Page 295 - The Toxicology of Fishes

Reactive Oxygen Species and Oxidative Stress 275

Although the focus of this chapter is on toxicities associated with oxygen species, the critical role played
by O in shaping and promoting advanced life forms as we know them is of fundamental importance.
2
The primary basis for the toxicity of oxygen lies in its propensity to undergo electron transfers that
yield reactive intermediates, here termed ROS. On the other hand, the ability of O to accept electrons
2
underlies its utility to aerobic organisms; for example, a key function of O is to serve as the terminal
2
electron acceptor in mitochondrial electron transport, which drives the production of high-energy ade-
nosine triphosphate (ATP). In this process, O is reduced to H O; this is a four-electron reductive process
2
2
that proceeds sequentially through the one-, two-, and three-electron products. These univalent reductions
of O to water are shown in Equation 6.1 to Equation 6.4:
2
O 2 + e → O 2 •– (6.1)
–
+
2
H
O 2 + e  → HO 2 (6.2)
•–
–
2
+
H
HO 2 + e  OH H O (6.3)
+
–
→·
2
2
+
H
→
·OH e  HO (6.4)
+
–
2
Sum: O 2 + e 4 – → H O (6.5)
2
The one-, two-, and three-electron products shown in Equations 6.1, 6.2, and 6.3, respectively, are the
superoxide anion radical, hydrogen peroxide, and the hydroxyl radical, respectively. The two radical
species are free radicals; as defined by Halliwell and Gutteridge (1999), a free radical “is any species
capable of independent existence that contains one or more unpaired electrons.” The existence of unpaired
electrons tends to make free radicals reactive, although the range of reactivity among free radicals is
•−
extensive. O , for example, is relatively weakly reactive, while ·OH is extremely reactive and is among
2
the most potentially deleterious compounds among radicals and ROS encountered in cells. Thus, ·OH
reacts indiscriminately with cellular constituents such as membrane lipids, proteins, and DNA. Due to its
reactivity, it has an in vivo lifetime measured in nanoseconds (Pryor, 1986). Interestingly, O itself is by
2
definition a free radical; it contains two unpaired electrons, each in a different π orbital and having parallel
spins. This last feature impedes the ability of O to act as an oxidant via accepting electron pairs from
2
typical molecules containing electron pairs with opposite spins. If it were not for this spin restriction, O 2
would qualify as an extremely potent ROS, and aerobic life undoubtedly would have a very different nature.
Although H O is not a radical, it is an important ROS. It is a weak reducing and oxidizing agent,
2
2
which lends to its use as a safe disinfectant at high concentrations. Its uncharged nature permits it to
cross cell membranes; however, its ability to serve as a precursor to ·OH at physiologically meaningful
levels underlies its significance to cellular oxidative stress. This ability of H O to generate ·OH via
2
2
•−
reaction with O was first postulated by Haber and Weiss (1934), who proposed the following reaction:
2
O 2 + H O 2 → · OH OH + O 2 (6.6)
+
–
•–
2
Reaction 6.6 is termed the Haber-Weiss reaction, and reaction rates in aqueous solution, although
thermodynamically favorable, are very low; however, as noted by Weiss in 1935 (Halliwell and Gut-
teridge, 1999), this reaction can be effectively catalyzed by transition metals such as iron, as shown in
Equations 6.7 and 6.8:
3+
Fe + O 2 → Fe 2+ + O 2 (6.7)
•–
2+ – 3+
+
Fe + H O 2 →· OH OH + Fe (6.8)
2
•– + –
·
Net: O 2 + HO 2 → OH OH + O 2 (6.9)
2