Page 224 - The Toxicology of Fishes

204 The Toxicology of Fishes

compound 4-methylumbelliferone was examined. Sulfonation of N-hydroxy-N-acetamidofluorene was
demonstrated in control and 2-acetamidofluorene (AAF)-treated guppy, indicating that the pathway to
the reactive ultimate carcinogen was intact (James et al., 2001). SULT activity was higher in guppy than
medaka, suggesting a possible biochemical basis for the higher sensitivity of guppy to the carcinogenic
effects of AAF. Rainbow trout, a species that is relatively insensitive to the carcinogenicity of AAF,
formed very little N-hydroxy-2-acetylaminofluorene (N-OH-AAF) or N-OH-AAF sulfate (Steward et
al., 1995).
Sulfonation of several xenobiotic substrates has been studied in the channel catfish. These include
3,7- and 9-hydroxybenzo(a)pyrene, benzo(a)pyrene-7,8-dihydrodiol, 2-naphthol, acetaminophen,
6-hydroxymethylbenzo(a)pyrene, 7-hydroxymethyl-12-methylbenz(a,h)anthracene, and p-nitrophenol.
Higher activity was found with the phenolic substrates than the alcoholic substrates (Tong and James,
2000). In contrast to the benzo(a)pyrene phenols, the dihydrodiol procarcinogen BaP-7,8-dihydrodiol
was a very poor substrate for sulfonation in the catfish intestine or liver (van den Hurk and James, 2000).
Others have shown in mammalian systems that BaP-7,8-dihydrodiol inhibits SULT but is a poor substrate
(Rap and Duffel, 1992). In vivo studies with the southern flounder, however, have shown that the sulfate
conjugate of BaP-7,8-dihydrodiol was formed to some extent and was excreted more rapidly than the
glucuronide (Pritchard and Bend, 1984).
The only other fish SULT that has been extensively studied for substrate selectivity is a recently cloned
zebrafish enzyme. The expressed zebrafish SULT had high activity with flavonoids, isoflavonoids, and
other phenolic natural products such as chlorogenic acid, catechin, epicatechin, and quercetin, as well
as with phenolic xenobiotics such as nonylphenol and naphthol (Sugahara et al., 2003a). The zebrafish
SULT was also active with dopamine, thyroxine, estrone, and dehydroepiandrosterone. Studies of
catalytic efficiency (V max /K ) showed that the endogenous substrate dopamine was a relatively poor
m
substrate for the expressed SULT enzyme compared with the natural product n-propyl gallate (Sugahara
et al., 2003a).

Tissue Distribution
Most studies of sulfonation have focused on activity in the liver, but in the catfish the intestine was
shown to be as active as liver in catalyzing the sulfonation of several substrates, including hydroxylated
benzo(a)pyrenes, dopamine, and 6-hydroxymethylbenzo(a)pyrene (James et al., 1997; Tong and James,
2000). Demonstration of activity in the intestine is important in the context of first-pass metabolism of
dietary carcinogens, which are often present at low levels. When low concentrations of substrate are
present in ingested material, biotransformation in the intestine becomes more important. The capacity
of the intestine for metabolism is fairly low, because the liver is a larger organ, but if only small amounts
of xenobiotic are present, as is the case in many environmental samples, the intestine and other extra-
hepatic organs that are portals of entry, such as gills and skin, become important sites of first-pass
metabolism.

Regulation of SULT
Not enough studies of SULT in fish have been conducted to have a good understanding of the number
of SULTs present or their regulation by endogenous or exogenous factors. In mammalian species, SULTs
are not induced by exposure to PAHs, and there are reports that AhR agonists such as PAHS repress
SULT activity (Runge-Morris and Wilusz, 1994). The few studies that have been conducted in fish show
no clear evidence of an effect of PAH-type inducing agents on SULT activity (James et al., 1997). In
catfish exposed to β-naphthoflavone, there was a slight loss of activity with 7-hydroxybenzo(a)pyrene
but no effect with the 3- and 9-hydroxy-BaP substrates. Evidence suggests that agents such as gluco-
corticoids and related substances cause a modest upregulation of SULT in mammals (Runge-Morris,
1998) and that tamoxifen upregulates SULT1A in intestine and liver of rats (Maiti and Chen, 2003), but
this has not been demonstrated in fish.
Recent investigations into the expression of phenol-type SULT in mummichog (Fundulus heteroclitus)
has revealed that this species has a remarkably low expression of this enzyme (Gaworecki et al., 2004).
The activity toward 3-hydroxybenzo(a)pyrene was very low compared to a variety of other fish species