Pyrrolizidine alkaloids of the lycopsamine type: biosynthesis of trachelanthic acid

Abstract

In order to investigate the biosynthesis of the carbon skeleton of trachelanthic acid a root culture of Eupatorium clematideum (Asteraceae) synthesising pyrrolizidine alkaloids (PAs) of the lycopsamine type was supplied with a mixture of [U- 13C 6]glucose and unlabelled glucose at a ratio of 1:20 (wt/wt). Trachelanthamine was isolated and was analysed by one- and two dimensional NMR spectroscopy. The 13C labelling pattern was interpreted by a retrobiosynthetic approach based on comparison with the labelling patterns of amino acids which were isolated from plant cell protein. The 13C labelling pattern of the necic acid carbon atoms 1′, 2′, 5′, 6′, and 7′ was identical with that of valine, thus indicating that trachelanthic acid and valine are biosynthesised in the same plant cell compartment via 2-oxoisovaleric acid as a plausible common intermediate. The isotopomer composition of the two carbon moiety comprising C3′/C4′ of the necic acid was identical to that of C5′/C6′ in trachelanthamine and to that of C3/C4 in valine, thus suggesting hydroxyethyl-TPP (activated acetaldehyde) as a common precursor. It follows that trachelanthic acid is biosynthesised by addition of a two carbon moiety from hydroxyethyl-TPP to 2-oxoisovaleric acid followed by a reduction step.