Page 318 - The Toxicology of Fishes

298 The Toxicology of Fishes

Mechanisms of Chemical-Mediated Oxidative Stress
As mentioned earlier, oxidative stress has been defined as “a disturbance in the prooxidant–antioxidant
balance in favor of the former, leading to potential damage” (Sies, 1985). Following this definition, two
broad sets of mechanisms by which pollutants can produce oxidative stress can be delineated: mecha-
nisms that enhance ROS production and mechanisms that reduce antioxidant capacity. Illustrative exam-
ples of both are described below.

Enhanced ROS Production
Mechanisms by which chemicals can enhance ROS (and in some cases NOS) production include redox
cycling, interactions with electron transport chains (notably in mitochondria and microsomes, as well
as chloroplasts in plants), and photosensitization. Additionally, some chemicals are metabolized to
•–
carbon-based free radicals that can donate unpaired electrons to O thereby generating O ; CCl 4
2
2
(described above) is a notable example. Moreover, some air pollutants occur as radicals themselves,
•
such as nitrogen dioxide, NO .
2
Redox Cycling
Redox cycling is perhaps the most common mechanism by which a diverse array of chemicals including
many environmental pollutants can generate intracellular ROS. Redox cycling chemicals include diphe-
nols and quinones, nitroaromatics and azo compounds, aromatic hydroxylamines, bipyridyliums, and
certain metal chelates, particularly of copper and iron (Di Giulio et al., 1989, Halliwell and Gutteridge,
1999). These include large classes of compounds of broad industrial use, many pesticides, ubiquitous
elements, and metabolic products of numerous pollutants. Examples of these redox active chemicals are
provided in Figure 6.6.
In the redox cycle, the parent compound accepts an electron from a reduced cofactor, such as NADH
or NADPH; this reaction is typically catalyzed by a reductase such as xanthine oxidase or cytochrome
P450 reductase (Kappus, 1986). Cytochrome P450 reductase normally functions to transfer electrons
from NADPH to cytochrome P450 via FAD and flavin-mononucleotide (FMN) contained in this reductase
(Chapter 4). However it is also a key catalyst in xenobiotic redox cycling in which the electron donated
by NADPH yields a radical of the xenobiotic. In the presence of O , the unpaired electron of the radical
2
•–
metabolite is donated to O , yielding O and regenerating the parent compound; importantly, the parent
2
2
compound can repeat this cycle until it is cleared or metabolized to an inactive product. In the course
of each redox cycle, two potentially deleterious events occur—a high-energy reducing equivalent is
+
expended (the oxidation of NADPH to NAD , for example), and an oxygen radical is produced. Moreover,
the proliferative nature of these potentially harmful outcomes associated with redox cycling (i.e., one
molecule of xenobiotic causing the oxidation of many molecules of NADPH and the production of many
•–
molecules of O ) is observed in other aspects of free radical biology, such as lipid peroxidation described
2
earlier. Redox cycling of some chemicals also occurs in the mitochondria (Doroshow and Davies, 1986;
Cadenas and Davies, 2000). A generalized redox cycle that includes associations with cellular toxicities
and antioxidant defenses is shown in Figure 6.7. It should be noted, however, that in some cases redox
cycling chemicals are toxic by mechanisms other than ROS production (Imlay and Fridovich, 1992);
thus, the in vitro capability of a chemical to cause oxidative stress is not a definitive demonstration of
an in vivo mechanism of toxicity.
Uncoupling or Inhibition of Electron Transport
Another important mechanism by which chemicals can enhance ROS production is through interactions
with electron transport chains, particularly those in mitochondria and endoplasmic reticula (microsomes)
in animals. As discussed earlier, these systems are important sources of ROS during normal aerobic
respiration due a degree of inherent uncoupling between NAD(P)H oxidation and substrate reduction.