The present-day cycads comprise the diverse, modified,
remnants of a much larger group of gymnosperms which
flourished in the Mezozoic era. The systematic position that
the cycads occupy in the botanical hierarchy is significant
in that they represent relatively unchanged survivors from
prehistory. The present global complement of 182 species
occur in tropical and mild temperate regions in both
hemispheres. Despite the density of proliferation of species,
about one-half of the extant taxa are considered endangered,
vulnerable or rare. Apart from characteristic features such
as differences in growth forms, variation in reproductive
structures and anatomical details, cycads are distinguished
from all other plant groups by the unique phytotoxins,
azoxyglycosides, which they possess.
The toxicity of cycads is well-documented in cases which
refer to both man and animals. Cycasin, which together with
macrozamin represent the major azoxyglycosides occurring in
cycads, has been reported to elicit responses similar to
those that have been observed during carcinogenicity,
mutagenicity and neurotoxicity assays. It has become apparent
that the mechanism by which azoxyglycosides manifest their
toxicity involves deglucosylation, by enzyme systems, which
releases the aglycone, methylazoxymethanol (MAM), and sugar
moieties. Metabolic activation of MAM succeeds
deglucosylation to generate methylene carbene units (: CH2)
which are capable of methylating macromolecules including
DNA, RNA and protein.
During this investigation, macrozamin was extracted from seed
kernels of Encephalartos transvenosus and cycasin was
tentatively identified in seed kernels of Cycas thouarsii .
The hexa-acetate derivative of macrozamin was prepared
whereas the tetra-acetate derivative of cycasin was not
secured in a pure form. The spectroscopic techniques employed
for identification include DV-absorption, infra-red and
nuclear magnetic resonance spectroscopy, all of which are
useful for detecting signals which arise as a result of the
azoxy function.
A kinetic study was carried out to determine the rate of
hydrolysis of macrozamin with 4M sulphuric acid, and to allow
calculation of the activation energy for the process. A
comparison of the kinetic parameters determined for the above
process with those derived for the hydrolysis of cycasin and
methylazoxymethanol under similar conditions followed. The
rates of hydrolysis increase in the order macrozamin,
cycasin, methylazoxymethanol since the molecules contain two,
one and zero glycosidic linkages respectively. Additional
glycosidic bonds are observed to decrease the rate of
reaction. Consequently, activation energies for hydrolysis of
the above molecules decrease in the above-stated order.
Macrozamin and cycasin were quantified in cycad material by
high performance liquid chromatography (HPLC), gas-liquid
chromatography (GLC) and by the chromotropic acid assay. The
results of the quantitative analysis has highlighted certain
limitations of the methods of detection, and has been found
to be applicable to a taxonomic evaluation and a proposal for
the biosynthesis of the azoxyglycosides. / Thesis (M.Sc.)-University of Natal, Durban, 1990.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/5402 |
| Date | January 1990 |
| Creators | Nair, Jerald James. |
| Contributors | Osborne, Roy. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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