Page 217 - The Toxicology of Fishes

Biotransformation in Fishes 197

O O
H 2N H 2N
OH OH
HBr
GST O NH
Br O NH
Br Br S O S O
NH NH
dibromoethane Br –
HO O HO O
glutathione conjugate episulfonium ion
reactive metabolite
FIGURE 4.14 Formation of a reactive metabolite by glutathione conjugation. Halogenated alkanes such as dibromoethane
can undergo glutathione conjugation then eliminate the second halogen to give a reactive episulfonium ion. This electrophilic
metabolite can react with cellular nucleophiles.

Regulation of GSTs
Many of the mammalian GST isoforms are inducible; however, GST regulation is complex, and expres-
sion is both developmentally regulated and tissue specific (e.g., some are expressed in brain and testis
only). Some GSTs are responsive to hormonal effects (growth hormone, thyroxine, insulin), and over
100 xenobiotic compounds have been shown to act as inducers (Hayes and Pulford, 1995). Moreover,
in humans and the rat (but not in fish), GSTP1 is overexpressed in hepatocytes during the process of
carcinogenesis. From structural and promoter analyses of GST genes (especially rGSTA2), various
functional regulatory elements have been identified (Table 4.15). Interestingly, nucleotide sequences
contributing to insulin responsiveness of GSTP and to specific expression of GSTs in liver and brain
have been identified. Involvement of NF-κB and AP-1 sites, c-fos, and c-jun are also implicated in
regulation. For a detailed discussion, the reader is referred to the review of Hayes and Pulford (1995).
An important facet of the GST pathway from an environmental perspective is the potential for induction
of GST isozymes on exposure to certain dietary and environmental chemicals. Typically, a modest
induction (twofold or less) of overall GST–CDNB activity is observed in fish exposed to prototypical
GST-inducing agents in the laboratory. Studies of GST–CDNB activities in fish may be complicated by
variations in diet, water temperature, gender, and reproductive cycling (Swain and Melius, 1984). Fur-
thermore, as discussed, GST–CDNB activity represents an integration of the activity of multiple isoforms,
and treatment effects on GST isoforms may not always be distinguishable by analysis of GST–CDNB
activity. The importance in ascertaining effects of inducing agents on multiple fish GST isoforms is
underscored by the fact that selective modulation of those GST isoforms with high specific activity toward
environmental toxicants or their metabolites may be missed if GST–CDNB activity is the only endpoint.
Ultimately, modulation of key GST isoforms that primarily contribute to the conjugation of environmental
agents (or their metabolites) will be the critical determinants of chemical susceptibility.

TABLE 4.15
Regulatory Elements and Inducer Responsiveness of Rat GST Genes
Element Inducers
XRE Planar polyaromatic hydrocarbons, dioxins, etc.
ARE Phenolic antioxidants, reactive oxygen species,
Michael reaction acceptors, quinones, epoxides, etc.
Barbie box element Barbiturates
GRE Synthetic glucocorticoids
GPE1 (TRE-related) Carcinogenesis
Cccgctc Insulin
HNF1 Liver expression