In vivo and in vitro experiments were performed in order to study the regulatory role of trans- cinnamic acid and its hydroxylated derivatives (p-coumaric acid, caffeic acid) on the deamination of phenylalanine catalyzed by PAL (EC 4.3.1.5). Trans-cinnamic acid inhibits growth and reduces the content of soluble proteins of anthocyanin-containing carrot cells grown in suspension. There is strong evidence from the polysomal patterns and from the effect of trans-cinnamic acid on protein synthesis in vitro that protein synthesis is inhibited. The kinetic data of PAL clearly demonstrate that trans-cinnamic acid inhibits the enzyme by a noncompetitive mechanism. On the contrary, ʟ-α-aminooxy-β-phenylpropionic acid (ʟ-AOPP), a competitive inhibitor of PAL, does not affect protein metabolism.
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