Page 229 - The Toxicology of Fishes

Biotransformation in Fishes 209

O O
O O
O O O

O O OCH 3
O O OCH 3
AFB 1 AFG 1
O O O O
O O O

O O OCH 3 O O OCH 3

AFB 2
AFG 3
FIGURE 4.22 Structures of naturally occurring aflatoxins.

Although the liver is a major site of biotransformation of BaP, biotransformation to active metabolites
in portals of entry, such as the intestine, gills, and skin, is also important. Studies in channel catfish
intestine have shown that benzo(a)pyrene was metabolized to several metabolites, including BaP-7,8-
dihydrodiol-9,10-epoxide, by intestinal microsomes and that metabolism was readily induced by expo-
sure to the AhR agonist β-naphthoflavone (James et al., 1997; Kleinow et al., 1998). The major form
of GST present in the intestine was a pi-class GST that had good activity with (+)-anti-BaP-7,8-diol-
9,10-oxide (Gadagbui and James, 2000). Similar studies carried out with the channel catfish intestinal
preparation in which (–)-BaP-7,8-dihydrodiol was infused showed that a major pathway for detoxification
of the (–)-BaP-7,8-dihydrodiol was glucuronidation. In the catfish, (–)-BaP-7,8-dihydrodiol was more
readily glucuronidated than sulfated, although the rate of glucuronidation of an environmentally relevant
concentration of (–)-BaP-dihydrodiol was much lower than the rate of glucuronidation of a similar
concentration of BaP phenols; the rate of conjugation of 2.5-µM (–)-BaP-7,8-dihydrodiol in catfish
intestinal microsomes was 0.025 ± 0.01 nmol/min/mg protein, whereas that of 1-µM 7-hydroxy-
benzo(a)pyrene was 0.35 ± 0.07 nmol/min/mg protein (James et al., 1997; van den Hurk and James,
2000). In induced catfish, metabolism of (–)-BaP-7,8-dihydrodiol to the ultimate carcinogen (+)-anti-
BaP-7,8-diol-9,10-oxide was increased, and the capacity of the intestine to detoxify the diol-epoxide by
glutathione conjugation was apparently overwhelmed, as induced fish had considerably higher DNA
adducts than controls (James et al., 1988). It is likely that species differences in the rate of conjugation
of BaP-7,8-dihydrodiol relative to the rate of activation by CYP to BaP-7,8-dihydrodiol-9,10-oxide are
factors in the species differences in sensitivity to this carcinogen.

Aflatoxin B 1
The aflatoxins represent a group of closely related difuranocoumarin compounds produced by the
common fungal molds Aspergillus flavus and Aspergillus parasiticus (Figure 4.22). Although fish may
be exposed to dietary aflatoxins by ingestion of contaminated feed, especially those with high levels of
cottonseed meal, most of the work surrounding the effects of this compound in fish is due largely to its
being an excellent model compound for hepatic carcinogenesis in rainbow trout. In the early 1960s, it
was discovered that aflatoxin contamination was responsible for an outbreak of hepatocellular carcinomas
in hatchery-reared rainbow trout (Oncorhynchus mykiss), a species for which background tumor rates
are very low (Halver, 1969). Of the various aflatoxins, aflatoxin B (AFB ) (Figure 4.23) is considered
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to be the most potent hepatocarcinogen and hepatotoxin (Eaton and Gallagher, 1994). Although the
carcinogenic potency of AFB in rats is extremely high, rainbow trout exceed rats in sensitivity to AFB 1
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hepatocarcinogenesis and are generally considered to be the most sensitive species to aflatoxin carcino-
genicity (Hendricks, 1994). A dietary exposure to 20 ppb AFB of the Shasta strain of rainbow trout for
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