Page 211 - The Toxicology of Fishes

Biotransformation in Fishes 191

TABLE 4.11
Immunochemical Analysis of GST Class Occurrence in Aquatic Animals
Anti-Rat GST Subunits
Anti-Plaice-GST Alpha Class Mu Class Pi Class
Species A B A1 A3 A4 M1 P1
Catfish ++ + ++++
Cod ++++ ± + NS ± ++ +++
Flounder ++++ + – – ± – –
Mullet
Plaice ++++ ++++ +++ – ++++ – –
Turbot ++++ +++ ± ± + ± –
Rainbow trout + ± – – ± – ++++
Sea bass
Sea trout + ± +++ + +++ +++ ++++
Salmon – + – ++ – ++++ –
Mussel ++++
Clam ++++
Source: Adapted from George, S.G., in Aquatic Toxicology: Molecular, Biochemical, and Cellular Perspectives.
Ostrander, G.K. and Malins, D.C., Eds., CRC Press, Boca Raton, FL, 1994, pp. 37–85.

arising from oxidation of lipids, nucleic acids, and proteins—for example, base propenals such as acrolein
from DNA oxidation; cholesterol oxide, fatty acid hydroperoxides, and hydroxynonenals from lipid
oxidation; and protein carbonyls from protein oxidation. Some isoforms can also exhibit a covalent
binding rather than a catalytic role with some compounds. Reactive metabolites of carcinogens (e.g.,
PAHs) may bind covalently in a suicide reaction that prevents their reaction with DNA; however, many
other neutral or lipophilic compounds that are not substrates, including steroid and thyroid hormones,
bile acids, bilirubin, fatty acids, and heme, may bind noncovalently in a reversible manner. Functionally,
the significance of this noncovalent binding is unknown, but, considering the high concentration of the
enzyme proteins in the cytosol, roles in intracellular transport, as a buffer for these compounds, and as
an efflux system (via the ATP-dependent glutathione conjugate efflux pumps GS-X, MOAT, and Mrp)
have been postulated.

GST Gene Structure
Glutathione S-transferases are widely distributed in nature, and essentially all eukaryotic species contain
multigene families, many of which contain further subfamilies of proteins. Both cytosolic and membrane-
bound forms are present (Board et al., 1997, 2000; Hayes and Pulford, 1995). They are generally
classified according to sequence homology and assigned to seven separate families of cytosolic enzymes
(designated class alpha, mu, omega, pi, sigma, theta, and zeta) and to two membrane families—microso-
mal and mitochondrial (kappa). The native cytosolic enzymes are present as dimers of 24- to 26-kDa
subunits, and a characteristic of the different families is that within each family the proteins contain
conserved amino acid residues that enable formation of both homo- and heterodimers of enzyme
subunits. The microsomal enzymes are trimers of approximately 15-kDa subunits and are integral
membrane proteins.
In lower vertebrates and invertebrates, few GSTs have been fully characterized, although on the basis
of the broad spectrum of catalytic activities found, immunochemical comparisons, and nucleotide
sequence homologies, the presence of multiple isoforms from a number of gene families in all phyla is
a certainty. On the basis of immunochemical cross-reactivity with antisera exhibiting family specificity,
proteins of the alpha, mu, pi, and theta-like families have been identified in many fish species (Table
4.11), and in three mollusk species pi-class enzymes are the major isoforms.
The relative abundances of these isoforms differ between species. GSTs are most abundant in the
liver. The predominant isoform in the cyprinids, salmonids, and gadoids is a pi-class homolog, and the