**3. Vitamin E as antioxidants and free radical scavengers**

The number of methyl groups on the chroman ring influence the antioxidative effects of the various isomers of vitamin E. Delta-tocopherol and alpha-tocopherol each have one and three methyl groups respectively. The alpha isomers of vitamin E are thought to have the highest antioxidant activity, followed by the beta, gamma and delta isomers. It is generally believed that the antioxidative of tocopherol is greater than tocotrienol. However, more recent studies have indicated that tocotrienol has higher antioxidant properties [42, 43].

As a free radical scavenger, vitamin E reacts with ROS indirectly in vitro. As tocopherol has been extensively studied compared to tocotrienol, its pathway during reaction with ROS is well-documented. This reaction will eventually lead to the formation of α-tocopheroxyl radical (α-Toc•) [44]. The antioxidative reaction goes on and on, until the final product i.e., lipid peroxide is formed. α-tocopherol is able to deliver hydrogen ion to peroxyl radical (LOO•) during lipid hydroperoxide formation (LOOH). The α-tocopherol radical formed (α-Toc•) will undergo a series of reactions with the peroxyl radical (LOO•) to eventually produce inert adducts (**Figure 3**) [45, 46].

#### **Figure 3.**

*Summary of whole antioxidative reaction of vitamin E. The figure was modified from Ref. [46]. The reactions involving tocotrienols are thought to be similar to that of tocopherols. Abbreviations:* α*-Toc•,* α*-tocopheroxyl radical;* α*-TocH,* α*-tocopherol; L•, lipid radical; L, low density lipoprotein; LH, unsaturated fatty acid; LO•, alkoxy radical; LOO•, lipid peroxyl radical; LOOH, lipid hydroperoxide.*

Vitamin E is also known to react with other radicals as well. The reaction of vitamin E with alkoxy radical produces α-tocopherol radical (α-Toc•). A previous report had also stated that the reaction between two α-tocopherol radical (α-Toc•) will produce inert non-radical dimers [47]. In terms of inhibition of lipid peroxidation, the alpha isomer of vitamin E has the highest potency, followed by the beta and gamma isomers which are equipotent, and the least potent is the delta isomer [46].

In in vivo situations, a variety of hydrophilic and hydrophobic antioxidant components exists and reacts with various moieties. It has been shown that the hydrophilic antioxidant ascorbic acid is able to reduce α-tocopherol radical (α-Toc•) back to α-tocopherol (α-TocH) [48].

The reactions involving tocotrienols are thought to be similar to that of tocopherols. In fact, a previous study had found that consumption of tocotrienol-rich rice bran oil decreased levels of plasma phospholipid hydroperoxide (PLOOH) in rats [49]. It must be remembered however that the supramolecular structures derived from various derivatives are too complex to be simulated by in vitro models to be compared with in vivo conditions [46].
