A study of the ecdysone content of some selected East African plants

Abstract

Ecdysone is a steroid hormone involved in the induction of moulting and metamorphosis in Arthropods. The initial discovery of the two zooecdysones , and B- ecdysones,was followed in 1966 by the discovery of these and other moulting-hormone like substances - the phytoecdysones - in plants. A total of about 30 phytoecdysones have since been isolated and chemically characterised and this number seems to be increasing rapidly. A lot of research has recently been centred around these complex compounds not only for academic reasons but by virtue of their possible application as "third generation insecticides". A brief study of some East African plants for mOQlting hormone activity has show'llthat Achyranthes aspera, Podocarpus gracilior and Cyathula polycephala are very active in assays using Sarcophaga Z strain. Bioassays were also carried out on Musca domestica, Antestiopsis orbitalis bechuana and Dermestes Beetle,but Sarcophaga Z strain were preferred for the bioassays because these insects were easy to rear and an unlimited numbers of carefully timed larvae could be obtained very easily. Using Schooley's Method, Ponasterone A with molecular formula C27H4406 (molecular wt. 46.4) and 8-ecdysone with molecular formula C27H4407 (molecular wt. 480) were isolated from Podocarpus gracilior leaves. The water-soluble, 8-ecdysone was also isolated from Achyranthes aspera. While both Ponasterone A and S-ecdysone are C27 compounds, Unknown I and Unknown II isolated from Cyathula polycephala appear to be C26 polyhydroxy steroids. The highly active Unknown I has M peak in the MS (direct inlet method) at 498 and this corresponds to a molecular formula C26R4409. From the mass spectrum of Unknown I the partial structure shown below has been determined: OR R 3 Rydroxyls RO OR HO If o The mass spectrum of Unknown II shows a peak at m/e 487. This may not necessarily be the molecular peak because many polyhydroxy steroids lose a molecule of water and this peak is recordetlabove mle 400 -instead of the molecular peak. This is then followed by the successive losses of water and methyl radicals. The mass spectra of Unknown II at 30 ev but 2 different temperatures is slightly different. Unknown II could be a mixture although it gives only one peak when run through LC and one spot when run on a TLC plate. The major peaks between m\e 200 - 350 at 180^0C indicate Unknown II to be steroidal and very much like many other ecdysones. Besides the moulting hormone activity some plants have been found to show antijuvenile activity. One such plant is Azadirachta indica (commonly known as the neem or nim tree) collected from the Kenya coast. Partially purified 70% aqueous methanol fraction of neem leaves from the Xad-2 beads has shown strong antijuvenile activity in tests carried out in Hilbert Roller's lab. The complete isolation and structure determination of this natural "antijuvenile hormone" will be carried out in the near future but we think it could be Azadirachtin, C35H440l6. Azadirachtin has been found to cause growth retardation and deformities in Plutella xylostella, Pieris brassicae. Spodoptera frugiperda and in Heliothis zea. A high dose of Azadirachtin causes death in Dysdercus fasciatus without further ecdysis (89). We also found that neem leaves as well as the neem berries cause deformed growth in Antestiopsis crbitalis bechuana. Azadirachta indica is thus the first plant in nature which shows antijuvenile activity .