Biosynthesis of gallic acid in Rhus typhina:: discrimination between alternative pathways from natural oxygen isotope abundance

The biosynthetic pathway of gallic acid in leaves of Rhus typhina is studied by oxygen isotope ratio mass spectrometry at natural oxygen isotope abundance. The observed delta(18)O-values of gallic acid indicate an O-18-enrichment of the phenolic oxygen atoms of more than 30parts per thousand above that of the leaf water. This enrichment implies biogenetical equivalence with oxygen atoms of carbohydrates but not with oxygen atoms introduced by monooxygenase activation of molecular oxygen. It can be concluded that all phenolic oxygen atoms of gallic acid are retained from the carbohydrate-derived precursor 5-dehydroshikimate. This supports that gallic acid is synthesized entirely or predominantly by dehydrogenation of 5-dehydroshikimate. (C) 2004 Elsevier Ltd. All rights reserved.