Page 562 - The Toxicology of Fishes

542 The Toxicology of Fishes

with the higher sensitivity of this species to BaP-induced hepatocarcinogenesis and DNA adduct forma-
tion in vivo. These observations may indicate that an enzyme different from CYP1A is responsible for
bioactivating BaP in fish. In this regard, BaP is more efficiently metabolized to the 7,8-diol by human
CYP1B1 than by CYP1A (Shimanda et al., 1999). Although CYP1B1 has been identified in fish (Godard
et al., 1999a,b), the possible role of this isozyme in BaP metabolism in catfish is not yet known. Additional
studies are required to delineate the mechanistic reasons why brown bullhead and channel catfish, as
well as other fish species, display differences in BaP-induced liver cancer.

2-Acetylaminofluorene
2-Acetylaminofluorene (AAF) is a carcinogen in most mammalian systems. AAF has been studied as a
model chemical to learn about the metabolism and mechanism of action of carcinogenic aromatic amines
and amides (Miller et al., 1961). In mammals, AAF is N-hydroxylated to N-OH-AAF, a proximate
carcinogenic metabolite (Miller et al., 1961; Weisburger and Weisburger, 1973). AAF can also be
deacetylated to 2-aminofluorene (AF), which can also form DNA adducts. On the other hand, ring
hydroxylation of AAF by other CYP450s results in the formation of water-soluble conjugates that are
excreted (Lotlikar and Hong, 1981). AAF can also be metabolized by flavin monooxygenases (Kitamura
et al., 1999). The sensitivity of different species to AAF-induced carcinogenesis is thought to be due in
part to the balance between metabolic activation to reactive, DNA-damaging agents and metabolic
deactivation/conjugation to water-soluble compounds which are eliminated from the organism (James
et al., 1994; King, 1978; Miller et al., 1961). Relative to most mammalian species, fish generally are
less sensitive to AAF-induced carcinogenesis, and some species (e.g., rainbow trout and medaka) are
more resistant to AAF-induced carcinogenicity than other fish (e.g., guppies) (James et al., 1994; Lee
et al., 1968). Exposing trout to AAF orally resulted in rapid elimination of AAF (Steward et al., 1994),
and in the bile most AAF metabolites (5-/7-/8-/9-OH-AAF) were conjugated with glucuronides. Only a
small fraction of AAF was identified as N-OH-AAF (the glucuronide conjugate). Similar results were
observed in isolated trout liver cells treated with AAF (Steward et al., 1995) and in trout (Elmarakby et
al., 1995) and medaka (James et al., 1994) liver microsomes incubated with AAF. Because mammalian
CYP1A2 evolved from relatively recent gene duplication, and a true CYP1A2 ortholog is not present
in trout (Morrison et al., 1995), these animals may be deficient in the ability to N-hydroxylate AAF,
which likely plays a role in resistance to AAF-induced carcinogenesis. In addition, exposing trout liver
cells to N-OH-AAF did not result in the formation of detectable DNA adducts (M. Miller, unpublished
observations), indicating efficient detoxification of this metabolite in trout. Furthermore, trout and medaka
appear to have relatively low activity of three other enzymes implicated in the metabolic deactivation/con-
jugation of AAF (N-hydroxy-AAF-sulfotransferase, glucuronyltransferase, and N-hydroxy-AAF acyl-
transferase) (Elmarakby et al., 1995; James et al., 1994). Deficiencies in AAF-activating enzymes,
coupled with a relatively high capacity of fish for ring hydroxylation (detoxification), likely contribute
to the resistance of most fish to the hepatocarcinogenic action of AAF.

Carcinogen Alteration of DNA

Chemicals previously classed as initiators and promoters now tend to be described as either genotoxic
or nongenotoxic agents. The former include compounds that are electrophilic and interact directly with
any of the four nucleotide bases within the DNA structure, forming adducts. The latter exert their
influence via epigenetic mechanisms and are discussed later.

Specific Characterized Adducts
AFB 1
Among fish and mammals, AFB mainly forms adducts with guanine (Figure 12.3), and 8,9-dihydro-8-
1
7
(N -guanyl)-9-hydroxy–AFB is the major adduct formed. Over time, this adduct forms open-ring
1
7
N -formamidopyrimidine adducts (Bailey et al., 1994; Croy et al., 1980). AFL also appears to form the