Biosynthesis of Erythrina alkaloids in Erythrina crista-galli

A precursor application system was developed to allow the study of Erythrina alkaloid formation in Erythrina crista-galli. Fruit wall tissue of this species was recognized as the major site of alkaloid biosynthesis. The application of radioactively and 13C-labelled potential precursors showed that the hitherto assumed precursor ( S)-norprotosinomenine was not incorporated into the Erythrina alkaloids. In contrast, ( S)-coclaurine as well as ( S)-norreticuline were metabolized to erythraline and erythrinine, respectively, suggesting that a coclaurine-norreticuline pathway is operative in Erythrina alkaloid formation. Feeding of [1- 13C]-labelled ( S)-norreticuline with subsequent NMR spectroscopy demonstrated that the resulting erythraline was exclusively labelled at position C-10. Therefore, the participation of a symmetrical intermediate of the diphenoquinone type in Erythrina alkaloid biosynthesis can be excluded.