Page 227 - The Toxicology of Fishes

Biotransformation in Fishes 207

Acetyl–coenzyme A
O
NH 2
N CH 3
N–Acetyltransferase H
Aminofluorene Acetamidofluorene
FIGURE 4.21 Acetylation of arylamines. The arylamine aminofluorene forms acetamidofluorene in a reaction catalyzed
by N-acetyltransferase. The cosubstrate is acetyl–coenzyme A.

Enzyme Specificity, Regulation, and Inhibition
Acetylation has been shown to be a major pathway of metabolism in fish of a number of xenobiotics
containing amino groups. These include therapeutic drugs such as sulfadimethoxine (Droy et al., 1989;
Kleinow et al., 1992) and pollutants such as aniline, chloroaniline, and quinolines (Birkholz et al., 1989;
Bradbury et al., 1993). Acetylation of the procarcinogen 2-aminofluorene leaves the molecule open for
further metabolism by N-hydroxylation and sulfonation to a reactive nitrenium ion, as described in the
section on sulfonation. Another pathway of activation of acetamidofluorene is N,O-acetyl transfer, leading
to the unstable N,O-acetyl metabolite. This is readily deacetylated to yield a nitrenium ion active
metabolite. Thus, although acetylation of amines may improve the water solubility and ease of excretion,
it may in some cases lead to the formation of more reactive metabolites. In mammalian species, the
N-acetyltransferase enzymes are not inducible but are expressed constitutively. The number of forms of
N-acetyltransferase in fish and their regulation have not been studied.

Toxicological Relevance
The previous sections have addressed the occurrence, regulation, and catalytic activities of biotransfor-
mation systems in fish. The following section focuses on three model compounds and chemical classes
and the contribution of biotransformation toward the toxicology of each compound. Benzo(a)pyrene was
selected as being representative of the large class of polynuclear aromatic hydrocarbons (PAHs). Because
of the use of the rainbow trout as a model organism for aflatoxin-induced hepatocarcinogenesis, signif-
icant work has been carried out characterizing the biotransformation of aflatoxin. Finally, due to the
significant potential for exposure to fish, the biotransformation of organophosphates and carbamates is
discussed.

Benzo(a)pyrene
The metabolic fate of PAHs in fish has been extensively studied because PAHs are common pollutants
of the aquatic environment and are carcinogens in fish as well as higher vertebrates (for reviews in fish,
see Buhler and Wang-Buhler, 1998; Buhler and Williams, 1989; Stegeman, 1981; Stegeman and Hahn,
1994). Benzo(a)pyrene (BaP), the model carcinogenic PAH, has received a great deal of attention and
has served as a model to better understand PAH biotransformation. This compound is not in itself
carcinogenic but becomes so upon biotransformation to reactive metabolites that form adducts to DNA
bases and initiate the process of mutagenesis and carcinogenesis. BaP can also be metabolized to products
that are not directly carcinogenic, such as quinones, phenols, and dihydrodiols (Figure 4.3). BaP possesses
a structural “bay region” that impairs enzymatic detoxification of specific stereoisomer metabolites; for
example, conversion of BaP to the (–)-7,8-dihydrodiol by consecutive reactions catalyzed by CYP1A
and epoxide hydrolase (EH), respectively, may lead to an additional CYP1A oxygenation at the 9,10
position, producing the ultimate carcinogen, (+)-anti-BaP-7,8-dihydrodiol-9,10-epoxide (Jerina et al.,
1984). These reactions are readily catalyzed by CYP1A and EH in fish liver (Buhler and Wang-Buhler,
1998; Buhler and Williams, 1989; Stegeman, 1981; Stegeman and Hahn, 1994). This particular metabolite
is recalcitrant to a second EH hydrolytic reaction due to steric hindrance resulting from the “bay region”
ring system; thus, the reactive intermediate is free to interact with DNA leading to potential mutagenesis
and carcinogenesis.