Chemistry Reference
In-Depth Information
are derived from L-tyrosine or its oxidation product L-dihydroxyphenylalanine
(L-DOPA). Mescaline (
N7
) originating from the latter amino acid is known to
occur in several cacti and is responsible for the hallucinogenic activity of pey-
ote (
Lophophora williamsii
, Cactaceae). Lophocerine is a tetrahydroisoquinoline
alkaloid derived from L-dopamine and found to occur in a different
Lophophora
species,
L. schotti
.
Condensation of two phenylethyl units derived independently from the
same or different aromatic amino acid(s) leads to a variety of benzyl-
tetrahydroisoquinolines, which, with additional structural modifications, produce
a diverse range of alkaloids. (
S
)-Reticuline occurring in several plant species
of Annonaceae is an important benzyl-tetrahydroisoquinoline alkaloid that acts
as a precursor to several pharmacologically active alkaloids such as papaverine
(
N8
), (
+
)-tubocurarine (
N9
), and morphine (
N10
). Papaverine and morphine
are known to occur in opium (
Papaver somniferum
, Papaveraceae) and are
responsible for its narcotic activity, whereas (
+
)-tubocurarine (
N9
) is a muscle
relaxant obtained from the arrow poison of the South American Indians,
curare (
Chondrodendron tomentosum
, Menispermaceae). Phenethylisoquinoline
alkaloids are similar structurally to benzylisoquinolines but as the name implies
contain a phenylethyl moiety instead of a benzyl moiety as the pendant aromatic
ring. Both (
S
)-autumnaline and the cyclized analog colchicine (
N11
) belonging
to this class have been found to occur in the seeds of autumn crocus (
Colchicium
autumnale
, Liliaceae).
1.7 MAXIMIZATION OF CHEMICAL DIVERSITY AND PRODUCTION
OF NATURAL PRODUCTS IN PLANTS
As is apparent from the foregoing discussion, plants produce a huge array of nat-
ural products, many of which are specialized secondary metabolites associated
with particular plant species and/or having to play important ecological roles.
It is likely that for diversification and survival of the plant kingdom, individual
plants had to develop the ability to perform
in vivo
combinatorial chemistry by
mixing and matching and evolving the genes required for different secondary
metabolite biosynthetic pathways (34, 35). With the elucidation of several sec-
ondary metabolic pathways in plants together with the advent of techniques for
the introduction of genes into plants and the availability of an increasing number
of genes, it has become possible to modulate and diversify secondary metabolite
production in transgenic plants and plant cell cultures.
Two general approaches for the production of long-chain polyunsaturated fatty
acids usually found in fish oil have been employed, both of which used 18 carbon
fatty acids endogenous to plants as the starting substrates (36). Soybean and
canola, the oilseed plants rich in omega-6 fatty acids, have been engineered to
produce omega-3 polyunsaturated fatty acids such as eicosapentaenoic acid (EPA)
and docosohexaenoic acid (DHA) (37, 38).
