Page 323 - The Toxicology of Fishes

Reactive Oxygen Species and Oxidative Stress 303

Nonenzymatic Antioxidant Defenses in Fish
The major nonenzymatic antioxidant defenses observed in mammals have also been studied in fish.
By far the most studied from a toxicological perspective is glutathione, which is present in fish tissues
at levels comparable to those observed in mammals (e.g., millimolar concentrations in liver tissue)
(Nimmo, 1987). As observed in mammals, tissue glutathione levels are often depleted after short-term
oxidant exposures but elevated after long-term exposures. Furthermore, glutathione depletion sensitizes
fish, like mammals, to the toxicity of prooxidant xenobiotics (Gallagher et al., 1992a). Other important
antioxidants, such as vitamin C, vitamin E, ubiquinone, and carotenoids, have been less studied relative
to toxicology in fish but likely play an important role (Bell et al., 2000; Olsen et al., 1999; Parihar
and Dubey, 1995; Payne et al., 1998; Tocher et al., 2002). In addition, important differences clearly
exist in terms of the ability of different fish species to synthesize vitamin C (Moreau-Regis and
Dabrowski, 1998). Although the majority of studies of nonenzymatic antioxidants in toxicology have
historically focused on glutathione, it is increasingly clear that other compounds are also important.
Studies of the total oxyradical scavenging capacity (TOSC) (Regoli and Winston, 1999; Winston et
al., 1998) of tissue homogenates from both aquatic and nonaquatic organisms have demonstrated that
compounds other than the classical antioxidants such as glutathione, vitamin C, and vitamin E can
contribute very significantly to the in vitro oxyradical scavenging capacity of those tissue preparations.
As in mammals, it is likely that diet plays a very important role in determining the nonenzymatic
antioxidant capacity of fish tissues (Mourente et al., 2000; Nakano et al., 1999; Olsen et al., 1999;
Pascual et al., 2003).

In Vivo Prooxidant Studies in Fish
As previously stated, the fact that a chemical produces ROS in an in vitro situation does not mean that
the same chemical will cause oxidative stress in vivo. The additional complications introduced by kinetics
of uptake, metabolism, and excretion in a living organism can be understood only by testing the effect
of a chemical in an in vivo system. Table 6.4 is a representative list of stressors shown to exert oxidative
stress either in intact fish or, in a few cases, in fish cells. It is important to bear in mind that not all
chemicals in the chemical classes listed in column 1 are expected to be prooxidants. Specific chemicals
that induced oxidative stress in the studies cited are listed in parentheses as examples. Generally speaking,
the chemicals shown to cause oxidative stress in mammals have also done so in fish.
Many chemicals are more or less toxic depending on other factors. An example that has received
considerable attention recently is phototoxicity, which is the greatly enhanced toxicity of specific
chemicals (especially many PAHs) in the presence of ultraviolet radiation. UV radiation can excite
an electron of such chemicals, and the resultant excited-state molecule is more reactive and often
ultimately more toxic. Empirical studies with fish have strongly supported the hypothesis that the
phototoxicity or photo-enhanced toxicity of PAHs resulting from the process of photosensitization
is mediated by oxidative stress (Choi and Oris, 2000; Weinstein et al., 1997). In addition, chemical
structure alterations produced by UV irradiation can produce compounds such as quinones and
phenols that are capable of redox cycling and interfering with electron flow in electron transport
chains of mitochondria (Huang et al., 1997; Tripuranthakam et al., 1999), both processes that can
lead to oxidative stress. Thus, phototoxicity represents a distinctive source of oxidative stress that
may be of particular importance in aquatic systems with clear water columns. It is important to note,
however, that most of the studies carried out thus far have been conducted under laboratory or
constrained field conditions, so the ecological relevance of phototoxicity is controversial (McDonald
and Chapman, 2002).

Differences between Fish and Mammals
As we have seen, many of the basic mechanisms of ROS production and damage are shared between
fish and mammals. Despite the overall similarities in antioxidant defenses, however, important differences
exist between fish and mammalian species, some of which are summarized here. ROS production in
microsomal fractions was higher in rainbow trout, as well as in two aquatic invertebrate species, with