Oxidative stress and Antioxidant Defense Mechanisms in Dentistry: A Literature Review.

This review paper aimed to evaluate the effect of oxidative stress in the prevalence of some dental diseases and the role of antioxidants in dentistry. Oxidative stress is a normal phenomenon, present in healthy human body help them to produce energy that they need for their life. During that, the formation of free radicals begins, including both reactive oxygen species "ROS" & also reactive nitrogen species "RNS". These radicals have a role in a many types of signalling pathways that regulate inflammation. Antioxidants constrain the formation of free radicals. Mitochondria, a small cell organelle acts to generate energy in the form of adenosine triphosphate "ATP". It combines oxygen & glucose to form carbon dioxide, water, & ATP. Free radicals result as by-products of this metabolic process & when natural defence apparatuses be unsuccessful, the level of free radicals begins to increase to levels where the capacity of the body is unable to neutralize them entering the body into the state of high oxidative stress, which means greater risk of numerous oral & systemic diseases. Oral tissues are liable to free radical damage in a unique manner due to the presence of mucus membranes that allow rapid absorption of constituents throughout their surfaces. In oral environment, infection of gingiva can produce oxidative stress. The rise in formation of free radicals by oxidative stress results in further destruction of cellular walls and oral tissues. The study results suggested that the use of antioxidants is an adjunct approach to dental therapy may be helpful in controlling the oral health status.


INTRODUCTION
In the healthy human body, oxidative stress is a normal phenomenon that helps to produce the energy necessary for their life (1) . Free radicals "a necessary evil" play an important role in life. They result, as byproducts, including reactive oxygen species "ROS" and reactive nitrogen species "RNS" triggering different signaling pathways inside the cell. (2) A small cell organelle called mitochondria acts to combines oxygen & glucose to form adenosine triphosphate "ATP", carbon dioxide and water (3) .
The levels of free radicals begin to increase where the body becomes unable to neutralize them, when an equilibrium is lost & when natural defense apparatuses failed. Certain people have insufficient micronutrient, and/or conditions with may increase the need "e.g., infection, stress, and pollution" will cause an additional reduction in their stores. (1) So bodies will enter into the state of high oxidative stress, which means higher risk of oral & systemic diseases. (4,5,6) . In order to scavenge the harmful effects of these free radicals, the body introduces variable mechanisms that will neutralize the elevated levels of free radicals and keep the cells safe against their toxic effects and help in prevention of diseases. (2) Substances that able to neutralize free radicals are called antioxidants. They do that by mean of donating an electron. The destructive effects of oxidative stress are usually coming from free radicals of oxygen which can be altered by enzymatic and / or non-enzymatic antioxidants. Very well-known examples of these antioxidants are vitamins (7,8) .

Salivary Antioxidants System:
The antioxidant guard consists of cellular protective enzymes and non-enzymatic antioxidant agents. Cells have an antioxidant defensive system for avoiding the excessive production of free radicals, besides establish repair of injury caused by free radicals.  Mechanisms of reactive oxygen species that cause tissue damage include: (31) a. Pro-inflammatory cytokines stimulation

Categorization of Antioxidants:
There are three classes of antioxidants: The primary one which control prevention of oxidants development; secondary antioxidants which act as scavengers of reactive oxygen species, and tertiary one which act as oxidized molecules renewal agents. They can also be divided as: (33) 1-Enzymatic antioxidants injuries. (35,36) Clinical studies showed that diseases of periodontium are associated with high lipid peroxidation in saliva (37) . ROS formed by established that they may be helpful in the disease investigations, that oxidative stress has a major role in its aetiology. (40) Oxidative Stress and dental caries: Dental caries is widely distributed worldwide. Reviews concern with the aetiology, pathology, and treatment of dental caries were available before. (41) Recently, the concept of oxidative stress is strongly connected with an inflammatory response in dentin, leading to teeth destruction. The action of oxygen free radicals and antioxidants on parotid gland was suggested in the theory of dental caries (42) . The relation between oxidative stress and dental caries is less obvious. Researches were mostly study the salivary oxidative stress in relation to caries of teeth were examining the antioxidant salivary biomarkers (43)(44)(45) but not the caries itself.
In a study analysed saliva collected from patients with active caries, a higher lipid peroxidation was identified in those patients compared to the control group. (46) The association between caries and total antioxidant capacity was confirmed by some researchers. (47) The Longevity.VoL. 2020, Article ID 5478708,