactions, but ferricyanide reduction, which requires photosystem II and photosystem I, cannot be restored by this procedure. Dibromothymoquinone (DBMIB), ” a well established inhibitor for use in studies of photosynthetic electron transport, has been shown to inhibit the flow of electrons between photosystems (PS) II and I, pre-sumably by interference with plastoqui-none function (1). Noncyclic electron trans-port from water to ferricyanide is inhibited more than 60 % by DBMIB concentrations of approximately 1 PM. Photosystem II-de-pendent noncyclic electron transport can be restored in DBMIB-poisoned chloro-I To whom correspondence should be addressed