Similarities between l-Phenylalanine and m-Fluorophenylalanine as Substrates for l-Phenylalanine Ammonia-Lyase

The ability of l -phenylalanine ammonia-lyase (E.C. 4.3.1.5) to metabolize dl -m-, dl -o- and dl -p-fluorophenylalanine in Avena sativa has been examined. Although all three amino acid analogues served as substrates for this enzyme, there was a marked difference in the behavior of the meta isomer from that of the para and ortho species. The Michaelis constant for the meta analogue was similar to that obtained for the natural substrate, l -phenylalanine, but distinct from the kinetic data for the para and ortho isomers. In addition, in vivo analyses demonstrated that both l -phenylalanine and the meta-fluoro-derivative served to protect against chlorogenic acid loss, whereas previous studies have shown that the para and ortho species depressed levels of this phenolic derivative. Finally, treatment of coleoptile apices with either the meta isomer or l -phenylalanine reversed dl -p-fluorophenylalanine stimulated growth and attendant reduction in chlorogenic acid content. These findings provide further clarification of the effects of fluorophenylalanines upon l -phenylalanine ammonia-lyase mediated biosynthesis of low molecular weight phenols in Avena.