Antioxidant and Biocidal Activities from Natural Sources: an Overview - Natural Antioxidants and Biocides from Wild Medicinal Plants - page 4

to the corresponding o -quinone (diphenolase
activity). Tyrosinase is responsible for brown-
ing in plants and is considered to be deleteri-
ous to the colour quality of plant-derived
foods and beverages. Tyrosinase also is one
of the key enzymes in insect metamorphosis
and is involved in sclerotization and moult-
ing regulation processes (Andersen, 1990).
Acetylcholinesterase (AChE), an
enzyme contained in nerve tissues, plays a
crucial role in the transmission of nerve
impulses. Free acetylcholine, the inactive
form, is bound to proteins and accumulates
at nerve endings in vesicles. As acetylcho-
line is consumed it is constantly replen-
ished by acetylation of choline. All these
processes occur when an impulse is trans-
mitted through a cholinergic synapse. Thus,
the process of synaptic transmission is an
involved biochemical cycle of acetylcho-
line exchange. AChE has a key role in this
cycle because inhibition of activity leads
to the accumulation of free acetylcholine
in the synaptic cleft, disrupting nerve
impulses. This is followed by convulsive
activity of the muscles that can be trans-
formed into paralysis; other features of self-
poisoning by surplus acetylcholine then
also appear. Some terpenoids are known to
inhibit AChE (Ryan and Byrne, 1988; Keane
and Ryan, 1999; Miyazawa et al ., 2000).
Resistance to some insecticides is known to
arise by modifications of AChE in insects
(Fournier et al ., 1994).
In addition to many flavonoids, stilbe-
noids, phenylpropanoids and phenolics
possessing tyrosinase inhibitory activity,
many of the same compounds also show
strong antioxidant activity in a series of
in vitro antioxidant assays such as DPPH,
ABTS, Trolox, TRAP, ORAC and FRAP. The
activity is principally due to the presence of
diverse moieties in the chemical structure
of the molecules, for instance orcinol or cat-
echol groups, or a hydroxyl group bonded
to an aromatic system (gallic acid and gal-
lates in general, resveratrol and other stil-
benes, phenylpropanoids, flavonoids, such
as quercetin, and other phenolic acids).
In these cases, it is possible to correlate anti-
oxidant activity with tyrosinase and AChE
inhibition, and IGR activity (Grundy and

Still, 1985; Baldwin et al ., 2001; Kessler and
Baldwin, 2002; Schultz, 2002; Kubo et al .,
2003a,b; Torres et al ., 2003; Guerrero and
Rosell, 2004, 2005).
In addition, many polyphenolic second-
ary compounds are ubiquitous in angiosperms
and have antifeedant effects on phytopha-
gous insects (Feeny, 1976; Rhoades and
Cates, 1976; Champagne et al ., 1989, 1992;
Simmonds, 2003). It has been assumed that
phenols bind to proteins, acting as nutri-
tional protein precipitating agents, thus
reducing their digestibility (Feeny, 1976;
Rhoades, 1979; Martin and Martin, 1982,
1983; Martin et al ., 1987; Ortego et al ., 1999).
Recent studies have demonstrated that
many plant species produce and accumu-
late a large variety of secondary metabo-
lites that provide defence against insect
predators (Berenbaum, 1989; Guella et al .,
1996; Marvier, 1996; Berenbaum, 2002).
One of the best known efforts has focused
on limonoids from the family Meliaceae
owing to their potent effects on insect
pests and their low toxicity to non-target
organisms (Koul and Isman, 1992; Kumar
and Parmar, 1996; Singh et al ., 1997).
Some examples are Azadirachta indica
(Meliaceae) and Derris elliptica (Fabaceae)
that produce the well-known insecticide
azadirachtin and other types of natural
compounds such as rotenone, respectively
(Gomes et al ., 1981; Kraus, 1993, 1995).
The main characteristics that account for
the successful use of these secondary
metabolites as natural insecticides are
mentioned above. These properties make
them less harmful to the environment than
many synthetic insecticides (Camps, 1988;
Berenbaum, 1989; Castillo et al ., 1998).
Although members of the family
Meliaceae are distributed worldwide, only
Melia, Toona, Cedrela and Swietenia species
have been studied in detail (Arnason et al .,
1987; Champagne et al ., 1992; Arnason et al .,
1993; Kraus et al ., 1993; Govindachari
et al ., 1995; Chan and Taylor, 1996; Céspedes
et al ., 2000). These plants have afforded a
number of limonoids such as azadirachtin,
gedunin, toosendanin, cedrelanolide, mexi-
canolide, odoratol, anthothecol, nomilin,
bussein and entandrophragmin. Azadirachtin