Thirty-five derivatives of cinnamic acid and related compounds were tested for inhibition against phenylalanine ammonia-lyase (PAL) derived from sweet potato, pea and yeast. Caffeic and gallic acids showed inhibition against PAL originating from higher plants, but not against yeast PAL. In contrast, yeast PAL was specifically inhibited by p-hydroxycinnamic and p-hydroxybenzoic acids. The results suggest that caffeic and gallic acids may act as regulatory substances in phenylpropanoid metabolism in higher plants. Inhibition experiments with synthetic cinnamic acid derivatives have revealed that the presence of a hydrophobic aromatic ring, α,β-double bond and carboxyl group is essential for inhibitory activity. 2-Naphthoic acid which fulfills these structural requirements showed a strong inhibition. The size and shape of the active site is discussed from structure-activity relationships of cinnamic acid derivatives. o-Chlorocinnamic acid, one of the strongest inhibitors found in this study showed an inhibitory effect on the growth of the roots of rice seedlings.
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