| dc.description.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 . | en |
