Interference of L-a-Aminooxy-/?-Phenylpropionic Acid with Phenylalanine Metabolism in Buckwheat

Phenylalanine Ammonia-Lyase, L-ar-Aminooxy-/?-phenylpropionic Acid, Aromatic Amino Acids, Shikimic Acid, Fagopyrum esculentum L-a-Aminooxy-/?-phenylpropionic acid (AOPP), a potent competitive inhibitor of phenylalanine ammonia-lyase (PAL), blocked light-induced phenylpropanoid synthesis in excised buckwheat hypocotyls and produced an up to 40-fold increase in the endogenous phenylalanine concentra­ tion, while the level of all other amino acids was hardly affected. After a 24 h incubation in the light in the presence of 0.3 or 1 mM AOPP phenylalanine alone constituted about 25% of the total soluble amino acids, compared to appr. 1% in the controls. In the presence of AOPP illuminated hypocotyls accumulated nearly 3 times more phenylala­ nine than hypocotyls kept in the dark, indicating an enhancing effect of light on the flow of carbon through the shikimate pathway. Exogenously added [14C] phenylalanine was extensively metab­ olized by control tissue, but accumulated in AOPP treated tissue. In the presence of AOPP radio­ activity from [14C] shikimate accumulated predominantly in phenylalanine, and the flow of shi­ kimate into tyrosine and phenylalanine was not affected by the inhibitor. Therefore, under these conditions no feedback control of phenylalanine and tyrosine synthesis from shikimate is apparent in buckwheat hypocotyls.