A free radical is an atom, molecule, or ion with at least one unpaired electron in its outermost shell. This configuration makes them highly unstable and reactive, as they constantly seek to gain or lose an electron to achieve stability. Among various free radicals, the hydroxyl free radical (OH•) stands out due to its exceptional reactivity and significant impact on biological systems.
What are Hydroxyl Free Radicals?
The hydroxyl radical is a specific type of free radical composed of one oxygen atom and one hydrogen atom, with an unpaired electron on the oxygen atom. Unlike other reactive oxygen species (ROS) such as hydrogen peroxide, it is not easily detoxified by enzymatic reactions within the body due to its extremely short half-life, typically less than a nanosecond (10^-9 seconds). This brief existence means it reacts almost immediately with any oxidizable compound in its immediate vicinity, rather than diffusing to an enzyme for neutralization. Its reactivity can initiate chain reactions that damage living tissues.
How Hydroxyl Free Radicals Form
Hydroxyl free radicals originate from both internal (endogenous) and external (exogenous) sources. Inside the body, they frequently arise as byproducts of normal cellular metabolism, especially during cellular respiration. Inflammatory responses also lead to their formation, as immune cells generate these compounds when combating pathogens.
Outside the body, exposure to various environmental elements induces their formation. Ionizing radiation, such as X-rays or gamma rays, can directly break down molecules to create these radicals. Air pollution, including volatile organic compounds, nitrogen oxides, certain chemicals, and tobacco smoke, also contribute. Additionally, the Fenton reaction, involving trace amounts of reduced transition metals, can catalyze hydrogen peroxide breakdown into hydroxyl radicals.
Effects on the Body
The uncontrolled actions of hydroxyl free radicals in the body lead to oxidative stress, an imbalance between the production of reactive species and the body’s ability to neutralize them. Hydroxyl radicals inflict damage on various biological macromolecules, fundamentally altering their structure and function.
They harm DNA, causing mutations and strand breaks that compromise genetic integrity. This DNA damage can disrupt normal cell processes and is linked to an increased risk of conditions like cancer. Proteins are also vulnerable, leading to loss of function and impaired cellular processes. Lipids, particularly those found in cell membranes, undergo lipid peroxidation when exposed to hydroxyl radicals, damaging the membrane structure and compromising cell integrity. This molecular damage contributes to broader physiological consequences, influencing aging and the development of chronic health conditions such as cardiovascular disease and neurodegenerative disorders.
Antioxidant Systems
The body has internal defense mechanisms to neutralize hydroxyl free radicals and other reactive oxygen species. These endogenous antioxidant systems include enzymes such as superoxide dismutase, which converts superoxide radicals into hydrogen peroxide, and catalase, which then breaks down hydrogen peroxide into water and oxygen. Glutathione peroxidase is another enzyme that plays a role in reducing oxidative stress by converting hydrogen peroxide into water.
Beyond these enzymatic defenses, dietary antioxidants also contribute to the body’s protective measures. Compounds like vitamin C, vitamin E, glutathione, and various flavonoids found in fruits and vegetables act by donating an electron to the free radical, thereby neutralizing its reactivity and preventing it from causing molecular damage. This combined approach of internal enzymatic systems and external dietary antioxidants helps maintain balance and supports cellular health.