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wide range of plant open-chain trypsin
inhibitors (Felizmenio-Quimio, 2001).
As stated before, the known cyclotides
are aligned according to the six conserved
Cys residues. The loops found in the back-
bone of these cyclic peptides correspond to
the regions located between these residues,
and overlapped in these loops are located
the amino acidic chains responsible for the
biological activities of cyclotides. According
to these data, some of the cyclotides present
in the 50% EtOH extract of
H. parviflorus
might have, according to its structure, a
trypsin inhibitory activity.
Whichever the mechanism of action
may be, these findings clearly demonstrate
that the outstanding stability of the cystine
knot structure makes it an excellent frame-
work on which a wide range of biological
activities can be added, generating potential
applications as insecticides in agriculture.
Furthermore, the ursolic acid and the b-sito-
sterol that have been isolated from many
plant species presented different degrees of
anti-alimentary activity on lepidopterous
insects. These compounds, present in the
CH
2
Cl
2
extract of
H. parviflorus
, could be in
part or totally responsible for the insecti-
cidal activity of these extracts.
bioavailability. The stability of linear pep-
tides can be improved by linking their N
and C termini as long as they are relatively
close. The pharmaceutical industry has
applied cyclization to stabilize the confor-
mation of small linear peptides but not
polypeptides and proteins (Craik, 2001).
The latter drawbacks could be overcome by
employing the cyclotides structure as a scaf-
folding for the development of novel drugs
with different activities (Craik
et al
., 2002;
Barry
et al
., 2004; Clark
et al
., 2006). The
cyclotides would also have interesting appli-
cations in agronomy (Alexander, 2001) because
they are potent insecticides and would also
serve as models for the development of com-
pounds with potential applications in agricul-
ture (Jennings
et al
., 2001, 2005).
Cyclic peptides are naturally occuring
miniproteins in plants, and their introduc-
tion in commercial crops present less tech-
nical difficulties than the transference of
bacterial genes, e.g. to cotton. According
to Göransson and Craik (2003), the gene
transference from one plant displaying
insecticidal activity to another is a meth-
odology worth exploiting. This technology
could be applied to a wide range of crops
including cotton, maize, soybean and, poten-
tially, wheat and rice cultures (Johensen and
Ho, 2004).
The global expenditure in the chemical
control of insect plagues is estimated to be
more than 3 billion US dollars per year. By
generating these products, a substantial part
of the traditional insecticide market would
be redirected towards the biotechnological
industry, as has occured with the first gener-
ation of transgenic plants that currently rep-
resent a market of 350 million US dollars.
13.6
Potential Uses of Cyclotides
As was observed, the cyclotides have a wide
range of biological activities and a high
stability owing to their chemical structure.
The applicability of linear peptides as
drugs is limited because of their suscepti-
bility to proteolytic cleavage and their low
References
Alexander, R., 2001. Choking on a knot: insecticidal properties of cyclotides.
Trends in Biochemical
Sciences
26, 588.
Anders, J.C., 2002. Advances in amino acid analysis.
Bio Pharm,
April 2002, 32-39.
Azcon-Bieto, J. & Talon, M., 1993.
Fisiología y Bioquímica Vegetal. Interamericana.
Mc Graw-Hill,
New York, pp. 237-262.
Bado, S., Mareggiani, G., Amiano, N., Burton, G. & Valerio, A., 2004. Lethal and sublethal effects of
withanolides from
Salpichroa origanifolia
and analogues on
Ceratitis capitata
.
Journal of
Agricultural and Food Chemistry
52, 2875-2878.
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