Biosynthesis of pseudoisoeugenols in tissue cultures of Pimpinella anisum. Phenylalanine ammonia lyase and cinnamic acid 4-hydroxylase activities

The genus Pimpinella contains pseudoisoeugenols, phenylpropanoids with a rare 2,5-dioxy substitution pattern on the phenyl ring. To study the biosynthesis of these compounds, we set up a leaf-differentiating tissue culture of Pimpinella anisum. These cultures mainly produce epoxy-pseudoisoeugenol-(2-methylbutyrate). To corroborate the biosynthetic pathway of epoxy-pseudoisoeugenol-(2-methylbutyrate) as proposed on the basis of investigations with 13C/14C-labelled precursors, the key steps of the pathway were investigated at an enzyme level. Experiments with cell-free homogenates clearly revealed that L-phenylalanine is converted to (E)-cinnamic acid by phenylalanine ammonia lyase and that (E)-cinnamic acid is converted to p-coumaric acid by cinnamic acid 4-hydroxylase. L-2-aminooxy-3-phenylpropionic acid, an analogue of L-phenylalanine, inhibited the incorporation of L-[3'-13C]phenylalanine into epoxy-pseudoisoeugenol-(2-methylbutyrate). Up to 2% of the precursor DL-[3'-13C]phenyllactate was incorporated into epoxy-pseudoisoeugenol-(2-methylbutyrate). Inhibition experiments with oxalacetic acid clearly showed that cinnamic acid is not formed by dehydration of phenyllactic acid in this leaf-differentiating tissue culture of P. anisum.