Page 296 - The Toxicology of Fishes

276 The Toxicology of Fishes

The net reaction, Equation 6.9, is the Haber-Weiss reaction. Equation 6.8 is referred to as the Fenton
reaction (Walling, 1975). Other transition metals present in cells, particularly copper, also can participate
in Fenton-like reactions. The abilities of iron and copper to participate in cellular ·OH generation, in
addition to their roles as essential nutrients, may underlie the existence of proteins (such as ferritin for
iron and ceruloplasmin and metallothionein for copper) that tightly regulate their cellular transport and fate.
In addition to these three O reduction products, several other ROS and related chemicals can play
2
important roles in oxidative stress. These include singlet oxygen, ozone, alkoxyl and peroxyl radicals, and
reactive nitrogen species, such as nitric oxide and peroxynitrite (Halliwell and Gutteridge, 1999). Addi-
tionally, carbon- and sulfur-centered free radicals occur and affect cellular function. Singlet oxygen (com-
monly denoted as O ) is an excited state of O at 22.4 kcal above ground state. In this form, the outer π
1
2
2
1
electrons are paired and in antiparallel spins; thus, O is not a free radical, but the spin restriction of ground-
2
state O is removed, making O more reactive than O . A major source of O in the environment is through
1
1
2
2
2
2
excitation of ultraviolet-absorbing chemicals, including many polycyclic aromatic hydrocarbons (PAHs).
This phenomenon has important ramifications in aquatic toxicology and is discussed later in this chapter.
Ozone (O ) also is not a free radical but is a much more powerful oxidizing agent than O . Ozone is
3
2
produced in the stratosphere by the photodissociation of O into oxygen atoms, which then react with
2
O to produce O . Tropospheric (ground-level) O is produced by sunlight-enhanced reactions between
2
3
3
nitric oxides and volatile organic compounds (VOCs); warmer temperatures and fossil-fuel combustion
produce nitric oxides, which enhance this process. While stratospheric O provides protection to organ-
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isms by reducing the solar ultraviolet radiation that reaches the Earth’s surface, ground-level O can be
3
toxic, particularly to animal respiratory systems and plant photosynthesis.
Alkoxyl (RO·) and peroxyl (ROO·) radicals are potent oxidants that often arise in cells subsequent to
·OH attack of organic chemicals. Oftentimes, such attack initially produces carbon-based radicals, which
under aerobic conditions react with O to produce RO· and ROO·. This process underlies membrane
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lipid peroxidation, a hallmark cellular toxicity associated with ROS that is described below.
Although not the focus of this chapter, reactive nitrogen species (RNS) also play important roles in
cellular physiology and toxicity and interact with ROS. Nitric oxide (NO·) is synthesized in many
organisms by an enzyme group known as the nitric oxide synthases (NOSs); these enzymes catalyze the
oxidation of arginine to produce NO· and citrulline (Muriel, 2000). Three forms of NOS have been
identified in mammals, referred to as type 1, constitutive, or neuronal NOS (cNOS, nNOS); type 2 or
inducible NOS (iNOS); and type 3 or endothelial NOS (eNOS). There is good evidence for conservation
of genes coding NOSs in other organisms, including fishes (Cox et al., 2001).
NO· has been studied predominantly from the standpoint of the normal physiological roles it plays.
These include acting as mediators of phagocytosis by macrophages and neutrophils and functioning as
a neurotransmitter to relax smooth muscle and thereby produce vasodilation (Ignarro, 2002; Neumann
et al., 2001; Toda and Okamura, 2003). NO· is not particularly reactive with nonradicals but is highly
reactive with other radicals (Halliwell and Gutteridge, 1999). Toxicities associated with NO·, observed
in some instances, are thought in large part to be due to its facile reaction with O . This reaction has
•–
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at least two potentially deleterious consequences: (1) the loss of functional NO·, thereby interfering with
vasodilation (and potentially leading to hypertension, for example), and (2) the production of peroxyni-
trite via the reaction:
O 2 + NO·→ ONOO – (6.10)
•–
Peroxynitrite is a very powerful oxidizing species that can elicit cytotoxicity through several mechanisms,
including those described below for ROS as well as by nitration of DNA bases and aromatic amino
acids (Beckman and Koppenol, 1996).
Endogenous Cellular Sources of Reactive Oxygen Species
Before describing mechanisms by which chemical pollutants can generate ROS and enhance oxidative
stress, it is important to discuss mechanisms by which these species arise normally. As alluded to earlier,