Impairment of Photosystem II Donor Side by the Natural Product Odoratol

The effect of odoratol, a natural protolimonoid, isolated from Cedrela odorata (Meliaceae) and two of its derivatives on different photosynthetic reactions of isolated spinach chloroplasts was investigated. This natural product is an inhibitor (150 μM) of oxygen evolution. Polarographic analyses of the photosynthetic partial redox reactions indicate that (a) photosystem I (PSI) activity is unaffected by odoratol, (b) there is a substantial inhibition of the electron flow of uncoupled photosystem II (PSII) as measured from water to silicomolybdate or diaminodurene, and (c) the electron flow from diphenyl carbazide to dichlorophenolindophenol of Tris-washed chloroplasts was insensitive. Collectively, these data suggest that the site of action of odoratol is located at the donor side of PSII. Comparison of chlorophyll a fluorescence induction curves of chloroplasts with authenticated donor side damage and those obtained from odoratol-treated samples further supports this interpretation. Comparative analyses using odoratol derivatives indicate that the diol moiety at positions 23 and 24 of the side chain is an important structural requirement for the inhibitory activity displayed by odoratol.