3-Hydroxy-3-phenylpropanoic acid is an intermediate in the biosynthesis of benzoic add and salicylic acid but benzaldehyde is not

Stable-isotape-labelled (H-2(6), O-18) 3-hydroxy-3-phenylpropanoic acid, a putative intermediate in the biosynthesis of benzoic acid (BA) and salicylic acid (SA) from cinnamic acid, has been synthesized and administered to cucumber (Cucumis sativus L.) and Nicotiana attenuata (Torrey). Analysis of the products by gas chromatography-mass spectrometry revealed incorporation of labelling into BA and SA, but not into benzaldehyde. In a separate experiment, 3-hydroxy-3-phenylpropanoic acid was found to be a metabolite of phenylalanine, itself the primary metabolic precursor of BA and SA. These data suggest that cinnamic acid chain shortening is probably achieved by beta -oxidation, and that the proposed "non-axidative" pathway of side-chain degradation does not function in the biosynthesis of BA and SA, in cucumber and N. attenuata.