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'-hydroxylase, Assays employed microsomes From suspension-cultured corn cells. Four of the analogues [(+)-8'-acetylenc-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-1 = 0.27 muM). Several analogues were competitive inhibitors of ABA. 8'-hydroxylase, 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'-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'-proprgyl-ABA and (+)8'-acetylene-ABA exhibited substantially stronger hormonal activity than (+)-ABA itself.