Page 232 - The Toxicology of Fishes

212 The Toxicology of Fishes

zebrafish is fourfold lower than that observed for the sensitive rainbow trout but fivefold higher than for
the rat. Despite the ability of zebrafish to activate AFB and to form persistent AFBO–DNA adducts,
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zebrafish appear to be quite resistant to the carcinogenic effects of AFB when administered by the
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dietary route (Troxel et al., 1997). Thus, the zebrafish studies suggest that mechanisms related to factors
other than the inherent ability to bioactivate and detoxify AFB may be involved; for example, it may
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be that the DNA-adducted cells do not go on to form initiated cells or that a lack of an initiation/promotion
progression is present in zebrafish.
As opposed to rodents, fish can rapidly convert AFB to a reductive metabolite, aflatoxicol (AFL)
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(Figure 4.23), by reducing the 1-keto-moiety via a cytosolic NADPH-dependent reductase (Salhab and
Edwards, 1977). Aflatoxicol can be further metabolized by 9α-hydroxylation to form AFLM (Figure
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4.23) (Lovel et al., 1988). Aflatoxicol is a potent frameshift mutagen and also elicits unscheduled DNA
synthesis in fibroblasts incubated with a rat liver postmitochondrial fraction (Stich and Laishes, 1975).
Aflatoxicol is approximately 50% as carcinogenic as AFB in trout (Schoenhard et al., 1981) and exhibits
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about 70% the mutagenicity of AFB in an in vitro trout liver activating system (Coulombe et al., 1982).
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Accordingly, the formation of aflatoxicol does not appear to be an important detoxification pathway for
AFB , especially as aflatoxicol may be rapidly converted back to AFB by a microsomal dehydrogenase
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(Salhab and Edwards, 1977), thereby increasing the physiological half-life of AFB (Loveland et al., 1977).
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Several of the products of AFB oxidative metabolism serve as substrates for phase II detoxification
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enzymes. As in the case of AFB epoxidation, there is extensive interspecies variation in phase II
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conjugation of AFB oxidative metabolites. In mammalian species, the primary pathway for AFB 1
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detoxification is through GST-mediated conjugation of AFBO with reduced glutathione (GSH). The
selectivity of GST isoenzymes toward AFBO serves as a critical determinant of differences among
mammalian species in susceptibility to AFB hepatocarcinogenesis (Degen and Neumann, 1981; Eaton
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and Gallagher, 1994; Eaton et al., 1990; Lotlikar et al., 1984; O’Brien et al., 1983; Roebuck and
Maxuitenko, 1994). Mouse liver cytosolic fractions have 50- to 100-fold greater AFBO conjugating
activity than rat even though both species have comparable amounts of GST activity toward 1-chloro-
2,4-dinitrobenzene (CDNB) (Monroe and Eaton, 1987). Accordingly, mice are resistant to the hepato-
carcinogenic effects of AFB when compared to rats (Wogan and Newberne, 1967), a difference reflected
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by 50- to 100-fold less AFB –DNA adduct formation by mice after in vivo AFB exposure (Monroe and
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Eaton, 1987). The high AFBO conjugating activity in mice is due to constitutive expression of an alpha
GST isozyme (mGSTA3-3) that has unusually high conjugating activity toward AFBO.
In contrast to mammals, GST-mediated AFB conjugation does not appear to be a significant route of
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AFB detoxification in fish. Species such as rainbow trout (Valsta et al., 1988), Coho salmon (Valsta et
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al., 1988), or channel catfish (Gallagher and Eaton, 1995) do not form appreciable amounts of
AFBO–GSH conjugates. The low capacity for AFBO–GSH conjugation in the presence of efficient AFB 1
epoxidation in the trout probably accounts for the high covalent binding index for AFB in trout relative
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to mammals (Bailey et al., 1984). Detectable, albeit low, GST–AFBO activity has been measured in liver
cytosolic fractions prepared from English sole (Pleuronectes vetulus) and starry flounder (Platichthys
stellatus) (Gallagher et al., 1998), but the significance of this activity toward AFB sensitivity has not
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been determined. English sole and starry flounder express a theta-class GST that shows relatively low
homology to the mouse alpha-class GST with high AFBO conjugating activity (mGSTA3-3) (Gallagher
et al., 1998). A similar form has also been cloned from largemouth bass, although its ability to conjugate
AFBO has not been tested (Doi et al., 2004). There is no evidence for the presence of a GST orthologous
to the mouse alpha-class GST form that rapidly conjugates AFBO in any fish species examined to date.
As in mammals, glucuronidation is an important pathway for the detoxification and excretion of
xenobiotics in fish (for a review, see George, 1994). The production of AFL represents a significant
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AFB detoxification pathway when a glucuronide is produced and excreted. Biliary AFB conjugates in
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rainbow trout, zebrafish, and Coho salmon are comprised mainly of AFM – and AFL –glucuronide
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conjugates (Loveland et al., 1984; Troxel et al., 1997). A similar metabolic profile appears to exist for
channel catfish (Gallagher and Eaton, 1995). Although the rate of glucuronidation of AFB metabolites
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by catfish liver has not been directly measured, channel catfish have high UDP–GT activities (Ankley
and Agosin, 1987; Short et al., 1988) and produce polar biliary AFB metabolites after oral AFB 1
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administration (Plakas et al., 1991). Sulfate conjugates were not detected in rainbow trout exposed to