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Biotransformation in Fishes 213

AFB (Loveland et al., 1984), which is consistent with other studies indicating that rainbow trout excrete
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xenobiotics as glucuronides, as opposed to sulfates (Bailey et al., 1984).
Rodent studies have demonstrated that a variety of dietary factors and synthetic antioxidants have
been shown to influence the carcinogenicity of AFB . The inducibility of fish hepatic phase II enzymes
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by synthetic antioxidants has been demonstrated in several species, including plaice (George, 1994),
channel catfish (Gallagher et al., 1992), largemouth bass (Hughes and Gallagher, 2004), and brown
bullheads (Henson et al., 2001). In the case of the synthetic antioxidant ethoxyquin, juvenile brown
bullheads fed a semipurified, antioxidant-free diet supplemented with ethoxyquin exhibited a significant
increase in hepatic cytosolic GST activity toward 1-chloro-2,4-dinitrobenzene (CDNB) relative to control
fish (Henson et al., 2001). Despite the ability of synthetic antioxidants to modulate GST and other phase
II enzyme expression, there is little evidence for chemoprotection of AFB carcinogenesis based on
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modulation of AFB biotransformation in fish. The rainbow trout tumor model has been used extensively
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to study the mechanisms of a number of dietary anticarcinogens, including AFB .
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Exposure to the natural product indole-3-carbinol (I3C) was effective as a chemoprotectant against
AFB tumorigenesis in rainbow trout (Takahashi et al., 1995). Although it was originally proposed that
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the mechanism of protection was mediated through the induction of CYP1A activity, studies concluded
that the protection was largely due to the fact that I3C undergoes breakdown in the gastrointestinal tract
to acid condensation products that act as blocking agents against AFBO–DNA adduct formation (Taka-
hashi et al., 1995).
In summary, AFB is an important dietary carcinogen that can serve as a model substrate for examining
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aquatic species’ differences in biotransformation in relationship to susceptibility to carcinogenesis. The
reader is directed to Chapters 5, 6, and 24 for additional information on the roles of biotransformations
in the mechanisms of action of environmental procarcinogens in fish.

Organophosphate Esters and Carbamates
Acetylcholinesterases are common targets for a host of xenobiotics, including S
various classes of pesticides. Biotransformation often plays a critical role in
facilitating or impeding the binding of these compounds to cholinesterases. One Rʼ ʼO P R
class of pesticides whose toxicology is significantly affected by biotransforma-
tion is the organophosphate ester insecticides. In general, organophosphate esters ORʼ
possess a phosphorothionate group bound to at least two alkyl ethers (see Figure
FIGURE 4.24 Structure
4.24). The phosphorothionate is usually activated through oxidative desulfuration
of organophosphate ester.
to an oxon, which irreversibly binds to the serine residues of the anionic site of
cholinesterase (see Figure 4.25). The inability of cholinesterase to regenerate because of this covalent
interaction results in irreversible binding and inactivation of the enzyme. The classical example is that of
parathion, which is activated to paraoxon which inactivates acetylcholinesterase (Figure 4.26). This
monooxygenation is classically catalyzed by cytochrome P450 isoforms, but depending on the electrone-
gativity of adjacent functional groups flavin-containing monooxygenases may also be involved in oxon
formation (Levi and Hodgson, 1992). Alternatively, various hydrolytic processes (i.e., dearylation of par-
athion) may also occur which detoxify the compound, as each hydrolytic derivative does not inhibit
cholinesterase. Hydrolysis may occur subsequent to or prior to desulfuration and is catalyzed by a host of
carboxylesterases (Maxwell, 1992). An additional detoxification reaction involves the conjugation of certain
compounds such as methyl parathion and methyl paraoxon with glutathione (Benke et al., 1974).
Although the toxicity of a host of organophosphates have been examined in fish, the biotransformation
of these compounds and elucidation of specific inhibitory metabolites have only been examined in a very
few cases. As these organophosphates demonstrate acute toxicity, it is likely that all are converted to
oxonic metabolites, even though few have been characterized chromatographically. The most studied
organophosphate ester in fish species is parathion. Several studies have reported paraoxon formation in
a range of fish species including channel catfish (Ictalurus punctatus) (Straus et al., 2000), bluegill sunfish
(Lepomis macrochirus), bullheads (Ictalurus melas), white flounder (Pseudopleuronectes americanus)
(Hitchcock and Murphy, 1971), sculpin (Leptacottus armatus) (Murphy, 1966), rainbow trout (Abbas et
al., 1996; Wallace and Dargan, 1987), and mosquitofish (Gambusia affinis) (Boone and Chambers, 1997).