Biosynthesis of gallic and ellagic acids with14C-labeled compounds inAcer andRhus leaves

The biosynthetic pathway for gallic and ellagic acids in young, mature and autumn leaves ofAcer buergerianum andRhus succedanea was examined by tracer experiments, and also by isotope competition, withd-shikimic acid-¹⁴C,l-phenylalanine-U-¹⁴C,l-phenyllactic acid-U-¹⁴C, gallic acid-G-¹⁴C and their unlabeled compounds. In young leaves of both plants, the incorporation rate of labeled shikimic acid into gallic acid was significantly higher than that of labeled phenylalanine, whereas in the mature and autumn leaves the latter was a good precursor rather than the former for the gallic acid biosynthesis. Therefore, two pathways for gallic acid formation, through β-oxidation of phenylpropanoid and through dehydrogenation of shikimic acid, could be operating inAcer andRhus leaves, and the preferential pathway is altered by leaf age. In both plants, the incorporation rate of labeled phenyllactic acid during a 24 hr metabolic period was almost the same as that of labeled phenylalanine. The incorporation ofd-skikimic acid-G-¹⁴C,l-phenylalanine-U-¹⁴C andl-phenyllactic acid-U-¹⁴C into ellagic acid was very similar to the case of the radioactive gallic acid formation. Furthermore, regardless of the presence of unlabeled shikimic acid and/or phenylalanine, incorporation of the radioactivity of labeled gallic acid into ellagic acid occurred at a very high rate, suggesting the reciprocal radical reaction of gallic acid for the ellagic acid formation. The incorporation of labeled compounds into ellagitanins was also examined and their biosynthesis discussed further.