thiolate anion (GS-), which then attacks the electrophilic center of lipophilic compounds (allelochemicals) to form GS-conjugates. This reaction leads to neutralization of reactive sites of these allelochemicals, making them water soluble and nontoxic, which are then excreted from the body [5]. According to where they are
located, insect GSTs are either microsomal, mitochondrial, or cytosolic [106]. These
enzymes have broad substrate specificity and are thus important in resistance against
a wide range of xenobiotics. Studies have provided evidence that GSTs take part in
detoxification of glucosinolates in Scaptomyza flava and Trichop lusiani [66].
4.2.4 UDP Glucosyl Transferases (UGTs)
UDP glucosyl-transferase belongs to the class of Phase II detoxification enzymes
functioning in detoxification, olfaction, endobiotic modulation, and sequestration.
These enzymes catalyze the transfer of sugar moieties to a wide range of lipophilic
plant secondary metabolites [107]. UGTs have wide substrate specificity and act on
terpenoids, coumarins, phenols, and flavonoids [108]. UGT conjugates glycoside
group to chemicals and facilitates excretion by making them more hydrophilic and
minimizing their detrimental effects [58].
UGTs are present in different body parts and are involved in various processes [109]. UGTs which are expressed in the olfactory mucosa are responsible
for olfactory processing and detoxification. In an example study from S. littoralis,
where males were exposed to pheromones and plant odorants, UGTs were downregulated. Whereas, the introduction of insecticide to antennae upregulated the level
of UGTs. Thus, this implied that UGTs protect the olfactory organ and play a role in
xenobiotic [110]. In a similar study, UGT facilitated odorant inactivation was
observed in D. melanogaster, as the glucurono-conjugated odorants did not elicit
any olfactory signals [110].
Along with this, UGTs were also expressed in the fat body, midgut, Malpighian
tubules, and antennae, suggesting the enzyme’s role in pheromone deactivation
[107, 111]. This was observed in B. mori, where the UGT genes were expressed in
the fat body, midgut, integument, testis, silk gland, and hemocytes of the fifth instar
larvae. The coregulation of UGTs with other detoxification enzymes such as cytochrome P450s has also been reported which may have evolved in the course of the
plant-insect warfare [112]. UGTs were also seen to detoxify naturally occurring
benzoxazinoid from Z. mays in Ostrinia furnacalis [113].
After detoxification and further processing so as to decrease hydrophobicity, there
exists Phase III detoxification. This includes ATP-binding cassette transporters
(ABCs), which aid in the efflux of the detoxified products from the cell. The
detoxified plant secondary metabolites can either be thrown out of the body or
sequestered for other purposes as discussed below.
4.3
Sequestration
As seen in the above sections, the toxic effects of allelochemicals are overcome by
insects with detoxification. These detoxified secondary metabolites are further
34
S. S. Zunjarrao et al.
located, insect GSTs are either microsomal, mitochondrial, or cytosolic [106]. These
enzymes have broad substrate specificity and are thus important in resistance against
a wide range of xenobiotics. Studies have provided evidence that GSTs take part in
detoxification of glucosinolates in Scaptomyza flava and Trichop lusiani [66].
4.2.4 UDP Glucosyl Transferases (UGTs)
UDP glucosyl-transferase belongs to the class of Phase II detoxification enzymes
functioning in detoxification, olfaction, endobiotic modulation, and sequestration.
These enzymes catalyze the transfer of sugar moieties to a wide range of lipophilic
plant secondary metabolites [107]. UGTs have wide substrate specificity and act on
terpenoids, coumarins, phenols, and flavonoids [108]. UGT conjugates glycoside
group to chemicals and facilitates excretion by making them more hydrophilic and
minimizing their detrimental effects [58].
UGTs are present in different body parts and are involved in various processes [109]. UGTs which are expressed in the olfactory mucosa are responsible
for olfactory processing and detoxification. In an example study from S. littoralis,
where males were exposed to pheromones and plant odorants, UGTs were downregulated. Whereas, the introduction of insecticide to antennae upregulated the level
of UGTs. Thus, this implied that UGTs protect the olfactory organ and play a role in
xenobiotic [110]. In a similar study, UGT facilitated odorant inactivation was
observed in D. melanogaster, as the glucurono-conjugated odorants did not elicit
any olfactory signals [110].
Along with this, UGTs were also expressed in the fat body, midgut, Malpighian
tubules, and antennae, suggesting the enzyme’s role in pheromone deactivation
[107, 111]. This was observed in B. mori, where the UGT genes were expressed in
the fat body, midgut, integument, testis, silk gland, and hemocytes of the fifth instar
larvae. The coregulation of UGTs with other detoxification enzymes such as cytochrome P450s has also been reported which may have evolved in the course of the
plant-insect warfare [112]. UGTs were also seen to detoxify naturally occurring
benzoxazinoid from Z. mays in Ostrinia furnacalis [113].
After detoxification and further processing so as to decrease hydrophobicity, there
exists Phase III detoxification. This includes ATP-binding cassette transporters
(ABCs), which aid in the efflux of the detoxified products from the cell. The
detoxified plant secondary metabolites can either be thrown out of the body or
sequestered for other purposes as discussed below.
4.3
Sequestration
As seen in the above sections, the toxic effects of allelochemicals are overcome by
insects with detoxification. These detoxified secondary metabolites are further
34
S. S. Zunjarrao et al.