Page 223 - The Toxicology of Fishes

Biotransformation in Fishes 203

TABLE 4.17
Sulfotransferase Activities in Different Fish Species
Activity
(pmole/
Species Preparation Substrate min/mg) Ref.
Guppy Liver cytosol 4-Methylumbelliferone 86 James et al. (2001)
(Poecilia 7-Hydroxy-2-acetylaminofluorene 121
reticulate) N-Hydroxy-2-acetylaminofluorene 8
Medaka Liver cytosol 4-Methylumbelliferone 130 James et al. (2001)
(Oryzias 3-Hydroxy-2-acetylaminofluorene 25
latipes)
Channel catfish Liver cytosol 9-Hydroxy-benzo(a)pyrene 600 Tong and James (2000)
(Ictalurus Intestinal cytosol 9-Hydroxy-benzo(a)pyrene 930 Tong and James (2000)
punctatus) Benzo(a)pyrene-7,8-dihydrodiol 2 van den Hurk and James
(2000)
Mummichog Liver cytosol 9-Hydroxy-benzo(a)pyrene 8 Gaworecki et al. (2004)
(Fundulus
heteroclitus)
Lamprey Liver cytosol Petromyzonol 232 Venkatachalam et al. (2004)
(Petromyzon
marinus)
Rainbow trout Liver cytosol N-Hydroxy-2-acetylaminofluorene 0.2 Elmarakby et al. (1995)
(Oncorhynchus Liver cytosol Thyroxine (T 4 ) 7.6 Finnson and Eales (1998)
mykiss) Liver cytosol 3,5,3′-triiodothyronine (T 3 ) 14 Finnson and Eales (1998)

Seven zebrafish SULT isozymes were bacterially expressed and purified and were investigated for
activity toward 17β-estradiol and five environmental estrogens. Three of the isozymes did not show any
activity, but the other four had various activities toward the substrates (Ohkimoto et al., 2003). The most
active isoform (SULT1 ST 2) had a high activity toward 17β-estradiol that was competitively inhibited
by bisphenol-A and 4-n-nonylphenol. Though the activity toward estrogenic compounds suggests that
this isozyme is homologous to the mammalian estrogen form (SULT1E), the amino acid sequence is
sufficiently different to exclude it from classification into this subfamily. This zebrafish isozyme also
had a high activity toward polyphenolic plant compounds, such as genistein, daidzein, and quercetin.
When these phytoestrogens were assayed together with 17β-estradiol, they clearly demonstrated com-
petitive inhibition (Ohkimoto et al., 2004).

Piscine SULTs
Much less is known about these enzymes in fish (Table 4.17). A SULT with activity toward the
endogenous substrate scymnol was found in shark liver, but no xenobiotic substrates were studied. The
shark SULT had a higher molecular weight (40,000) than mammalian SULT (Macrides et al., 1994).
SULT was purified from hepatic and intestinal cytosol of the channel catfish (Falany et al., 1990).
Catalytically active fractions from liver and intestine contained proteins of 41,000 MW that cross-reacted
with an antibody to the human phenol-sulfating form of phenol sulfotransferase (P-PST). Internal
sequence from the liver enzyme indicated it was in the SULT1 family (Tong and James, 2000). The
isolated intestinal and hepatic enzymes had nanomolar K values with 9-hydroxybenzo(a)pyrene as the
m
substrate and were also active with several other substrates.

Reactions and Substrate Specificity
The specificity of sulfonation by substrate has been studied in a few fish species. Sulfonation of
N-hydroxy-N-acetylaminofluorene and 3-hydroxyacetylaminofluorene (procarcinogenic and noncarcino-
genic metabolites, respectively of acetamidofluorene) has been studied in small fish used in carcinoge-
nicity testing (see Figure 4.19). As well as the acetamidofluorene metabolites, sulfonation of the model