Introduction
Reactive oxygen species and reactive nitrogen species are generated as a result of cellular redox reactions in our body which involve utilization of oxygen as energy source. At low and moderate levels, they are highly beneficial for our body where as in high levels they generate oxidative stress which leads to development of several ailments such as cancer, ageing, cardiovascular diseases etc.When there is imbalance between production and neutrilization of free radicals oxidative stress arises. Free radicals cause severe tissue damage in the form of peroxidation of phospholipid membranes and also damage to DNA and protein. This oxidative stress can be counteracted by production of antioxidants either in situ or supplemented through foods/diets. These antioxidant systems act as free radical scavengers that work either by preventing the generation or repairing the damage caused by these free radicals.
Biological systems protect themselves against the damaging effects of activated species by several means. These antioxidant systems are classified as enzymatic and non enzymatic antioxidants. Glutathione (GSH), superoxide dismutase (SOD), catalase, glutathione peroxidase (GPx) are enzymatic systems that are important cellular defence system against oxidative stress. SOD and catalase are the most important enzymatic defence mechanisms against reactive oxygen species (ROS) toxic effects.
Role of enzymatic antioxidants
SOD is considered a primary defence as it prevents further generates free radicals by causing the dismutation of superoxide anion. Superoxide dismutase (SOD) is an enzyme that removes the superoxide (O2-) radical, repairs cells and reduces the damage done to them by superoxide, the most common free radical in the body. SOD is found in both the dermis and the epidermis, and is key to the production of healthy fibroblasts (skin-building cells). It plays a critical role in the defense of cells against the toxic effects of oxygen radicals. SOD competes with nitric oxide (NO) for superoxide anion, which inactivates nitric oxide to form peroxynitrite. Therefore, by scavenging superoxide anions, SOD promotes the activity of nitric oxide.
Catalase is a common enzyme found in living organisms. Its functions include catalyzing the decomposition of hydrogen peroxide to water and oxygen. Catalase has one of the highest turnover rates of all enzymes; one molecule of catalase can convert millions of molecules of hydrogen peroxide to water and oxygen per second. Catalase collaborates with the removal of H2O2 formed during the reaction catalyzed by SOD.
GSH is by far the most important antioxidant in most mammalian cells. This ubiquitous tripeptide, γ-Glu-Cys-Gly, performs many important cellular functions. In particular, the thiol containing moiety is a potent reducing agent. Reduced glutathione (GSH) normally plays the role of an intracellular radical scavenger and is the substrate of many xenobiotic elimination reactions. An early response to oxidative stress is the depletion of cellular soluble and protein bound thiols (glutathione-GSH) and vulnerable sulfhydryl group. GSH systems may have the ability to manage oxidative stress with adaptational changes in enzymes regulating GSH metabolism. As GSH is an important antioxidant molecule, its depletion leads to the increase of oxidative stress. GPx is a selenium-containing metalloenzyme that catalyses the oxidation of GSH by H2O2 to form water and oxidized glutathione. They are a group of selenium dependent enzymes. Four of its isoforms include Cytosolic GPx,Plasma GPx, Phospholipid hydroperoxide PHGPx and Gastrointestinal GPx-GI. All GPx require GSH as cofactor and secondary enzymes, such as glutathione reductase and glucose-6 phosphate dehydrogenase for proper functioning. G-6-PDH generates Total antioxidant status (TAS)/Total antioxidant activity (TAA)/Total antioxidant capacity (TAC) is measure of the amount of free radicals scavenged by a test solution. It indicates the ability of sampled individual to resist oxidative stress and can be used to evaluate the antioxidant capacity.
Lipoid acid, glutathione, L-ariginine, coenzyme Q10, melatonin, uric acid, bilirubin, metal-chelating proteins, transferrin are considered as metabolic non enzmatic antioxidants as they are produced during metabolic processes in the body. Vitamin E, vitamin C, carotenoids, trace metals (selenium, manganese, zinc), flavonoids, omega-3 and omega-6 fatty acids, etc are considered as nutrient non enzymatic antioxidants as they are supplemented through foods.
Role of non enzymatic antioxidants
Vitamin E is a fat-soluble and has pronounced antioxidant properties. They react more rapidly than polyunsaturated fatty acids with peroxyl radicals and hence act to break the chain reaction of lipid peroxidation. In addition to its antioxidant role, vitamin E might also have a structural role in stabilising membranes. The antioxidant function of vitamin E is to trap peroxyl radicals and to break the chain reaction of lipid peroxidation in cell membranes and lipoproteins. Vitamin E only reduces the formation of secondary radicals and doesn’t prevent the initial formation of carbon centred radicals in a lipid rich environment.
Vitamin C is an important water soluble antioxidant and it can be oxidized in the extracellular environment in the presence of metal ions to dehydroascorbic acid, which is transported into the cell through the glucose transporter. Its primary antioxidant partners are Vitamin E and the carotenoids as well as working alone with the antioxidant enzymes. Vitamin C along with Vitamin E raises glutathione levels in the cell by regenerating α-tocopherol from αtocopherol radicals in membranes and lipoprotein. It also protects protein thiol group against oxidation. In cells, it is maintained in its reduced form by reaction with glutathione, which catalyzes by protein disulfide isomerase and glutaredoxins. Vitamin C is a reducing agent and can reduce and thereby neutralize, ROS such as hydrogen peroxide.
Lipoic acid (LA) is readily digested, absorbed and is rapidly converted to Dihydrolipoic acid (DHLA) by NADH or NADPH in most tissues. DHLA neutralizes free radicals it is known to regenerate Vitamin C which is even better than GSH and Vitamin E from their oxidized forms. DHLA has metal chelating properties which help the body to get rid of accumulated ingested toxins.
Caretenoids are particularly efficient scavengers of singlet oxygen, but can also trap peroxyl radicals at low oxygen pressure. They prevent in vivo lipid peroxidation and are characterized with pro-vitamin A activity.
Thus these enzymatic and non-enzymatic antioxidants counteract the toxic effects of free radicals in body and an appropriate balance of enzymatic and non-enzymatic antioxidant defense is necessary for withstanding the destruction caused by the active oxygen species and maintaining the health of the animal.Thus these antioxidant systems play a major role in combating oxidative stress in the body and promotion of health and immunity.
Dr. Pallavi Khajuria, Dr. Gurpreet Singh and Dr. Chetna Mahajan
1 Assistant Professor, 2 Associate Professor, 3 Assistant Professor
Department of Veterinary Physiology and Biochemistry
College of Veterinary Science, Rampura Phul
Guru Angad Dev University of Veterinary and Animal Sciences