title: Inhibitors of abscisic acid 8'-hydroxylase.
abstract: Structural analogues of the phytohormone (+)-abscisic acid (ABA) have been synthesized and tested as inhibitors of the catabolic enzyme (+)-ABA 8'-hydroxylase. Assays employed microsomes from suspension-cultured corn cells. Four of the analogues [(+)-8'-acetylene-ABA, (+)-9'-propargyl-ABA, (-)-9'-propargyl-ABA, and (+)-9'-allyl-ABA] proved to be suicide substrates of ABA 8'-hydroxylase. For each suicide substrate, inactivation required NADPH, increased with time, and was blocked by addition of the natural substrate, (+)-ABA. The most effective suicide substrate was (+)-9'-propargyl-ABA (K(I) = 0.27 microM). Several analogues were competitive inhibitors of ABA 8'-hydroxylase, of which the most effective was (+)-8'-propargyl-ABA (K(i) = 1.1 microM). Enzymes in the microsomal extracts also hydroxylated (-)-ABA at the 7'-position at a low rate. This activity was not inhibited by the suicide substrates, showing that the 7'-hydroxylation of (-)-ABA was catalyzed by a different enzyme from that which catalyzed 8'-hydroxylation of (+)-ABA. Based on the results described, a simple model for the positioning of substrates in the active site of ABA 8'-hydroxylase is proposed. In a representative physiological assay, inhibition of Arabidopsis thaliana seed germination, (+)-9'-propargyl-ABA and (+)-8'-acetylene-ABA exhibited substantially stronger hormonal activity than (+)-ABA itself.
authors.affiliation: Plant Biotechnology Institute, National Research Council of Canada, 110, Gymnasium Place, Saskatoon, Saskatchewan S7N 0W9, Canada. adrian.cutler@nrc.ca
authors.forename: A J
authors.initials: AJ
authors.lastname: Cutler
authors.forename: P A
authors.initials: PA
authors.lastname: Rose
authors.forename: T M
authors.initials: TM
authors.lastname: Squires
authors.forename: M K
authors.initials: MK
authors.lastname: Loewen
authors.forename: A C
authors.initials: AC
authors.lastname: Shaw
authors.forename: J W
authors.initials: JW
authors.lastname: Quail
authors.forename: J E
authors.initials: JE
authors.lastname: Krochko
authors.forename: S R
authors.initials: SR
authors.lastname: Abrams
mesh_terms: D000040:Abscisic Acid / Q000819:agonists / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism; D001667:Binding, Competitive; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000737:chemistry / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000138:chemical synthesis / Q000737:chemistry* / Q000378:metabolism; D018525:Germination; D006900:Hydroxylation; D007700:Kinetics; D006899:Mixed Function Oxygenases / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000378:metabolism; D010937:Plant Growth Regulators / Q000138:chemical synthesis / Q000737:chemistry / Q000378:metabolism; D010940:Plant Proteins; D012639:Seeds / Q000201:enzymology / Q000502:physiology; D013379:Substrate Specificity
chemical_list: D065607:Cytochrome P-450 Enzyme Inhibitors; D004791:Enzyme Inhibitors; D010937:Plant Growth Regulators; D010940:Plant Proteins; D000040:Abscisic Acid; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; C107203:abscisic acid 8'-hydroxylase