Page 212 - The Toxicology of Fishes

192 The Toxicology of Fishes

0.1
ZebrafishGSTA
FHMinnow
PufferGSTA
Medaka
Rho class
plaiceGSTA1
SeaBream
LMBass
PlaiceGSTA
ZebrafishOmega
Omega class
PufferOmega
ZebrafishAlpha Alpha class
CatfishPi
SocksalmonPi
Pi class
TroutPi
ZebrafishPi
ZebrafishMU2
ZebrafishMU1 Mu class
MedakaMU
ZebrafishTheta
Theta class
PufferTheta
WFlounderAJ605273
PufferCA844840
Microsomal
MedakaAV669487
Plaicetrypsinogen

FIGURE 4.12 Phylogenetic relationships of cloned fish GSTs; derived from deduced amino acid sequences using Clustal
W (zeta and mitochondrial kappa isoforms omitted).

major isoform in flatfish, mullet, and bass is now designated as Rho-class (Konishi et al., 2005). From
nucleotide and protein sequence data, genes for all nine families have been identified in fish to date. A
phylogenetic analysis of the sequenced fish genes is shown in Figure 4.12. Table 4.12 shows for each
species the number of GST isoforms assigned to each family.
From purification studies, pi-class GSTs have been identified as the major hepatic and intestinal GSTs
in the livers of salmonids, brown bullhead, channel catfish, and lamprey. These piscine pi-class enzymes
display high activities with BaP epoxides and diol epoxides. The most highly characterized piscine GSTs
are from the plaice, a pleuronectid flatfish, where purification and cloning studies identified a multigene
cluster of enzymes originally designated as GSTA. These are most closely related to the mammalian,
insect, and plant theta-class enzymes and have now been designated as a separate class, Rho. They
appear to be unique to fish. Their genetic organization appears to be the same in plaice, flounder, sea
bream, and largemouth bass. The primary role of these theta-like enzymes is in oxidative defense. GST-A
is most active toward hydroxynonenals, and GSTA1 (or GSTR2) exhibits a high peroxidase activity with
organic peroxides. They are the major isoforms in most tissues of flatfish. Homologs have been identified
by cDNA sequencing, purification, or immunochemical cross-reactivity in a number of other fish species,