Page 305 - The Toxicology of Fishes

Reactive Oxygen Species and Oxidative Stress 285

Vitamin E
Vitamin E is a highly effective lipid soluble scavenger of peroxyl radicals and hence a key protectant
against membrane damage via lipid peroxidation (Niki and Matsuo, 1993). It appears to be a dietary
requirement of all animals. Vitamin E is actually a mixture of several tocopherols and tocotrienols; the
major component acting in animal cells is α-tocopherol (Figure 6.3) (Diplock, 1985). α-Tocopherol (in
addition to other tocopherols and tocotrienols) is effective at protecting membranes from oxidative attack
by lipid peroxyl radicals (LOO·) because it is more reactive with these radicals than are membrane lipids
1
•–
and proteins. Vitamin E components are also effective scavengers of O , ·OH, and O . As with ascorbate,
2
2
α-tocopherol itself becomes a radical species upon reacting with an oxygen radical (Figure 6.3). α-Toco-
pherol can be regenerated from its radical form by mechanisms including reduction of the tocopherol
radical by ascorbate; this reaction appears to represent an important cooperation between these vitamins
to maintain active α-tocopherol within membranes (Traber, 1994).
Carotenoids
Carotenoids are a large group (over 600 described) of plant pigments, some of which are absorbed from
the diet in many animals (Krinsky, 1993). Carotenoid structure generally includes about 40 carbons,
most of which occur in alternating single and double bonds, a feature that allows for electron delocal-
ization and absorbance of light in the visible range (Britton, 1995). Either or both ends of this carbon
chain are often modified into cyclic rings, which may also contain oxygen groups; several carotenoids
are illustrated in Figure 6.3. Carotenoids are very lipophilic and virtually insoluble in water. Many,
including the most abundant carotenoid observed in humans, β-carotene (Figure 6.3), serve as precursors
for vitamin A (retinol) and retinoic acid, which play important roles in cell growth, differentiation, and
development. Some, such as astaxanthin (Figure 6.3), contribute to the red coloration of some fishes,
including male sticklebacks, for which this coloration plays a role in sexual preference by female
sticklebacks (Barber et al., 2000).
In terms of antioxidant function, the best established role for carotenoids is that of a scavenger of
singlet oxygen in plant chloroplasts (Telfer et al., 1994); O is generated at high rates during photosyn-
1
2
thesis. The ability of carotenoids to scavenge a variety of oxygen radicals has been demonstrated in vitro
(Liebler and McClure, 1996). Although it is generally believed that they also a serve a significant
antioxidant function in animals in vivo, this has not been firmly established.
Other Antioxidants
A number of endogenous compounds with various well-characterized functions other than acting as an
antioxidant have been shown to be also capable of scavenging ROS in various in vitro systems. These
include bilirubin, estradiol, lipoic acid, coenzyme Q (ubiquinol), and uric acid (Halliwell and Gutteridge,
1999); however, the significance of in vitro studies suggesting an antioxidant function to the in vivo
situation is oftentimes difficult to gauge.
Another potentially important function of some compounds, other than direct ROS scavenging, is
regulation of metals that catalyze ROS production (via Fenton-like chemistry, for example). Iron and
copper are the two essential transition metals that naturally occur in vertebrates at relatively high con-
centrations, and if they are free in cells they can readily enhance ROS generation, lipid peroxidation, and
other manifestations of oxidative stress. Consequently, cells have mechanisms to carefully regulate them,
keeping them available for incorporation into appropriate metalloproteins, for example, while preventing
them from participating in destructive oxidative reactions, such as Fenton reactions and autooxidations.
For iron, perhaps the most important regulatory protein is ferritin (Harrison and Arosio, 1996; Orino
et al., 2001). Most intracellular iron is bound to ferritin, which forms a large shell-like structure comprised
in mammals of 24 subunits each of about 20,000 molecular weight and distinguished into two types: H
and L. Up to 4500 iron ions can be stored in the core of a protein. H subunits are involved in iron
detoxification by virtue of ferroxidase activity that oxidizes Fe to Fe . L subunits facilitate nucleation
3+
2+
3+
of Fe within the core of ferritin for long-term storage. A similarly structured ferritin has been purified
from the marine fish Dasyatis akajei (Kong et al., 2003).