Impairment of photosystem II donor side by the natural product odoratol

The effect of odoratol, a natural protolimonoid, isolated from Cedrela odor ata (Meliaceae) and two of its derivatives on different photosynthetic reac tions of isolated spinach chloroplasts was investigated. This natural produ ct is an inhibitor (150 mu M) of oxygen evolution. Polarographic analyses o f the photosynthetic partial redox reactions indicate that (a) photosystem I (PSI) activity is unaffected by odoratol, (b) there is a substantial inhi bition of the electron flow of uncoupled photosystem II (PSII) as measured from water to silicomolybdate or diaminodurene, and (c)the electron flow fr om diphenyl carbazide to dichlorophenolindophenol of Tris-washed chloroplas ts was insensitive. Collectively, these data suggest that the site of actio n of odoratol is located at the donor side of PSII. Comparison of chlorophy ll a fluorescence induction curves of chloroplasts with authenticated donor side damage and those obtained from odoratol-treated samples further suppo rts this interpretation. Comparative analyses using odoratol derivatives in dicate that the diol moiety at positions 23 and 24 of the side chain is an important structural requirement for the inhibitory activity displayed by o doratol.