Jg. Romagni et al., The phytotoxic lichen metabolite, usnic acid, is a potent inhibitor of plant p-hydroxyphenylpyruvate dioxygenase, FEBS LETTER, 480(2-3), 2000, pp. 301-305
The lichen secondary metabolite usnic acid exists as a (-) and a (+) enanti
omer, indicating a alpha or beta projection of the methyl group at position
9b, respectively, (-)-Usnic caused a dose-dependent bleaching of the cotyl
edonary tissues associated with a decrease of both chlorophylls and caroten
oids in treated plants whereas no bleaching was observed with the (+) enant
iomer, (-)-Usnic acid inhibited protophorphyrinogen oxidase activity (I-50
= 3 mu M), but did not lead to protoporphyrin IX accumulation. Bleaching ap
pears to be caused by irreversible inhibition of the enzyme 4-hydroxyphenyl
pyruvate dioxygenase by (-)-usnic acid (apparent IC50 = 50 nM), (C) 2000 Fe
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