Page 299 - The Toxicology of Fishes

Reactive Oxygen Species and Oxidative Stress 279

(Fridovich, 1995). Interestingly, the blue crab (Callinectes sapidus) and other marine crustaceans that
rely on Cu-dependent hemocyanins for O transport contain no CuZnSOD, but have MnSOD in both
2
mitochondria and cytosol (Brouwer et al., 1997). Vertebrate MnSODs have a molecular mass of about
92,000, and they usually contain four protein subunits and 2 or 4 Mn ions per enzyme. MnSODs are
insensitive to inhibition by both cyanide and diethyldithiocarbamate. Despite these differences between
MnSOD and CuZnSOD, both catalyze essentially the same reaction (Equation 6.13).
Extracellular SOD is also a CuZnSOD present in mammals (unknown for other vertebrates), but it
has a far higher molecular weight than cytosolic CuZnSOD (135,000 vs. 32,000). ECSOD is a tetrameric
glycoprotein in which each subunit contains one Cu and one Zn ion. It is particularly abundant in the
extracellular space of blood vessels, bound to heparin sulfate, the glycosaminoglycan component of
heparin sulfate proteoglycans that interact with various proteins on cell surfaces (Fukai et al., 2002).
The large mass of this tetrameric protein and its affinity for cell surfaces prevents filtration by the kidneys
•–
(Fridovich, 1998). In blood vessels, ECSOD appears to play a critical role in protecting NO· from O .
2
NO· produced by endothelial cells stimulates smooth muscle relaxation; thus, diminished ECSOD
expression or activity may play a role in cardiovascular diseases (Fukai et al., 2002).
Another SOD found in bacteria, algae, and higher plants (but not observed in animal tissues) contains
iron; hence, it is termed FeSOD. Most FeSODs contain two subunits, and each enzyme molecule has
one or two iron ions. The bacterium Escherichia coli also contains a hybrid dimeric SOD, with one
subunit from MnSOD and the other from FeSOD; this hybridization is facilitated by the similar amino
acid sequences shared by the two forms (Fridovich, 1995).

Catalases
•–
Although SODs very effectively reduce cellular O concentrations, the downside of this activity is that
2
one of the two products produced is H O . Hydrogen peroxide can be damaging in its own right, and
2
2
in the presence of transition metals such as copper and iron it can also serve as the precursor to highly
reactive ·OH. Most aerobic organisms and all vertebrates possess two enzyme systems that metabolize
H O : the catalases and peroxidases (including glutathione peroxidases found in vertebrates, discussed
2
2
below). Peroxidases generally can act on a variety of organic peroxides as well as H O , whereas catalases
2
2
are largely restricted to H O .
2
2
Vertebrate catalases are large proteins with a molecular mass of 120,000 and consisting of four subunits,
each containing a ferric heme group at its active site (Reid et al., 1981). The heme groups are deep
within the subunits and accessed by narrow channels, which accounts for the substrate specificity of
catalase. They catalyze a reaction conceptually similar to that catalyzed by SOD, a dismutation reaction
(Equation 6.14). Aminotriazole is an effective catalase inhibitor (Darr and Fridovich, 1986).
2HO → O 2 + 2HO (6.14)
2
2
2
The major cellular location of vertebrate catalase is in an organelle known as the peroxisome. Perox-
isomes function in the β-oxidation of fatty acids, and H O is produced as a byproduct of this process;
2
2
thus, catalase prevents damage to peroxisomes and also impedes the movement of H O to other locations
2
2
in the cell (due to its uncharged nature, H O traverses organelle membranes more readily than other
2
2
ROS). The role that catalases play in the metabolism of H O produced outside of peroxisomes appears
2
2
to vary among tissues. The next enzyme discussed, glutathione peroxidase, has a broader distribution
within cells and likely plays a more important role in clearing hydrogen (and other) peroxides produced
in some tissues, such as liver; however, under conditions elevating H O production in some tissues
2
2
including erythrocytes (that do not contain peroxisomes), lung, and eye, catalase activity may be
particularly important (Halliwell and Gutteridge, 1999).
Glutathione Peroxidases and Transferases
Glutathione peroxidases (GPXs) provide another mechanism by which animals can detoxify H O 2
2
(Arthur, 2000); moreover, they can also reduce fatty acid peroxides (LOOH), an important manifestation
of oxidative stress described later. Four selenium-dependent GPXs have been identified in mammals: