INTRODUCTION

An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions that damage cells. Antioxidants terminate these chain reactions by removing free radical intermediates, and inhibit other oxidation reactions. They do this by being oxidized themselves, so antioxidants are often reducing agents such as thiols, ascorbic acid or polyphenols.

Although oxidation reactions are crucial for life, they can also be damaging; hence, plants and animals maintain complex systems of multiple types of antioxidants, such as glutathione, vitamin C, and vitamin E as well as enzymes such as catalase, superoxide dismutase and various peroxidases. Low levels of antioxidants, or inhibition of the antioxidant enzymes, cause oxidative stress and may damage or kill cells.

An antioxidant is a substance that when present in low concentrations relative to the oxidizable substrate significantly delays or reduces oxidation of the substrate. Antioxidants get their name because they combat oxidation. They are substances that protect other chemicals of the body from damaging oxidation reactions by reacting with free radicals and other reactive oxygen species within the body, hence hindering the process of oxidation. During this reaction the antioxidant sacrifices itself by becoming oxidized.

ENZYMES

VITAMINS

Antioxidant Role

Superoxide dismutase (SOD)

Mitochondrial Cytoplasmic

Extracellular

Dismutates O2• to H2O2

Catalase Dismutates H2O2 to H2O

Glutathione peroxidase (GSH.Px) Removes H2O2 and lipid peroxides

Alpha tocopherol Breaks lipid peroxidation

Lipid peroxide and O2• and •OH scavenger

Beta carotene Scavenges •OH, O2•and peroxy radicals

Prevents oxidation of vitamin A

Binds to transition metals

Ascorbic acidDirectly scavenges O2•, •OH, and H2O2

Neutralizes oxidants from stimulated

neutrophils

Contributes to regeneration of vitamin E

CLASSIFICATION OF ANTIOXIDANTS  

ANTIOXIDANTS IN BIOLOGICAL SYSTEM

1. ANTIOXIDANTS IN RELATION TO LIPID PEROXIDATIONA. PREVENTIVE OXIDANTSE.g. catalase, glutathione peroxideB. CHAIN BREAKING ANTIOXIDANTSE.g. superoxidase dismutase, vitamiv –e, uric acid2. ANTIOXIDANTS ACCORDING TO THEIR LOCATIONA. PLASMA ANTIOXIDANTSE.g. beta carotene, ascorbic acid, bilirubin, uric acid, ceruloplasmin,

transferringB. CELL MEMBRANE ANTIOXIDANTSE.g. alpha-tocopherol3. INTRACELLULAR ANTIOXIDANTSE.g. sod, catalase, glutathione peroxidase4. ANTIOXIDANTS ACCORDING TO THEIR NATURE AND

ACTIONA. ENZYMATIC ANTIOXIDANTSE.g. sod, catalase, glutathione peroxidase, glutathione reductase

B. NON-ENZYMATIC ANTIOXIDANTS

1. NUTRIENT ANTIOXIDANTSE.g. Carotenoids (beta-carotene), alpha-tocopherol, ascorbic acid,

selenium2. METABOLIC ANTIOXIDANTSE.g. glutathione, ceruloplasmin, albumin, bilirubin, transferring,

ferritin, uric acid

DRUGS HAVING ANTIOXIDANT PROPERTIES Xanthine oxidase inhibitor (allopurinol, folic acid) NADPH inhibitors (adenosine, calcium channel blockers) Inhibitorsofironredoxcycling(deferoxmine,apotransferin,ceruloplasmi

n) Some Non-steroidal anti-inflammatory agents Oral anti-diabetic agents (8) like (metformin, gliclazide & troglitazone) Statins (9) (atorvastatin, simvastatin, pravastatin & rosuvastatin) Probucol

MODE OF ACTION OF ANTIOXIDANTS

There are four routes:

Chain breaking reactions, e.g. alpha-tocopherol which acts in lipid phase to trap "ROD" radical.

Reducing the concentration of reactive oxygen species e.g. glutathione.

Scavenging initiating radicals e.g. superoxide dismutase which acts in aqueous phase to trap superoxide free radicals.

Chelating the transition metal catalysts: A group of compounds serves an antioxidant function by sequestration of transition metals that are well-established pro-oxidants. In this way, transferrin, lactoferrin, and ferritin function to keep iron induced oxidant stress in check and ceruloplasmin and albumin as copper sequestrants.

ANTIOXIDANT SYSTEM

Enzymatic and non enzymatic oxidant system

The enzymatic antioxidants include the Superoxide dismutase (SOD), which catalyses the conversion of O2

⁻ to H2O2 and H2O; catalase, which then converts H2O2 to H2O and O2; and glutathione peroxidase, which reduces H2O2 to H2O.

