Picrate as an Inhibitor of Photosystem II in Photosynthetic Electron Transport

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Abstract
2,4,6-Trinitrophenol (picric acid) is an effective inhibitor of photosynthetic electron transport (pI50 value 6.82). By means of artificial donor and acceptor systems its site of inhibition was located at the reducing side of photosystem II [in spinach]. Picric acid uncouples photophosphorylation, but a much higher concentration is required than for inhibition of electron transport (pI50 value 4.40). By use of radioactivity labeled [U-14C]picric acid (spec. act. [specific activity] 2.1 Ci/mol) a binding constant of 0.25 .mu.M and a number of binding sites of 2.84 nmol/mg chlorophyll was measured. This corresponds to 1 molecule of picric per 394 molecules of chlorophyll or 1 molecule per electron transport chain. Picric acid is competitively displaced from the thylakoid membrane by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (and functionally related compounds) and phenolic inhibitors. In this property, picric acid, despite its phenol character, is distinct from other phenolic inhibitors.