Page 831 - The Toxicology of Fishes

Toxicology of Synthetic Pyrethroid Insecticides in Fish: A Case Study 811

C N
O
Cl C O C
O
H
Fenvalerate

C N
O
Cl C O C
O H

OH
FIGURE 20.3 The primary detoxification reaction for fenvalerate in fish is ring-hydroxylation at the 4′ position which is
followed by conjugation to produce a glucuronide conjugate.

O C N
Cl O
C O
Cl
Cypermethrin

O C N
Cl O
C O
Cl
OH

O
O HO C
Cl
C OH O
Cl
OH
FIGURE 20.4 The metabolic pathway for detoxification of cypermethrin proceeds via the 4′-hydroxylation reaction in
fish and higher vertebrates. The hydrolysis reaction shown occurs rapidly in birds and mammals, but not in fish; this
deficiency contributes to the enhanced toxicity of pyrethroids in fish.

(Figure 20.3). Products of ester hydrolysis were rarely found, except as reported in one abstract (Figure
20.4). One study on the toxicity of the oxidation products showed that the largely intact ones retained
some toxicity, although less than the parent. The alcohol and acid moieties that result from ester
hydrolysis are of minimal toxicity to any animals. The importance of esterases for detoxification of
pyrethroids by mammals and birds is notable because the metabolite profile in those organisms reveals
ample hydrolysis and oxidation products. Studies by Glickman et al. (1982) showed that the exposure
of rainbow trout to permethrin did not result in more toxicity when an esterase inhibitor was added,
which reflects a lack of esterase involvement in detoxification of the pyrethroid in that species. The fish
species investigated by several labs all seemed to be deficient in hydrolysis capability (Bradbury and
Coats, 1989a). This is most likely a significant contributing factor for the susceptibility of fish to synthetic
pyrethroids.