The nonenzymatic antioxidants include the lipid-soluble vitamins, vitamin E and vitamin A or provitamin A (beta-carotene), and the water-soluble vitamin C and GSH. Vitamin E has been described as the major chain-breaking antioxidant in humans

ANTIOXIDANT PROTECTION SYSTEM

ENDOGENOUS ANTIOXIDANTS Bilirubin Thiols, e.g., glutathione, lipoic acid, N-acetyl

cysteine NADPH and NADH Ubiquinone (coenzyme Q10) Uric acid Enzymes:

copper/zinc and manganese-dependent superoxide Dismutase (SOD)

iron-dependent catalase selenium-dependent glutathione peroxidase

DIETARY ANTIOXIDANTS Vitamin C Vitamin E Beta carotene and other carotenoids and oxycarotenoids e.g., lycopene and lutein Polyphenols e.g., flavonoids, flavones, flavonols, and Proanthocyanidins METAL BINDING PROTEINS Albumin (copper) Ceruloplasmin (copper) Metallothionein (copper) Ferritin (iron) Myoglobin (iron) Transferrin (iron)

ROS NEUTRALIZINGANTIOXIDANTS Hydroxyl radical - vitamin C, glutathione, flavonoids, lipoic

acid Superoxide radical -vitamin C, glutathione, flavonoids, SOD Hydrogen peroxide - vitamin C, glutathione, beta carotene,

vitamin E, CoQ10, flavonoids, lipoic acid Lipid peroxides - beta carotene, vitamin E, ubiquinone,

flavonoids, glutathione peroxidase

MECHANISM OF ACTION OF SOD

2O2 + 2H + SOD ---------- H⁻ ⁺ 2O2 + O2

SOD is considered fundamental in the process of eliminating ROI by reducing (adding an electron to) superoxide to form H2O2. Catalase and the selenium-dependent glutathione peroxidase are responsible for reducing H2O2 to H2O.

MECHANISM OF ACTION GLUTATHIONE PEROXIDASE

Glutathione peroxidase reduces H2O2 to H2O by oxidizing glutathione (GSH) (Equation A). Rereduction of the oxidized form of glutathione (GSSG) is then catalysed by glutathione reductase (Equation B). These enzymes also require trace metal cofactors for maximal efficiency, including selenium for glutathione peroxidase; copper, zinc, or manganese for SOD; and iron for catalase.

H2O2 + 2 GSH -------------- GSSG + 2 H2O (equation A)GSSG + NADPH + H+ ----------------- 2 GSH + NADP+ (equation B)

MECHANISM OF ACTION CATALASE Catalase is a common enzyme found in nearly all living

organisms that are exposed to oxygen, where it functions to catalyze the decomposition of hydrogen peroxide to water and oxygen. Catalase has one of the highest turnover numbers of all enzymes; one molecule of catalase can convert millions of molecules of hydrogen peroxide to water and oxygen per second.

.

.

ANTIOXIDANT VITAMINS

MECHANISM OF ACTION

Alpha tocopherol (vitamin E) ASCORBIC ACID (VITAMIN C) BETA CAROTENE FERRITIN

ALPHA-LIPOIC ACID

OTHER ANTIOXIDANTS RETINOIDS CoQ10: Uric acid: ALBUMIN:

SELENIUM

BILURUBIN

ANTIOXIDANTS IN HYPERGLYCEMIA AND DIABETES

Diabetes is a metabolic disorder with multiple etiology, characterized by hyperglycemia with disturbances of protein, fat, carbohydrate metabolism resulting from inadequate secretion of insulin and inadequate distribution of insulin to target cell or both. In the hyperglycemic condition glucose toxicity, glucose auto oxidation, oxidative phosphorylation will occur and leads to reactive oxygen species and finally free radicals are formed.

These free radicals will involve oxidative stress, there by free radicals lowers the activity of enzymes called catalase, superoxide dismutase and glutathione peroxidase and pancreatic beta cell destruction takes place and leads to diabetic complications. Supplementation of antioxidants exogenously will lead to fight against free radicals and lowers or inhibit their activity. These antioxidants there by involving normal functioning of pancreas and leads to insulin secretion and avoiding diabetic complication. Simply by administration of antioxidants will normalize hyperglycemic condition and avoiding diabetic complications.

