The biosynthesis and production of hypoxoside in Hypoxis hemerocallidea Fisch. and Mey. in vivo and in vitro.
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Date
1989
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Abstract
Hypoxoside, a phenolic diglucoside, with a diarylpentane-type
structure, is thought to be the medicinally active constituent of corm
extracts of Hypoxis hemerocallidea Fisch. & Mey. which are reputed to
alleviate the symptoms of prostate hypertrophy and urinary infections.
The biosynthes is and production of this unique phytochemical were
investigated in H. hemerocallidea using both in vivo and in vitro
systems.
It was found, in root-producing callus, that [l4]C-phenylalanine and
[14]C-t-cinnamic acid were efficient precursors for hypoxoside in
comparison to [14]C-sodium acetate and [14]C-acetyl coenzyme-A, which were
not incorporated into the phenolic compound. Thus, at least one aryl
moiety of hypoxoside was derived, via phenylalanine and t-cinnamic
acid, from the shikimate pathway. The acetate pathway did not appear
to be involved in the biosynthetic process. The data supports the
hypothesis that the molecule is formed from two cinnamate units with
the loss of a carbon atom, in opposition to the proposal that the
molecule is derived from head-to-tail condensation of acetate units
onto a propenylic moiety.
Despite the structural similarities between hypoxoside and caffeic and
p-coumaric acids, these two hydroxycinnamic acids were not efficient
precursors for hypoxoside in vivo or in vitro. A number of reasons
are put forward to explain this finding.
It was found that the greatest concentration of hypoxoside was located
in the corms of intact plants. The major biosynthetic site of the
molecule was also found to be located in this organ. Since the roots did accumulate the phytochemical to a small extent, the biosynthetic
potential of these organs has not been disregarded. That of the
leaves has been, however.
The report by PAGE (1984) that the upper region of the corm contained
a greater con cent ration of hypoxoside than the lower portion, is
substantiated in this study, where this region was found to be more
biosynthetically active than the lower half. Light microscopic and
electron microscopic studies revealed that starch storing cells, which
accumulated phenolics in their vacuoles, contained seemingly
synthetically active tubular endoplasmic reticulum in their cytoplasm.
A greater number of these cells were concentrated in the upper region
as opposed to the lower half of the corm. It is postulated that these
cells are the site for biosynthesis and accumulation of hypoxoside.
The shikimate pathway, from which the precursors for hypoxoside are
derived, was found, through the exposure of intact plants to
[14]C-carbon dioxide, to be located mainly in the leaves. It is
postulated from the above study and one in which [14]C-phenylalanine,
[14]C-t-cinnamic acid, [14]C-p-coumaric acid and [14]C-caffeic acid were
applied to intact plants, that phenylalanine and/or cinnamic acid are
the transported form of the shi kimate derivatives. p-Coumaric and
caffeic acids, which are metabolically more stable, are envisaged to
be the sequestering forms.
The investigation of the seasonal production of hypoxoside revealed
that most of the synthesis and accumulation occurred after the corms
had broken winter dormancy and after the flush of leaf growth had slowed down. During dormancy the production of hypoxoside appeared to
cease.
The in vjtro studies, where the effects of light, temperature,
nutrients, plant growth regulators and supply of potential precursors,
on hypoxoside production by root-producing callus were investigated,
indicate that this metabolite is not simply a "shunt" metabolite. A
number of factors other than precursor availability enhanced, or
reduced the jn vjtro production of this phytochemical. Furthermore,
production of the phytochemical and growth were not always
antagonistic.
Hypoxoside, the biosynthesis of which requires a more thorough
investigation, is, however, according to this investigation, a typical
secondary metabolite in many respects.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1989.
Keywords
Biosynthesis., Plant tissue culture., Hypoxis hemerocallidea., Metabolism, Secondary., Hypoxoside., Theses--Botany.