Free Radicals & Antioxidants- Current status

Dr. T.R.RAMANUJAM. M.D.,

Professor & Head, Dept of Pharmacology,
Sri Ramachandra Medical College & Research Institute,   
Porur ,   
Chennai - 600 116 
SOUTH INDIA. 

• Introduction
• What are Free radicals & Antioxidants
• Invivo origin of free radicals
• Sources of oxidative stress in human pathophysiology
• Major targets of oxidative activity
• Antioxidants
• General principles of Antioxidant therapy
• indications for Antioxidant therapy 
• Current status of Antioxidants

Introduction

Recent years have witnesses increased interest in the role of free radical oxidative damage in human diseases and aging. Free radical oxidative stress has a probable role in the pathogensis of variety of human disease which has led to the use of agents that can supplement the natural antioxidant defenses. Various natural antioxidant enzymes, vitamins and even synthetic agents with antioxidant enzymes, vitamins and even synthetic agents with antioxidant properties have a potential role in various conditions like Diabetes mellitus, inflammatory diseases, reperfusion injury (as in Myocardial injury), atherosclerosis cancer and Ageing as per the recent studies. Much interest has been shown on the antiaging effect of these antioxidants and extensive research in underway for agents which can simulate natural superoxide dismutase, Glutathione peroxidase, Glutathione reductase and Glutathione S transferse etc., The involvement of oxidative radical attack in ageind & in pathogensis leading to acceleration of aging such as Down’s syndrome, Alzheimers dis, atherosclerosis, Diabetes nellitus is highly probable. According to Ying Free radical damage constitute a part of deleterus network which also comprises of mitochondrial defects, glycation and Calcium dyshomeostasis. This review includes the essentials of understanding the important facts about the role of Oxidative radicals, possible role in human pathogensis of certain disorders, probable targets, the different types of antioxidants their probable indications etc., It is too Early to conclude the definitive benets of antioxidant therapy and the clinicians should exert restraint in over indulgence of prescribing of antioxidant therapy without any rationale. Further controlled trials are needed before concluding their definitive role.

What are Free radicals & Antioxidants

Free Radicals are energetically unstable atoms or molecules capable of independent existence containing one or more unpaired electrons in their outer orbitals which are highly reactive and short lived species whose stability is achieved by removal of electrons (i.e., Oxidation of) surrounding molecules to produce an electron pair.
However, the remainder of the attacked molecule then possess an unpaired electron and has therefore become a free radical by this way may initiate a chain sequence of electron transfer (Redox reaction) 

(The reaction of F.R. with non radical species)

SOURCE OF FREE  RADICALS:

In aerobic cells, incomplete reductions of  O₂  in the mitochodrial electron transport chain releases superoxide anion radicals in the cytosol. The superoxide readical is relaticely unreactive but may interact with trasition metal icons such as iron, Cu to produce highly reactive and damaging hydroxylated radicals. Powerful metal binding reactions exist to prevent its participation in redox reactions. The enzyme Xanthine oxidase may also a source of superoxide formation during reperfusion of ischemic tissues.

Inflammatory cells like macrophages, neutrophils produce H₂O₂  and hypochlorous acid as a means of bacterial killing, which may damage innocent bystander cells and be responsible for much of the damage associated inflammatory processes. Certain type of drug toxicity ex. Paracetamol induced hepatotoxicity is due to the promotion of Free radical formation during their metabolism and similarly cigratte smoking induces free radical formation by virtue of its Gas phase rich in free radicals.

Superoxide anion (O₂^.-) is the most widely produced Free Radical. It is a  negatively charged monoradical resulting from the monovalent reduction of molecular O₂

And essentially produced enzymatically by;

• NADPH oxidases (phagocytosis)
• Mit Cyt oxidase (cell respiration)
• Liver Cyt P₄₅₀ (oxid metabolism of Xenobiotics)
• Xanthine oxidase (ishemic reperfusion)

In the presence of protons , its dismutation leads to O₂ + H₂O₂

              2H⁺

2O₂            H₂O₂ + O₂

             SOD

This reaction is highly favoured by Superoxide dismutase.

Hydroxyl radical (^. OH) is also produced in tissues and several times more reactive than O2.- and leads to easily to the formation of new free radicals. It can be formed from H₂O₂

Which in the presence of Ferrous (Fe ⁺⁺) ions leads to OH^- and ^.OH, according to Fenton reaction.

X-Fe⁺⁺ + H₂O₂  X.Fe³⁺ + OH^- + ^.OH

H₂O₂   which is not a free radical has nevertheless has a high oxidative capacity via this reaction. H₂O₂ is able to cross biologic membrane and to induce cellular damages by a fenton reaction, resulting in the formation of ^.OH free radical.

Peroxyl (ROO^.) and alcoxyl (RO^.) free radicals are also synthesizes essentially from polyunsaturated fatty acids either in a direct and controlled way or in an indirect and uncontrolled way. Peroxyl (ROO^.) free radicals result from the action of oxyganases (COX or LOX i.e., Cycloxoygenase and Lipoxygenases) and lead to the eicosanoid pathway and in case of alcoxyl (RO^.) result from the action .OH, leading to the formation of an oxygen free radical ROO^. This constitutes the initial phase of Lipid peroxidation.

Invivo origin of free radicals

Oxygenated Free Radicals

O2.-	NADPH oxidation
	Cytochrome oxidases
	Cytochrome P450
	Xanthine oxidase
.OH	X-Fe2 + H2O2
ROO.	COX, LOX
	PUFA
RO.	ROOH

 

Free Radicals or  Pre Oxidants

Defense System

O2.- superoxide anion

Superoxide dismutaseMn SODCu, Zn SOD

.OH (Hydroxyl radical)

Vit C, Glutathoine Taurine, Uric acid

ROO. (peroxy radical)

Tocopherols Ubiquinone

1o₂ (singlet Oxygen)

Carotenoids

H2O2 (hydrogen Peroxide)

Catalase Se Glut peroxidase Glut reductase

ROO (hydroperoxides)

Se Glut peroxidase Glutathione reductase

Transition metals (Fe 2+, Cu+)

Chelators

The defense systems – In addition to the continuous production of free radicals, our body possesses several defense systems that are constituted of enzymes and radical scavengers. These latter ones are easily oxidisable compounds which are present either in the cytosol (e.g.) ascorbic acid and glutathione) or in membranes (alpha tocopharols) The above table sums up these defense systems and their protective mechanisms.

These defense systems, called first line defense systems are not totally efficient, since almost all cell components undergo free radical damages. Therefore second defense systems are involved. They are constituted of repair systems for biomolecules that have been damaged by radical attacks.