**2. Antioxidants and free radicals**

Antioxidants can exist in either water-soluble or water-insoluble forms. Vitamin C, the prototypical representative of water-soluble antioxidant, can be found in cellular fluids. Vitamin E, which comprise the tocopherols and tocotrienols, is the typical water-insoluble antioxidant and can mostly be found in cellular membranes [1, 2]. Antioxidants can further be classified as enzymatic or non-enzymatic entities. Interaction between antioxidants and free radicals will result in the inactivation of free radicals damaging effects on our bodies. The human body defence mechanism against the actions of free radicals would usually involve the enzymatic actions of antioxidants which would result in the reduction of lipid peroxidation levels [25].

Free radicals are molecules with one or more unpaired electrons. Free radicals cause damage when they react with other molecules to find electrons to pair with their unpaired electrons. The other molecules then lost their electrons, causing them to become free radicals themselves, thus creating a chemical chain reaction of free radical production [26]. Oxidative and chemical stress in the body due to pollutants, xenobiotics and certain foods can expedite the formation of free radicals [22, 27, 28]. The free radical chain reaction may cause damage to cellular homeostasis due to its potential in causing alterations in the lipid, protein and DNA structure [27]. The damaged molecules may initiate mutation and growth of tumours. Several studies throughout the last few decades have suggested that oxidative stress plays a role in the development of many conditions, including cancer, cardiovascular disease, neurodegenerative disorders and inflammatory diseases such as arthritis [29].

The mitochondria is the chief target of free radical damage due to its preponderance to produce reactive oxygen species (ROS). The metabolic processes occurring within the mitochondria e.g., the electron transport chain may cause leakage of electrons. These electrons may in turn react with water to form ROS such as the superoxide radical, or via an indirect route the hydroxyl radical. These radicals then damage the mitochondria's DNA and proteins, and these damaged components in turn are more liable to produce ROS by-products. Thus a positive feedback loop of oxidative stress is established that, over time, can lead to the deterioration of cells and later organs and the entire body [30].

The most highly reactive free radical species are the hydroxyl radical (OH), hydrogen peroxide (H2O2), superoxide anion radical (O2-), and peroxynitrite radical (ONOO-) [31–33]. The hydroxyl (OH) radical is the most active oxygen species amongst the others, whereby it could potentially cause serious biological perturbations and peroxidation of lipid-based cellular membranes. Superoxide radical (O2-) inflicts severe damage after interaction with various biological cellular components [34]. H2O2 toxicity is due to the oxidation of proteins, membrane lipids and DNA by the peroxide ions [35]. Peroxynitrite radical is an oxidant and nitrating agent. Because of its oxidizing properties, peroxynitrite inflicts damage to various cellular components e.g. DNA and proteins. Peroxynitrite formation in the body has been attributed to the reaction of the superoxide free radicals with the nitric oxide free radicals [36, 37].

Lipid peroxidation is the oxidative damage caused by free radicals when they attack the polyunsaturated fatty acids (PUFAs) of the cell membranes. Lipid peroxides or lipid oxidation products are the results of this oxidation process. These fatty acid radicals are unstable molecules that react immediately with molecular oxygen, thus creating a peroxyl-fatty acid radical. This radical is also an unstable species and reacts towards other free fatty acids to produce more lipid peroxides, or cyclic peroxides if they reacted with each other. This cycle can linger on continuously as the new fatty acid radical reacts in the same way. Reactive aldehydes such as malondialdehyde (MDA) and 4-hydroxynonenal (HNE) would be the end products of lipid peroxidation, which are potentially mutagenic and carcinogenic. For example, MDA reacts with DNA to create DNA adducts [38–40].

When a free radical reacts with an inert molecule, a new radical molecule is formed, which is why the process is called a "chain reaction mechanism". The free radical reaction terminates when two radicals react and produce a non-radical species. Certain molecules within the cells can accelerate the termination of lipid peroxidation by neutralizing free radicals, thus protecting the cellular membrane from being damaged. Such molecules are known as antioxidants, of which the vitamin E tocotrienols are important examples [41].
