Page 210 - The Toxicology of Fishes

190 The Toxicology of Fishes

O
H 2N
OH
CI glutathione

O 2 NO 2 glutathione S– O NH
transferase S O
1–chloro–2,4–dinitrobenzene
O 2N NH
NO 2
HO O
NH 2
S O
γ–glutamyl
transpeptidase
OH
O 2N NO 2
dipeptidase
O NH 2
cysteine conjugate
S O
N-acetyltransferase
H 3C NH
S O NH
O 2N NO 2
OH
O 2N NO 2 HO O
Mercapturic acid
FIGURE 4.11 Glutathione conjugation of 1-chloro-2,4-dinitrobenzene. This compound is a substrate for several distinct
glutathione S-transferase enzymes and is commonly used to measure glutathione transferase activity. The product has a
strong ultraviolet absorbance at 344 nm, and its formation is readily followed spectrophotometrically. Following glutathione
conjugation, the conjugate is converted in a three-step reaction to the N-acetylcysteine conjugate (mercapturic acid).

of thyroid hormones demonstrated that glucuronidation plays an important role in maintaining a homeo-
static thyroid status (Finnson and Eales, 1999). Other environmental toxicants that inhibit UGTs are
chlorinated hydrocarbons. Pentachlorophenol and other compounds in pulp mill effluents were shown
to inhibit UGT activity in rainbow trout (Castren and Oikari, 1987). Hydroxylated PCBs form another
group recognized as potent inhibitors of phenol-type UGT in channel catfish (van den Hurk et al., 2002).
Inhibition of phenol-type UGT by PCB metabolites may hinder the detoxification of procarcinogens, as
was demonstrated with benzo(a)pyrene-7,8-dihydrodiol in channel catfish (James et al., 1994).

Glutathione S-Transferases
Overview
The glutathione S-transferases (GSTs; EC 2.5.1.18) are a supergene family of phase II enzymes that
provide cellular protection against the toxic effects of a variety of endogenous and environmental
chemicals. These dimeric enzymes are ubiquitously distributed and comprise approximately 2 to 4% of
total cytosolic proteins in liver. The most important reaction catalyzed by all isoforms is the conjugation
of the tripeptide glutathione (gamma-glutamyl-cysteinyl-glycine) with an electrophilic center that can
be a C, N, or S atom; these are present in arene oxides, aliphatic and aromatic halides, and α,β-unsaturated
carbonyls. Following formation of the glutathione conjugate, the metabolite may undergo two separate
amino acid cleavage reactions followed by N-acetylation to form mercapturic acid derivatives (Figure
4.11). The substrate specificity of the GSTs is extremely broad. Notable toxic xenobiotic electrophilic
compounds that are conjugated and of toxicological interest include carcinogens and their metabolites,
such as aflatoxin B , benzo(a)pyrene, 7,12-dimethylbenzanthracene, 5-methylchrysene, and pesticides
1
(e.g., alachlor, atrazine, DDT, lindane, methylparathion). A number of GSTs also catalyze biosynthetic
reactions of the leukotrienes and prostaglandins, and others act as organic peroxidases and steroid
isomerases. A major role in endogenous metabolism is the detoxification of products of oxidative stress