The Antioxidant Enzymes Network of The Skin

The physiological cellular respiration located at mitochondria is always comes with the byproduct of the aerobic metabolism where a small fraction of the oxygen is constantly converted to superoxide anions, hydrogen peroxide, hydroxyl radicals, and other ROS. Although within a certain local concentration range, ROS play important roles in regulating many cellular functions and acting as a secondary messenger to activate specific transcription factors such as NF-κB and AP-1. Aging-associated respiratory function decline can result in enhanced production of ROS in mitochondria. An excess production of ROS, particularly with aging, is harmful to cells that ROS or free radicals can damage proteins, DNAs, cell membranes. To cope with the ROS, body have an internal mechanism of protection and is equipped with a network of protective antioxidants. They include enzymatic antioxidants such as glutathione peroxidase, superoxide dismutase, and catalase, and nonenzymatic low-molecular-weight antioxidants such as vitamin E isoforms, vitamin C, glutathione (GSH).

The antioxidant enzymes network (of the skin) include superoxide dismutase (SOD), catalase (CAT), glutathione system of enzymes, peroxiredoxins, thioredoxin system and methionine reductase. Most antioxidant enzymes are redox enzymes that the active form is an efficient reducing agent for scavenging reactive oxygen species after which the active form become oxidized by ROS and is inactivated. To regenerate the active form of the enzyme, the enzyme system usually has a corresponding reductase (e.g. thioredoxin reductase, glutathione reductase) to regenerate the active enzyme. The activities and concentrations of these antioxidant enzymes and the concentrations of small-molecular-weight antioxidants in blood and tissue cells are altered (mostly declined) in the aging process.

Superoxide dismutases (SODs) are a class of closely related enzymes that catalyze the conversion of the ROS – superoxide anion into oxygen and hydrogen peroxide which is then transformed to water by glutathione peroxidases or by catalase. SOD enzymes are present in almost all cells and in extracellular fluids. Superoxide dismutase enzymes (SOD) require metal ion as cofactors which depend on which isoform it is. Metal ion cofactors can be copper, zinc, manganese or iron. The copper/zinc SOD is present in the cytosol while manganese SOD is present in the mitochondrion. The mitochondrial isozyme seems to be the most biologically important of these three. Copper/zinc SOD (SOD1) may be less important for preserving the longevity.

Catalases are enzymes that catalyze the conversion of hydrogen peroxide (the byproduct of the reaction by SOD) to water and oxygen, using either an iron or manganese cofactor. This protein is localized in peroxisomes in most cells. Catalase is an unusual enzyme since, although hydrogen peroxide is its only substrate, its cofactor is oxidized by one molecule of hydrogen peroxide and then regenerated by transferring the bound oxygen to a second hydrogen peroxide molecule. Despite its apparent importance in hydrogen peroxide removal, humans with genetic deficiency of catalase suffer few ill effects. The natural interaction – synergy – between superoxide dismutases and catalases constitute body’s the most effective system of free radical control .

The glutathione system of antioxidant enzymes include glutathione peroxidases (GPx), glutathione reductase, and glutathione ”S”-transferases. Glutathione peroxidase is another of the body’s major protectors against free radicals – peroxide. The effects of excess cellular peroxidation is diverse and detrimental and must be limited to maintain cellular health. This antioxidant enzyme consists of the amino acid Glutathione and the trace mineral ‘Selenium’. Glutathione peroxidase is an enzyme containing four selenium-cofactors that catalyzes the breakdown of hydrogen peroxide and organic hydroperoxides. Cell membranes consist primarily of lipids which are very susceptible to damage by free radicals, especially peroxide radicals. Lipid peroxides have proven to be toxic. Glutathione peroxidase prevents destruction of cell membranes by removing several classes of lipid peroxides. There are at least four different glutathione peroxidase isozymes. Glutathione peroxidase 1 is the most abundant and is a very efficient scavenger of hydrogen peroxide, while glutathione peroxidase 4 is most active with lipid hydroperoxides. Glutathione ”S”-transferases are particularly abundant in the liver and has strong activity with lipid peroxides removal. Glutathione peroxidases are used primarily to prevent skin related diseases and skin aging. It seems that glutathione peroxidase 1 is dispensable and does not adversely affect life span if it is deficiency.

Peroxiredoxins (Prxs) are peroxidases that catalyze the reduction of hydrogen peroxide, organic hydroperoxides, as well as peroxynitrite. They are divided into three classes: typical 2-cysteine peroxiredoxins; atypical 2-cysteine peroxiredoxins; and 1-cysteine peroxiredoxins. These enzymes share the same basic catalytic mechanism, in which a redox-active cysteine (the peroxidatic cysteine) in the active site is oxidized to a sulfenic acid by the peroxide substrate. Over-oxidation of this cysteine residue in peroxiredoxins inactivates these enzymes, but this can be reversed by the action of sulfiredoxin. Peroxiredoxins seem to be important in antioxidant metabolism and decelerate aging.

The thioredoxin system contains the 12-kDa protein thioredoxin and its companion thioredoxin reductase. The active site of thioredoxin consists of two neighboring cysteines, as part of a highly conserved CXXC motif, that can cycle between an active dithiol form (reduced) and an oxidized disulfide form. In its active state, thioredoxin acts as an efficient reducing agent, scavenging reactive oxygen species (ROS) and maintaining other proteins in their reduced state. After being oxidized, the active thioredoxin is regenerate d by the action of thioredoxin reductase, using NADPH as an electron donor.

Methionine reductase is a unique enzyme that has the ability to remove an extremely toxic free radical known as “Hydroxyl Radical”. The hydroxyl radical is commonly formed through reactions involving heavy metals and other less toxic free radicals, such as mercury reacting with hydrogen peroxide. The hydroxyl radical can damage any type of organic tissue and is considered to be the most dangerous free radical. Hydroxyl radicals are also the main toxins generated by exposure to excessive radiation. With their ability to damage any type of tissue, symptoms directly related to hydroxyl radical induced tissue damage are difficult to identify. It seems that hydroxyl radicals are also formed during exercise in oxygen starved closed rooms or in an auto exhaust filled polluted environments.

Generally, the activities and capacities of antioxidant systems of tissue cells (of the skin) are declined with age, resulting in the accumulation of oxidative damage in the aging process. Although many researchers studied aged-related changes in body’s natural antioxidant defense system, not all results are consistent. The activity of MnSOD located in the mitochondria was found to increase significantly during aging in various tissues. Increased MnSOD resulted in decrease in mitochondrial mass, accumulation of intracellular hydrogen peroxide, and induction of mRNA levels of matrix metalloprotainase-1, – a matrix degrading enzyme that degrade collagen or elastin fibers/extracellular matrix fibers in the skin connective tissue. This suggests that even though superoxide anions produced by mitochondria aerobic metabolism may be scavenged by MnSOD, but hydrogen peroxide thus accumulated in mitochondria may still increase oxidative stress during the aging process. Properly regulated level of of MnSOD is important for cells to cope with oxygen radical-mediated molecular and cellular damage during the aging process. Alterations in the levels of Cu/ZnSOD, CAT, and GPx are also important factors in the aging process. Research indicate that appropriate relative amounts of free radical scavenging enzymes is important for the cellular resistance to oxidative stress. Nutritional programs that contain nutritional factors effective in enhancing the activity of antioxidant enzyme systems have proven to preserve longevity and prevent skin aging.

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