Page 213 - The Toxicology of Fishes

Biotransformation in Fishes 193

TABLE 4.12
GST Gene Family Homologs Identified by Cloning in Fish Species
Gene Class
Species Alpha Mu Omega Pi Rho Theta Microsomal
Bass 2 2
Catfish 1 1
Flounder 1 2 2 1 2
Medaka 1 1 1
Plaice 2
Pufferfish 1 1 1 2 2 2
Sea bream 1
Zebrafish 3 3 2 1 1 2 1
Note: Numbers represent quantity of homologs.

and quantitatively they also represent the major isoforms in bass and mullet livers (Gallagher et al.,
2000; Martinez-Lara et al., 1997).

Reactions and Substrate Specificity
Before cloning studies and in the early days of purification, attempts were made to identify diagnostic
substrates or reactions that may be diagnostic of the presence of certain isoenzymes in different tissues
and species. Table 4.13 shows the preferred substrates (apart from 1-chloro-2,4-dinitrobenzene [CDNB])
of the different GST subunits in the rat on which this approach is based. It is important to note that a
relatively small number of GST isozymes possess ketosteroid isomerase activity and catalyze the
conversion of ∆5-3-ketosteroids to ∆4-3 ketosteroids.
It must be realized that great caution must be exercised when using catalytic activities (CDNB) to
evaluate GST expression, as the activity toward any one substrate is a function of both the overlapping
substrate specificities of the different isoforms and their different abundances within tissues; thus, such
predictions may be unreliable. Consider, for example, the use of activity with ethacrynic acid (ETHA)
which is diagnostic of pi-class GSTs but can also be conjugated by an alpha-class enzyme (GSTA4) in

TABLE 4.13
Selected Characteristic Reactions of the Rat GST Gene Families
Class Subunit Activities in Addition to CDNB Conjugation
Alpha rGSTA1 Cholesterol epoxide, aflatoxin B 1 8,9-epoxide; androsterone 3,17-dione,
prostaglandin H2 isomerase, carcinogen binding
rGSTA3 Aflatoxin B 1 8,9-epoxide; peroxidase (cumene hydroperoxide)
rGSTA4 ETHA, NBC, 4-hydroxynonenal
rGSTA5 Aflatoxin B 1 8,9-epoxide, peroxidase (cumene hydroperoxide)
Mu rGSTM1 BSP, DCNB, NBC, BaP 4,5-oxide
rGSTM2 NBC, 4-hydroxy dodecenal (4-OH nonenal), BPDE, tPBO, leukotriene A4
rGSTM3 Very high activity with CDNB
rGSTM6 4-OH nonenal, NBC, DCNB (testis and brain)
Omega — Prostaglandin D synthase
Pi rGSTP1 DNA hydroperoxides, acrolein, BPDE (no ENPP, ETHA, DCNB, CuOOH)
Sigma rGSTS1 —
Theta rGSTT1 ENPP, BaP-4,5-oxide (no CDNB)
rGSTT2 ETHA, tPBO, menaphthyl sulfate, CuOOH (no CDNB)
Membrane GST — N-Acetylcysteine, leukotriene C 4 binding
Note: Diagnostic activities are italicized. Substrates: BaP, benzo(a)pyrene; BPDE, benzo(a)pyrene diol ep-
oxide; BSP, bromosulphalein; CDNB, 1-chloro-2,4-dinitrobenzene; DCNB, 1,2-dichloro-4-nitroben-
zene; ENPP, 1,2-epoxy-3-(p-phenoxy)propane; ETHA, ethacrynic acid; NBC, p-nitrobenzyl chloride;
tPBO, trans-4-phenyl-3-buten-2-one.