Kinetics of Phenylalanine Ammonia-Lyase and the Effect of L-α-aminooxy-β-Phenylpropionic Acid on Enzyme Activity and Radicle Growth in Germinating Lettuce Seeds

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The effects of different concentrations of L-α-aminooxy-β-phenylpropionic acid (AOPP), an analog of L-phenylalanine, on the activity of phenylalanine ammonia-lyase (PAL, EC 4.3.1.5) and the growth of radicles in 24 h old germinating lettuce (Lactuca sativa L.) seeds were investigated. AOPP causes a significant inhibition of PAL activity in the seeds (85% inhibition at 10^<-4> M). It also causes a stimulation of radicle growth at that concentration. The results show that the inhibition of PAL by AOPP may be due to an irreversible binding of the inhibitor to the enzyme leading to its inactivation. AOPP also inhibits ethylene biosynthesis in germinating lettuce seeds which could probably explain the stimulation of radicle growth in these seeds. The enzyme shows typical Michaelis-Menten kinetics. The K_m for L-phenylalanine is 4.2 × 10<-5> M. The enzyme does not show any tyrosine ammonia-lyase activity. Various substrate analogs such as D-phenylalanine, p-fluorophenylalanine, β-phenyllactic acid, tryptophan and the product of the enzyme reaction, trans-cinnamic acid, inhibit the enzyme competitively. A number of intermediates and endproducts of the phenylpropanoid pathway, except chlorogenic acid, do not show any inhibition.
日本植物生理学会の論文