GLUCOSE TOXICITY GLUCOSE AUTOOXIDATION METABOLISM OF GLUCOSAMINE OXIDATIVE PHOSPHORYLATIONSHIP MALONDIALDEHYDE FFAS

THE ISLET, ANTIOXIDANT ENZYMES AND GSH METABOLISM

The generation of ROS per se seems likely to be a physiological event that contributes to the regulation of general metabolic health, such as mediating phagocyte killing of bacteria. Another physiological function is to modulate transcription of genes, such as nuclear factor-B (NF-B), a well-known redox-sensitive transcription factor. However, the elegantly designed host-defense mechanisms against ROS provided by antioxidant enzymes and GSH synthesis imply that ROS levels need to be finely regulated to keep good radicals from going bad and thereby avoid oxidative damage to cellular processes.

VITAMINS IN DIABETES Administration of the antioxidants, for example the vitamins and

along free amino acids, gets a better reaction to insulin and can supply extra benefits to the proposed reduction of oxidative stress in tissues. All vitamins are involve to increase in insulin action was mainly due to an improvement in non-oxidative glucose metabolism.

The alpha tocopherol (vitamin) is to prevent the peroxidation of membrane phospholipids and avoids pancreatic cell membrane damage through its antioxidant activity. It also decreases the incidence of cataract. The ascorbic acid (vitamin-c) is a water – soluble chain breaking antioxidant and it scavenges free radicals and reactive oxygen molecules which are produced during metabolic pathways of detoxification. Moreover ascorbic acid alone can act as a “prooxidant” or reducing agent to react with copper or iron salts. Ferric iron formed by the reaction, Fe2++ H2O2

HO+ ∙OH + Fe3+ are converted by ascorbic acid to ferrous ion. Ferrous ion is therefore recycled to promote the conversion of more hydrogen peroxide to ∙OH.

The antioxidant function of beta carotene is due to its ability to quench singlet oxygen, scavenge free radicals and protect the cell membrane lipids from the harmful effects of oxidative degradation. The quenching involves a physical reaction in which the energy of the excited oxygen is transferred to the carotenoids, forming an excited state molecule.

Alpha lipoic acid is also called lipoic acid or thioctic acid, is a powerful antioxidant that helps cells convert glucose to energy, detoxifies the body and helps stabilize blood sugar. It has been called as “universal antioxidant” because it is both water and fat soluble and thus can penetrate tissues. It helps increase the body’s supply of glutathione, the most abundant natural antioxidant so that free radicals are escorted out of the body before they can damage to cells.

ROLE OF SUPEROXIDE DISMUTASE, GLUTATHIONE PEROXIDASE, CATALASE

In the hyperglycemic condition ROS formation takes place and free is generated and leads to destroy pancreatic beta cells and may leads to diabetic complications. The role of SOD is eliminating reactive oxygen intermediate (ROI) by reducing superoxide to form hydrogen peroxide and catalase, selenium-dependent glutathione peroxide are responsible for reducing hydrogen peroxide to water.

The role of glutathione peroxide reduces hydrogen peroxide to water by oxidizing glutathione. Rereduction of the oxidize form of glutathione is then catalysed by glutathione reductase.

ROO-, H2O2, 1O2, NO, ONOO-, HOCl

The catalase role in diabetes is to catalase the decomposition of hydrogen peroxide to water. Catalase has one of the highest turn over numbers of all enzymes, one molecule of catalase can convert millions of molecules of hydrogen peroxide to water and oxygen per second.

SELENIUM It is a natural antioxidant which protects pancreatic beta cell by

supporting the body’s natural defenses and scavenging harmful free radicals. Selenium greatly reduce occurrence of diabetic complications.

ANTIOXIDANTS ROLE IN DIABETIC COMPLICATIONS Anti-oxidants are involved to reduce diabetic complications because they

are involved to fight or destroy free radicals. The antioxidants which are scavenges are oxidants or free radicals. These free radicals in hyperglycemia by glucose toxicity, glucose auto oxidation and oxidative phosphorylation produces ROS and releases free radicals and these are involved in oxidative stress and leads to destroy the pancreas. The beta cells are inhibited by their function and destroyed by free radicals.

The above process will prone to diabetic complications so the antioxidants role here is to inhibit activity of free radicals and avoiding diabetic long-term complications.

CONCLUSION

In the hyperglycemic state elevation of ROS (reactive oxygen species) by glucose toxicity, glucose autooxidation, oxidative phosphorylation which forms free radicals and these involved in oxidative stress. These free radicals destroy or dysfunction of pancreatic beta cells and leads to diabetic complications like retinopathy, neuropathy, nephropathy, and cardiopathy. Antioxidant plays an important role to prevent diabetic complications because these are fight or destroys free radicals or oxidants and also enhances the insulin secretion and insulin sensitization. So antioxidants like catalase, glutathione peroxide, superoxidase dismutase, albumin, bilirubin, uric acid, selenium along with vitamins are very much necessary for avoid long-term diabetic complications.