p-Hydroxyphenylpyruvate dioxygenase is a herbicidal target site for β-triketones from Leptospermum scoparium

beta-Hydroxyphenylpyruvate dioxygenase (HPPD) is a key enzyme in tyrosine catabolism and is the molecular target site of beta-triketone pharmacophores used to treat hypertyrosinemia in humans. In plants, HPPD is involved in the biosynthesis of prenyl quinones and tocopherols, and is the target site of beta-triketone herbicides. The beta-triketone-rich essential oil of manuka (Leptospermum scoparium), and its components leptospermone, grandifforone and flavesone were tested for their activity in whole-plant bioassays and for their potency against HPPD. The achlorophyllous phenotype of developing plants exposed to manuka oil or its purified beta-triketone components was similar to that of plants exposed to the synthetic HPPD inhibitor sulcotrione. The triketone-rich fraction and leptospermone were approximatively 10 times more active than that of the crude manuka oil, with I-50 values of 1.45, 0.96 and 11.5 mu g mL(-1), respectively. The effect of these samples on carotenoid levels was similar. Unlike their synthetic counterpart, steady-state 02 consumption experiments revealed that the natural triketones were competitive reversible inhibitors of HPPD. Dose-response curves against the enzyme activity of HPPD provided apparent I-50 values 15.0, 4.02, 3.14, 0.22 mu g mL(-1) for manuka oil, triketone-rich fraction, leptospermone and grandifforone, respectively. Flavesone was not active. Structure activity relationships indicate that the size and lipophilicity of the side-chain affected the potency of the compounds. Computational analysis of the catalytic domain of HPPD indicates that a lipophilic domain proximate from the Fe (2+) favors the binding of ligands with lipophilic moieties. (c) 2007 Elsevier Ltd. All rights reserved.