Thomas Hartmann
Institut für Pharmazeutische Biologie der Technischen UniversitätBraunschweig,
Mendelssohnstrasse 1, D-38106 Braunschweig, Germany
Pyrrolizidine alkaloids (PAs) of the senecionine type comprise morethan 100 structures. Most of these structures can be regarded as derivativesof senecionine N-oxide. Senecio species which display quite differentPA patterns all synthesize senecionine N-oxide as first and common PA.Its biosynthesis takes place in a highly specific and well regulated mannerexclusively in the roots. Senecionine N-oxide is translocated from theroots into the shoots where its selective conversion into species specificalkaloid pattern occurs. These conversions comprise simple reactions includingposition-specific dehydrogenations, hydroxylations, epoxidations and O-acetylations.The generation of species specific PA patterns from senecionine N-oxideis exemplified in studies with different Senecio species, e. g.S. vulgaris, S. vernalis, S. jacobaea (two chemotypes),S. erucifolius, S. inaequidens. It is suggested that variationin the activities of the enzymes responsible for these conversions arecausal to intraspecific variation in PA patterns between populations. Spontaneouslyoccuring changes in the activities of these enzymes, i.e. mutations ofthe underlying genes, provide the mechanistic basis for chemical diversityupon which processes of natural selection may act.