4 ALKALOIDS IN CHEMOSYSTEMATICS
Broadly speaking the alkaloids
invariably occur across the entire plant kingdom. In reality, alkaloids are
usually found more abundantly in plants specifically belonging to the Dictoyledones
than in either the Monocotyledones or the non-flowering plants.
However, it has been observed that amongst the Pteridophyta and Gmnospermae,
only the Lycopodiaceae family happens to synthesize these compounds to
any reasonable extent. The Lycopodium alkaloids are essentially the quinolizidine
derivatives; and usually have the ring structure as displayed by Lycopodine
below:
(15R)-15-Methyllycopodan-5-one;
(C16H25NO).
One school of thought believes
that the alkaloids are confined rarely in dicotyledon orders classified before
the Centrospermae, thereby establishing linkage of these simplier flowering
plants with the Gymnospermae. The distribution and occurrence of
alkaloids is found to be quite uneven among the remaining orders. Thus, an
order rich in such compounds could be preceded and followed by an order wherein
alkaloids are not synthesized at all. Such an erratic distribution may be
noticed even more distinctly and apparently in certain plant families also. Therefore,
one may safely conclude that alkaloids by themselves are absolutely spinless in
establishing phylogenetic relationships either between orders or between
families within an order. Of course, there exists some exceptionally few cases,
for instance: Centrospermae which has been duly illustrated below.
Such natural orders that are
found to be rich in alkaloids are: Centrospermae, Gentianales, Magnoliales
and Ranunculaloes belonging to the class of Dicotylendones; and
Liliales, and Orchidales belonging to the category of Monocotylendones.
It has been reported duly tht both Papaveraceae and the absolutely
unrelated Apocynaceae contain the largest number of these secondary
metabolites. Surprisingly, each and every species of Papaveraceae thoroughly
studied till date comprises of alkaloids; whereas, the Apocynaceae contains
a rather more prominent diversity of complex indole alkaloids. There
are a host of other plant families in which alkaloids usually occur more
predominantly. and frequently are, namely: Amaryllidaceae, Compositae,
Leguminosae, Liliaceae, Loganiaceae, Orchidaceae, Ranunculaceae,
Rubiaceae, Rutaceae and Solanaceae. However, the Amaryllidaceae alkaloids
are found to be specific only to that family, that contains no other variety.
A broad survey of literature
has adequately proved the fact that while applying the chemosystematics to the
classification of plants, the biosynthetic pathway is certainly more vital and
important than that of the end-product. It may be further examplified by
considering, quinine, a quinoline derivative obtained in Rubiaceae, is
biosynthesized from tryptophan by a biosynthetic pathway very similar to that
forming the complex indole alkaloidal characteristic features of the family.
Besides, quinoline derivatives are also found in the Rutaceae, but in
this particular instance they are biosynthesized from anthranilic acid; and
also by a pathway that is very specific to this family.
Lastly, one may draw an
inference that the strategic application of alkaloid biosynthesis to classification
is made rather complicated by both convergence and divergence. Some typical
examples of convergence and divergence are given below:
(a) Examples of
Convergence: The synthesis of the tropane alkaloids from hygrine is onaccount of convergence. It normally occurs in a plethora of unrelated plant families, such as: Convolvulaceae (Convolvulus species), Cruciferae (Cochleavia arctica); Erythroxylaceae (Erythroxylum coca); Euphorbiaceae (Phyllanthus discoidens); and Solanaceae (includes several genera).
(b) Examples of
Divergence: Papillionoideae exhibit several examples of divergence;
because, members of this subfamily synthesise virtually a wide spectrum of
secondary metabolites.
The Papillionoideae
alkaloids are found to exhibit no apparent relationship either in molecular
structure or in their respective biosynthetic pathways. Hence, the spiroamine
alkaloids found in Erythrina species have virtually little
structural relationship to the pyrrolizidine alkaloids that are
characteristic of Crotolaria species, and are biosynthesized from
tyrosine; whereas, the pyrolizidine alkaloids are normally found to
originate from ornithine.
Source:Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar
Source:Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar
