Chemistry Reference
In-Depth Information
because other formulations are regarded as being too hazardous. Granules ensure
slow release of the active ingredient under field conditions, which can have the ben-
efits of longer-term control of pests and of reduced environmental hazards. However,
there have occasionally been environmental problems with granules because (1) birds
sometimes consume them; and (2) when fields are flooded, the active ingredient may
be dissolved in the water of large puddles on the land surface, making it available to
animals and birds (Hardy 1990).
Similar to OPs, CBs tend not to be very persistent in food chains, and do not
undergo biomagnification with passage along them. Some of them (e.g., aldicarb and
carbofuran) are systemic, and so may be taken up by insects feeding on plant sap.
This may occur with nontarget species feeding on weeds, as well as pest species
feeding on crops.
10.3.4 T
o x i c i T y
CBs, like OPs, act as inhibitors of ChE. They are treated as substrates by the enzyme
and carbamylate the serine of the active site (Figure 10.8). Speaking generally, car-
bamylated AChE reactivates more rapidly than phosphorylated AChE. After aging
has occurred, phosphorylation of the enzyme is effectively irreversible (see Section
10.2.4). Carbamylated AChE reactivates when preparations are diluted with water,
a process that is accelerated in the presence of acetylcholine, which competes as a
substrate. Thus, the measurement of AChE inhibition is complicated by the fact that
reactivation occurs during the course of the assay. Carbamylated AChE is not reac-
tivated by PAM and related compounds that are used as antidotes to OP poisoning
(see Box 10.1).
Carbamates vary greatly in their toxicity to vertebrates. Some examples are given
in Table 10.4. The most striking feature of the data is the very high acute toxicity of
the two systemic carbamates aldicarb and carbofuran; carbaryl is far less toxic than
either of these two compounds to mammals, birds, or fish. Propoxur is substantially
less toxic to rats and birds. The toxicity to mammals and birds looks broadly similar
for the four compounds represented here. However, a comparison of the toxicity of
20 CB insecticides to the red-winged blackbird (
Agelaius phoeniceus
) and starling
(
Sturnus vulgaris
) with toxicity to the rat showed that in over 85% of cases the car-
bamate was more toxic to the bird than to the rat (Walker 1983). A possible factor
here is the relatively low monooxygenase activity found in these and many other
species of birds compared with the activity in the rat (see Chapter 2). Detoxication
O
H
OC
N
OH
CH
3
O
H
ChE
OH
+
ChE
+
OC
N
CH
3
Carbaryl
Carbamylated
enzyme
I-Naphthol
fIgure 10.8
Carbamylation of cholinesterase.
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