Desire DALOZE-1, Jean-Claude BRAEKMAN-1, Sabine
LECLERCQ-1, Christine DEVIJVER-1, Jacques M. PASTEELS-2
1-Laboratory of Bio-organic Chemistry (CP 16Q/07) and
2-Laboratory of Cellular and Animal Biology (CP 160/12), Free
University of Brussels, Faculty of Sciences, Av. F. D. Roosevelt,
50, B-1050 Brussels - Belgium.
It is now well recognized that some groups of insects autogenously produce alkaloids, which are mostly used for defence and communication purposes. Considering the number and variety of alkaloids already isolated from insects (particularly ants and ladybird beetles3, it is intriguing that nearly nothing is known concerning their biosynthesis. We have initiate a research programme to fill this gap and will present the first results of our investigations.
Due to the usually low incorporation ratios in insects, it is necessary to feed 14C-labelled precursors. The labelled alkaloids are then isolated, diluted with "cold" material and submitted to a degradation scheme to determine the distribution of radioactivity in the molecule.
This methodology has been applied to several structural types of insect alkaloids, such as the 2-methyl-6-alkylpiperidines (e. g. solenopsin A, 1) from the fire ants (Solenopsis spp.)-1, the 5-alkyldecahydropyrido[1,2-c]pyrrolo[1',2'-a]pyrimidines (e. g. tetraponerine 8, 2)-2 and the 5-alkyldecahydrodipyrrolo[1,2-a:1',2'-c]pyrimidines (e. g. tetraponerine 6, 3] from Tetraponera sp. ants, and the 2-methylperhydro-9b-azaphenalenes (e. g. coccinelllne, 4) from ladybird beetles (Coccinellini)-3.
The results obtained up to now indicate that the solenopsins (1) and the 2-methylperhydro-9b-azaphenalene alkaloids (4) have a polyacetate origin. In contrast, both tetraponerine 8 (2) and tetraponerine 6 (3) have a mixed biogenetic origin. In 2, the pyrrolidine ring derives from L-glutamic acid, via L-ornithine and putrescine, whereas the remaining twelve carbon atoms chain comes from the combination of six acetate units. Surprisingly, in 3, preliminary experiments suggest that the two pyrrolidine rings derive from putrescine and the seven remaining carbon atoms from a polyacetate precursor.
The present results, together with those of Attygalle et al.4 on the biosynthesis of the azamacrocyclic alkaloid epilachnene from the ladybird Epilachna varivestis point to the prominence of the polyketide / fatty acid pathway in insect alkaloid biosynthesis. However, the biosynthetic origin of many more insect alkaloids should be established before this assumption could be accepted as generally valid. Literature