Inhibitors of abscisic acid 8 '-hydroxylase

Structural analogues of the phytohormone (+)-abscisic acid (ABA) have been synthesized and tested as inhibitors of the catabolic enzyme (+)-ABA 8'-hyd roxylase, Assays employed microsomes From suspension-cultured corn cells. F our of the analogues [(+)-8'-acetylenc-ABA, (+)-9'-propargyl-ABA, (-)-9'-pr opargyl-ABA, and (+)-9'-allyl-ABA] proved to be suicide substrates of ABA 8 '-hydroxylase. For each suicide substrate, inactivation required NADPH, inc reased with time, and was blocked by addition of the natural substrate, (+) -ABA. The most effective suicide substrate was (+)-9'propargyl-ABA (K-1 = 0 .27 muM). Several analogues were competitive inhibitors of ABA. 8'-hydroxyl ase, of which the most effective was (+)-8'-propargyl-ABA (K-i = 1.1 muM). Enzymes in the microsomal extracts also hydroxylated (-)-ABA at the 7'-posi tion at a low rate. This activity was not inhibited by the suicide substrat es, showing that the 7'-hydroxylation of (-)-ABA was catalyzed by a differe nt enzyme from that which catalyzed 8'-hydroxylation of (+)-ABA. Based on t he results described, a simple model for the positioning of substrates in t he active site of ABA 8'-hydroxylase is proposed. In a representative physi ological assay, inhibition of Arabidopsis thaliana seed germination, (+)-9' -proprgyl-ABA and (+)8'-acetylene-ABA exhibited substantially stronger horm onal activity than (+)-ABA itself.