Page 198 - The Toxicology of Fishes

178 The Toxicology of Fishes

The toxicological significance of FMO-catalyzed biotransformation reactions has not been extensively
examined in fish. Recent studies with organoselenides and FMO from sharks have indicated that FMO
may be involved in the oxidation and initiation of redox cycling in these species (Schlenk et al., 2003);
however, most studies examining the toxicological roles of FMO have examined pesticides. The oxy-
genation of the pesticides thiobencarb and eptam in striped bass (Morone saxatilis) by hepatic FMO
was shown to lead to the formation of a reactive-intermediate that covalently bound protein sulfhydral
groups (Cashman et al., 1990; Perkins et al., 1999); however, protein binding was not observed in vivo
by thiobencarb. S-Oxygenation of aldicarb to the sulfoxide by FMO significantly increased the inhibition
of acetylcholinesterase in rainbow trout (Oncorhynchus mykiss) (250-fold) and Japanese medaka (Oryzias
latipes) (40-fold) (El-Alfy and Schlenk, 2002; Perkins et al., 1999). Elevated toxicity has been observed
in FMO-containing fish which can activate aldicarb to the more potent sulfoxide compared to species
that lack FMO and convert aldicarb to the less toxic sulfone or hydrolytic metabolites (Perkins and
Schlenk, 2000; Schlenk, 1995).
Enhanced sulfoxidation may possibly explain the enhanced toxicity of aldicarb in higher salinity observed
in medaka and trout, as FMO expression has been shown to be directly correlated to salinity in medaka
(Larsen and Schlenk, 2001; Schlenk and El-Alfy, 1998). Studies comparing the effects of salinity on aldicarb
toxicity in trout and striped bass indicate that salinity significantly enhances the toxicity of aldicarb in trout
but not in striped bass (Wang et al., 2001). In striped bass, aldicarb sulfoxide formation and FMO expression
were unchanged by salinity, whereas salinity increased aldicarb sulfoxide formation, cholinesterase inhi-
bition, and FMO expression in rainbow trout. Consequently, understanding factors that affect the expression
patterns of FMO is important when considering species-specific sensitivities to xenobiotics and differential
responses of organisms to environmental factors such as salinity and temperature regimes.

Monoamine Oxidases
Monoamine oxidases catalyze the oxidation and eventual elimination of alpha carbon groups from
secondary amines. Monoamine oxidases have been characterized in several fish species, with most
occurring in trout. Given the critical importance in catecholamine metabolism, most studies have focused
on its endogenous role in the neurophysiology of fish. In contrast to terrestrial vertebrates, which have
two forms of the enzyme (MAO A and MAO B), fish appear to only have a single form that is genetically
distinct from terrestrial vertebrates. Although no specific studies have examined the role of MAO in
xenobiotic biotransformation in fish, the effects of various organic and inorganic pollutants on enzyme
activity has been examined (Senatori et al., 2003).

Alcohol and Aldehyde Dehydrogenases
Alcohol dehydrogenase (ADH) catalyzes the oxidation of alcohols to aldehydes, which are subsequently
+
converted to acids by aldehyde dehydrogenase (ALDH). NAD is a cofactor for each enzyme. A class 3
ADH cDNA was first identified in sea bream, in which its expression was observed in all tissues as well
as eggs and embryos. Expression decreased during early embryonic development but increased fourfold
from day 1 to 21 after hatching, indicating that the maternal ADH mRNA is present in the eggs and
embryos but diminishes as development occurs, allowing the larval tissue to express its own ADH
(Funkenstein and Jakkowiew, 1996). An additional ADH3 cDNA was also identified by RT-PCR in
zebrafish (Danio rerio) (Dasmahapatra et al., 2001). Expression of the gene in zebrafish embryos appeared
to correspond with temporal variations in zebrafish susceptibility to ethanol toxicity. In cod, an ADH
enzyme was purified that displayed structural similarities to ADH3, but functionally it was more like
ADH1. Ethanol was an excellent substrate for the purified enzyme, and 4-methylpyrazole was a strong
inhibitor (K = 0.1 µM) (Danielsson et al., 1992). Allylic and acetylenic alcohols appear to be bioactivated
i
through oxidation by trout liver ADH and may also act as inhibitors (Bradbury and Christensen, 1991).
ALDH has been observed in all tissues of numerous fish species (Nagai et al., 1997). Similar to CYP1A,
ALDH has been observed in mammals to be regulated by the Ah receptor. Studies in the dab (Limanda
limanda), the sea bass (Dicentrarchus labrax), and the rainbow trout failed to observe increases in ALDH
activities following 3-MC or BNF treatment (Le Maire et al., 1996); however, expression was significantly
elevated in liver tumor tissues from adult rainbow trout treated with aflatoxin (Parker et al., 1993).