Chapter 25: Oxygen Toxicity and Free Radical Injury

Reactive oxygen species including superoxide anion, hydrogen peroxide, and hydroxyl radicals are continuously produced during normal aerobic metabolism, enzyme-catalyzed reactions, and exposure to external stressors such as ionizing radiation and environmental toxins. These unstable molecules damage critical cellular components including membrane lipids through chain-reaction peroxidation, protein structures through amino acid modification, and nuclear and mitochondrial DNA through direct chemical attack, contributing to aging processes and various pathological conditions. Reactive nitrogen-oxygen species form when nitric oxide, an important signaling molecule, reacts with superoxide and other radicals, amplifying oxidative stress and extending cellular injury beyond traditional free-radical mechanisms. The chapter details the body's multilayered antioxidant defense system, which includes enzymatic defenses such as superoxide dismutase that catalyzes superoxide conversion, catalase that degrades hydrogen peroxide, and glutathione peroxidase that neutralizes organic peroxides, complemented by dietary antioxidants including vitamins E and C, carotenoids, and flavonoids that scavenge free radicals. Clinical applications demonstrate how oxidative damage underlies neurodegenerative diseases like Parkinson's disease and amyotrophic lateral sclerosis, cardiac ischemia-reperfusion injury that paradoxically worsens heart tissue following restoration of blood flow, chronic inflammatory conditions, and age-related retinal degeneration. Understanding the balance between oxidative stress and antioxidant capacity provides crucial insights into disease mechanisms and therapeutic strategies targeting free-radical-mediated pathology.