Submitted to: Federation of European Biochemical Societies Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/2/2000
Publication Date: N/A
Interpretive Summary: A natural product from lichens, usnic acid, was found to cause bleaching in lettuce seedlings. From the chemical structure of this compound, we determined that it was similar to the structure of a synthetic class of herbicides that inhibit the enzyme p-hydroxyphenylpyruvate dioxygenase (HPPD). Using data from the enzyme assays, we found that usnic acid was a more potent inhibitor than the synthetic chemical, sulcotrione. This is the first example of a natural product inhibiting this enzyme.
Technical Abstract: Abstract The lichen secondary metabolite usnic acid exists as an (-) and an (+) enantiomer, indicating a alpha or beta projection of the methyl group at position 9b, respectively. (-)-Usnic caused a dose- dependent bleaching of the cotyledonary tissues and a decrease in the amounts of chlorophylls and carotenoids in treated plants whereas little phytotoxic activity was associated with the (+) enantiomer. (-)-Usnic inhibited protophorphyrinogen oxidase activity (I50 = 3uM), but did not lead to protoporphyrin IX accumulation. Bleaching appears to be caused by irreversible inhibition of the enzyme 4-hydroxyphenylpyruvate dioxygenase by (-)-usnic acid (apparent IC50 =50 nM).