Variability of phenylpropanoid precursors in the biosynthesis of phenylphenalenones in Anigozanthos preissii
Introduction
Phenylphenalenones are characteristic constituents of the Haemodoraceae plant family (Cooke & Edwards, 1980). They accumulate in roots of Anigozanthos preissii and have also been found in root cultures of that species (Hölscher & Schneider, 1997). Recent biosynthetic studies demonstrated the symmetrical incorporation of two molecules of cinnamic acid and p-coumaric acid into anigorufone (1) (Hölscher & Schneider, 1995a) and hydroxyanigorufone (2) (Hölscher & Schneider, 1995b), respectively. The involvement of diarylheptanoids in the phenylphenalenone biosynthesis was shown by feeding a labelled precursor of that type, 1-phenyl-7-(3,4-dihydroxyphenyl)-hepta-1,3-dien-5-one, carrying a catechol moiety (Hölscher & Schneider, 1995a). It has been suggested that the catechol portion undergoes oxidation to an o-quinone, generating an excellent dienophile capable of involvement in a proposed Diels–Alder cycloaddition, resulting in the phenylphenalenone ring system (Bazan et al., 1978, Schmitt & Schneider, 1999). However, details of the biosynthetic pathway are still hypothetical. Until now, for example, it was uncertain whether the second hydroxyl group of the catechol moiety in the diarylheptanoid intermediate, originated from caffeic acid or, alternatively, whether it was introduced by hydroxylation at a later stage of the biosynthesis. In this paper, we describe in vivo feeding experiments using labelled phenylpropanoids, having different substitution patterns in the phenyl ring, and how these precursors are biosynthetically incorporated into phenylphenalenones.
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Results
Previous feeding experiments using labelled phenylalanine have established the ready incorporation into phenylphenalenones in Haemodorum corymbosum (Thomas, 1971) and Lachnanthes tinctoria (Edwards, Schmitt & Weiss, 1972). Specific incorporation of label from [1-]phenylalanine into C-7 but not C-6 of lachnanthoside aglycone was observed in L. tinctoria (Harmon, Edwards & Highet, 1977). In our biosynthetic studies, two molecules of [1-]phenylalanine were shown to be specifically
Discussion
The experiments described here clearly demonstrate that in Anigozanthos preissii two phenylpropanoids are being utilized to form the phenylphenalenone skeleton. Equal distribution of labelling from [1-]phenylalanine into C-6 and C-7 of the phenylphenalenones provided evidence for symmetric incorporation of early precursors into both halves of the molecule. Symmetric incorporation of cinnamic acid and p-coumaric acid into anigorufone (1) and hydroxyanigorufone (2), representing the major
Plant material, synthesis and administration of labelled compounds
Root cultures of Anigozanthos preissii (L.) were initiated as previously described (Hölscher & Schneider, 1997) and grown in liquid MS medium (Murashige & Skoog, 1962) (100 ml in 300 ml conical flasks) at 22°C on a gyratory shaker (100 rpm) under permanent light (4.4 μmol m−2 s−1). Three days before administration of the precursors the cultured roots (about 15 g) were transferred to fresh medium. [1-]Phenylalanine (99% ) was from Deutero GmbH, Kastellaun, Germany. [2-]Phenylpropanoic
Acknowledgements
The authors wish to thank Dr. N. Oldham, Jena, for recording mass spectra and for improving the English of the manuscript. The Deutsche Forschungsgemeinschaft (Bonn) is gratefully acknowledged for providing the NMR spectrometer, and for other financial support. This investigation was supported by the Fonds der Chemischen Industrie (Frankfurt/Main).
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Present address: Institute of Biological Chemistry, Washington State University, Pullman, WA 99164-6340.