Page 319 - The Toxicology of Fishes

Reactive Oxygen Species and Oxidative Stress 299

DNA STRAND BREAKS, OXIDIZED DNA

LIPID
PEROXIDATION HO ˙ O 2

ENZYME
INACTIVATION
Parent
Fe Compound
FAD NADP +
•– red
O Flavin
2 REDOX
Superoxide CYCLE Enzyme
Catalase Dismutase O 2 FAD ox NADPH + H +
H O + O 2 H O 2
2
2
O 2
Radical
Glutathione Metabolite
Peroxidase
GSH Pentose
NADP + Phosphate
GSSG Cycle
H O
2
COVALENT BINDING
NADPH + H + TO NUCLEIC ACIDS
METABOLISM COVALENT BINDING
TO PROTEINS
Pentose
Phosphate (ENZYME INACTIVATION)
Cycle
FIGURE 6.7 An overview of ROS generation by redox cycling, key enzymatic antioxidant defenses, and cellular targets
of ROS. (Adapted from Kappus, H., in Oxidative Stress, Sies, H., Ed., Academic Press, London, 1985, pp. 273–310.)

Chemicals can enhance ROS production by these chains in several ways. Planar halogenated aromatic
hydrocarbons (PHAHs) such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and certain polychlorinated
biphenyls (PCBs) that bind to the AhR and upregulate expression of genes for biotransformation enzymes
including CYP1A (see Chapter 4) have been observed to enhance ROS generation in mitochondria,
microsomes, and whole cells (Nebert et al., 2000; Park et al., 1996). In the case of TCDD-enhanced
H O production in mice mitochondria, this effect was found to be dependent on TCDD binding to the
2
2
AhR but independent of CYP1A activity (Senft et al., 2002). Studies with 3,3′,4,4′-PCB and 3,3′,4,4′,5-
PCB (PCB 77 and PCB 126, respectively, both established AhR ligands) in the marine fish scup
(Stenotomus chrysops) demonstrated the ability of these AhR agonists to potently induce CYP1A and
at higher doses to uncouple microsomal electron transfer, resulting in increased ROS production and
inactivation of CYP1A (Schlezinger and Stegeman, 2001; Schlezinger et al., 1999). In this case, the
production of ROS was thought to be associated with interactions of the active site of the enzyme with
these chlorinated chemicals that are recalcitrant to metabolism and thereby uncouple normal electron
flow to the substrate (i.e., PCB), resulting in ROS.
Inhibition of electron transport in mitochondria can also lead to production of ROS; blockage of the
flow of electrons leaves mitochondrial proteins in a highly reduced state, which can lead to the reduction
of oxygen and ROS production. Chemicals known to produce ROS by this mechanism include rotenone,
an insecticide (Li et al., 2003); antimycin A, an antibiotic frequently used to study inhibition of
mitochondrial electron transport (Chen et al., 2003); 1-methyl-4-phenylpyridinium (MPTP), a compound
used to study Parkinson’s disease (Smith and Bennett, 1997); certain oxidatively modified PAHs
(Tripuranthakam et al., 1999); and cadmium (Wang et al., 2004). In some cases, however, xenobiotic-
mediated uncoupling of electron transport in mitochondria can result in reduced production of ROS
(Kowaltowski and Vercesi, 1999; Turrens, 1997).