Chemistry Reference
In-Depth Information
A
Malathion is an example of an organophosphorus insecticide,
which works by inhibition of the enzyme acetylcholinesterase,
responsible for the hydrolysis of the neurotransmitter acetyl-
choline. Inhibition of the enzyme allows the build-up of lethal
concentrations of acetylcholine, convulsions and death. Malathion
is a weak inhibitor of the enzyme and in humans is hydrolysed
to the corresponding acid, which also has a low biological activity.
In insects, malathion is oxidised to malaoxon which is 10 000
times more active than the parent compound. This causes an
increase in levels of
acetylcholine, which kills the insect (see
Figure 5.10).
O
O
Humans
OC
2
H
5
OC
2
H
5
CH
3
S
CH
3
S
O
O
O
O
P
P
OH
OC
2
H
5
CH
3
CH
3
O
S
O
S
Insects
O
OC
2
H
5
CH
3
S
O
O
P
OC
2
H
5
CH
3
O
O
Figure 5.10
The metabolism of malathion.
This example illustrates two important points. First,
malathion is a selectively toxic compound in that it kills insects
without harming humans. Second, different species may
metabolise drugs in different ways and extreme care must be exer-
cised when extrapolating results from one species to another,
notably from animal toxicity data to humans.
