The conversion of L-phenylalanine into benzoic acid on the thylakoid membrane of higher plants.

The conversion of L-phenylalanine into benzoic acid and other aromatic carboxylic acids was investigated in Nasturtium officinale (watercress), Astilbe chinensis, and Hydrangea macrophylla in vivo and in vitro. Comparative feeding experiments with radioactively labelled L-phenylalanine and cinnamic acid administered to intact leaf discs of A. chinensis indicated a rapid formation of benzoic acid from L-phenylalanine, whereas cinnamic acid was a poor precursor. Using a pulse-chase labelling technique followed by a fractionation of the tissue into subcellular components, chloroplasts could be identified as the predominant, if not exclusive, site of benzoic acid formation in A. chinensis. Experiments in vitro with chloroplasts and thylalkoids of N. officinale, H. macrophylla, and A. chinensis demonstrate the capacity of thylakoid membranes to catalyze the degradation of L-phenylalanine to benzoic acid. The results obtained upon stimultaneous incubation with [4'-3H]L-phenylalanine and [3-14C]cinnamic acid lead to the hypothesis that the reaction of L-phenylalanine to benzoic acid proceeds via a cinnamic acid pool which is different from that of soluble cinnamic acid.