Page 298 - The Toxicology of Fishes

278 The Toxicology of Fishes

Additionally, H O in the presence of iron within the bacterium may produce cytotoxic ·OH via the
2
2
Fenton reaction (see Equation 6.8). The enzyme myeloperoxidase occurs in neutrophils but not mac-
rophages (Hampton et al., 1998). This enzyme, first isolated from human pus, is a nonspecific heme-
–
containing peroxidase that in the presence of H O catalyzes the oxidation of chloride ion (Cl ) to
2
2
hypochlorous acid (HOCl), a potent bactericidal oxidant.
Although the generation of ROS by these phagocytic cells is generally beneficial (to the host organism,
anyway!), chronic inflammation can produce damage in local tissues and can contribute to diseases
associated with oxidative stress, including arthritis and some cancers.

Antioxidant Defenses
As described above, the generation of ROS is an inescapable part of aerobic life. To flourish, all aerobic
organisms have evolved a diverse array of mechanisms to minimize impacts due to ROS, and healthy
organisms generally can cope well with the normal flux of ROS associated with respiration, certain
enzyme activities, autooxidations, phagocytosis, and so forth. These mechanisms include enzyme systems
that act to remove ROS, low-molecular-weight compounds that directly scavenge ROS (in animals, some
produced endogenously and others obtained from the diet), and proteins that act to sequester prooxidants,
particularly iron and copper. A number of these mechanisms, particularly some antioxidant enzymes,
have been highly conserved in the course of evolution. Among vertebrates, most mechanisms are very
similar, with differences among phyla generally being more quantitative than qualitative in nature; thus,
extensive research in this area with mammalian models has greatly informed work with fishes. The
following discussion of antioxidant defenses is condensed from studies with various models; a later
section describes fish-specific studies.

Antioxidant Enzyme Systems
Superoxide Dismutases
A superoxide dismutase (SOD) was first isolated from bovine blood by McCord and Fridovich (1969);
since that time, several forms of this enzyme have been identified and characterized (Fridovich, 1995).
•–
•–
These enzymes accelerate the dismutation of O by accepting an electron from one O and passing it
2
2
to another, yielding the following net reaction:
O 2 + O 2 → H O 2 + O 2 (6.13)
•–
•–
2
The observations that practically all aerobic organisms contain SOD, that many organisms including
vertebrates produce several distinct SOD proteins, and that these enzymes enhance a reaction that occurs
rapidly nonenzymatically support the theory that O plays an important role in oxidative stress, despite
•–
2
its low reactivity vs. some other ROS (notably, ·OH).
Vertebrates contain three distinct forms of SOD: copper/zinc (CuZnSOD), manganese (MnSOD), and
extracellular (ECSOD). CuZnSOD occurs predominantly in the cytosol, with some present in lysosomes
and the nucleus. Eukaryotic CuZnSODs have a molecular mass of about 32,000 and contain two protein
subunits, each containing one Cu and one Zn ion. At physiological pH, the uncatalyzed dismutation of
–1 –1
5
•–
O has a rate constant on the order of 5 × 10 M s ; bovine erythrocyte CuZnSOD accelerates this
2
reaction to about 1.6 × 10 M s , effectively reducing the half-life of cellular O by several orders of
•–
–1 –1
9
2
magnitude. CuZnSOD is highly sensitive to inhibition by cyanide, a characteristic useful in distinguishing
its activity from MnSOD in analytical assays. Diethyldithiocarbamate is also a potent inhibitor of
CuZnSOD.
Manganese SOD in animals is highly associated with mitochondria; it also occurs in bacteria and
plants. Amino acid sequences of MnSODs across these phyla are quite similar (and distinct from those
of CuZnSODs), consistent with the endosymbiotic theory that proposes that mitochondria evolved from
a symbiosis between an early CuZnSOD-containing eukaryote and a MnSOD-containing prokaryote