Free Radicals: Formation, Effects and Defenses against It

Read this article to learn about Free Radicals. After reading this article you will learn about: 1. Formation of Free Radicals 2. General Effects of Free Radicals 3. Defences against free Radical Attack.

Formation of Free Radicals:

Free radicals are basically formed as accidental and deliberate by-products and phagocytosis from oxygen, H₂O₂, etc. Some of them are superoxide (O₂⁻), hydroxyl (OH^.), etc.Independent on the identity of the cel­lular O₂-activator, the initial reduction product is always a super-oxide radical.

SOD, which is present in all cells, destabilizes O₂⁻ by a diffusion limited dis-mutation of two O₂^−• to H₂O₂ and O₂ (Figure 31.1). Due to cellular compartmentation of SOD, O₂ radicals are not expected to be total­ly metabolized to H₂O₂.

This is also the case with H₂O₂ which is a substrate for both catalase and glutathione-peroxidase (GSH- POD). H₂O₂ which escapes detoxification by the latter enzymes may become subject to the trans­fer of a further electron thereby forming OH^• rad­icals; these are the most reactive metabolites known in biological systems.

Due to its extremely high reaction potential, diffusion of OH^• from its site of generation can be excluded. Consequently respective reaction prod­ucts are expected to be formed in close proximity to OH^• generation sites. The most frequent reac­tion product stems from an interaction of OH^• with membrane-phospholipids.

The first step in this re­action sequence is a hydrogen-abstract from the divinyl-methane structure of polyunsaturated fat­ty acid and the subsequent addition of an O₂-molecule, resulting in the formation of peroxylradical.

This peroxyl-radical can enter a chain reac­tion forming an alkoxylradical by a bimolecular reaction and O₂ in an excited state (singlet state). Vitamin E in cooperation with Vitamin C is able to terminate self-sustaining chain reaction either by a direct interaction with OH^• or by the chemi­cal reduction of propagating organic radical in­termediates.

The excited state of singlet oxygen (¹O₂) is unstable and will lead to photo-emission by mono- or dimol reactions. Alternatively, the energy can be transmitted directly to other mole­cules such as carbonyl groups, which are in turn transferred into an excited state (singlet or triplet state).

Presence of transition metals is essential for free radical formation in most cases. Antioxidants present in foods counter the action of oxygen free radicals which are produced under a number of circumstances. They are also formed as a part of body’s normal metabolic process. Synthetic (xenobiotic) chemi­cals, radiation, x rays, pollution and even stress can produce these damaging entities.

The chemi­cals known to produce free radicals include chlo­rinated hydrocarbons, aromatic hydrocarbons, industrial acids, solvents, most pesticides and her­bicides, preservatives in foods, printing pigments and inks and other industrial chemicals, fragrances and perfume vehicles, cosmetic vehicles and cos­metics, pollutants in air and water, many if not all pharmacological agents used in medicine and anesthetics, which have a profound effect in pro­ducing radicals in the central nervous system.

Even the transitional metal catalysts iron and copper, which are ubiquitous, have a most powerful gen­erating effect on chain initiating radicals. Chemi­cal mobilization of fat stores under various condi­tions such as lactation, exercise, fever, infection and even fasting, can result in increased radical activity and damage in particular to the immune and nervous systems.

Under conditions of continuing and excessive emotional stress, higher levels of the hormones adrenaline and nor adrenaline are secreted by the adrenal glands.

As a natural part of their metabol­ic processing, these stress hormones are oxidized to simpler molecules and in doing so become free radicals. It is possible, through this increased pro­duction of hormone radicals that stress increased biological degenerative processes occur resulting in wide-spread molecular, cell and tissue damage.

In the living organism, free radical chain re­actions are produced normally in the mitochon­drial respiratory chain, liver mixed function oxi­dases, by bactericidal leucocytes, through xanthene oxidase activity, by atmospheric pollutants, and from transitional metal catalysts, drugs and xenobiotic chemicals.

Superoxide is formed by the reaction of oxy­gen with an electron.

O₂ + e⁻ →O₂^−•

Superoxide on reaction with hydrogen per­oxide yields hydroxyl radical with the liberation of oxygen.

O₂^−• + H₂O₂ → OH^• + OH⁻ + O₂

Oxygen on reaction with 2 electrons yields hydrogen peroxide.

O₂ + 2e⁻ + 2H⁺ →H₂O₂

Hydrogen peroxide is itself uncharged but is called reactive oxygen species (ROS) due to its highly reactive nature. Two superoxide ions may also react together to yield hydrogen peroxide and oxygen. This is known as dis-mutation reaction.

20₂^−• + 2H⁺ →H₂O₂+O₂

Hydrogen peroxide in the presence of transi­tion metals like Fe²⁺ forms hydroxyl radical which is known as Haber Weiss reaction.

H₂O₂+ Fe²⁺ —> OH^• +OH⁻+ Fe³⁺

General Effects of Free Radicals:

Free radicals decompose enzymes, sequester pro­teins, scavenge different metabolic compounds. But they are hard to study due to their short life span. Their life-span may range from a few nano­seconds to one or two seconds only.

However during the period of time they are present, they can do untold damage to biological systems in­cluding DNA, macromolecules such as proteins, lipids and intracellular organelles such as mito­chondria, Golgi apparatus and lysosomes.

They are also induced by several external agents. A clas­sic example is carbon tetrachloride (CCI₄). It is believed that CCI₄ causes hepatic toxicity by free radical attack. Lipids are most sus­ceptible to free radical attack. They cause exten­sive lipid peroxidation. Lipids are present in the cell membrane in the form of poly unsaturated fatty acids or PUFA.

Lipid peroxidation triggers a chain of degenerating events. It directly leads to decomposition of membrane. Indirectly, they lead to formation of other highly reactive harmful al­dehydes. Peroxidation is also believed to be responsible for atherosclerosis.

It has less effect directly on proteins. But formation of lens crystalline proteins in the eye by peroxidation is believed to be caused by peroxidation of lipid leading to cataract.

It has a direct and more expansive effect on DNA which may be the cause of cancer. Its ef­fect on DNA may be considered to be causing indirect effect on the protein constituents of the cell.

Defences against Free Radical Attack:

Defences against the free radical attack may be basically categorized into two:

1. Prevent the production of free radicals.

2. Intercept or scavenge the produced free radi­cals.

One way of preventing the production of free radicals is to sequester transition mental ions required for their formation. Hydrogen peroxide, li­pid peroxides formed during lipid peroxidation may be removed by catalase and glutathione per­oxidase (GSH) enzymes found in peroxisomes and cytosol respectively leading to prevention of fur­ther damage.