<p>However, the roles and importance of the various forms of vitamin E are presently unclear,^{<a href="https://en.wikipedia.org/wiki/Antioxidant#cite_note-113">[113]</a>}^{<a href="https://en.wikipedia.org/wiki/Antioxidant#cite_note-114">[114]</a>}&nbsp;and it has even been suggested that the most important function of &alpha;-tocopherol is as a&nbsp;<a href="https://en.wikipedia.org/wiki/Cell_signaling" title="Cell signaling">signaling molecule</a>, with this molecule having no significant role in antioxidant metabolism.^{<a href="https://en.wikipedia.org/wiki/Antioxidant#cite_note-Azzi-115">[115]</a>}^{<a href="https://en.wikipedia.org/wiki/Antioxidant#cite_note-116">[116]</a>}&nbsp;The functions of the other forms of vitamin E are even less well understood, although &gamma;-tocopherol is a&nbsp;<a href="https://en.wikipedia.org/wiki/Nucleophile" title="Nucleophile">nucleophile</a>&nbsp;that may react with&nbsp;<a href="https://en.wikipedia.org/wiki/Electrophile" title="Electrophile">electrophilic</a>&nbsp;mutagens,^{<a href="https://en.wikipedia.org/wiki/Antioxidant#cite_note-Brigelius-109">[109]</a>}&nbsp;and tocotrienols may be important in protecting&nbsp;<a href="https://en.wikipedia.org/wiki/Neuron" title="Neuron">neurons</a>&nbsp;from damage.^{<a href="https://en.wikipedia.org/wiki/Antioxidant#cite_note-117">[117]</a>}</p>

<h2>Oxidative challenge in biology</h2>

<p>The use of oxygen as part of the process for generating metabolic energy produces reactive oxygen species.^{<a href="https://en.wikipedia.org/wiki/Antioxidant#cite_note-Raha-57">[57]</a>}&nbsp;In this process, the superoxide anion is produced as a&nbsp;<a href="https://en.wikipedia.org/wiki/By-product" title="By-product">by-product</a>&nbsp;of several steps in the&nbsp;<a href="https://en.wikipedia.org/wiki/Electron_transport_chain" title="Electron transport chain">electron transport chain</a>.^{<a href="https://en.wikipedia.org/wiki/Antioxidant#cite_note-58">[58]</a>}&nbsp;Particularly important is the reduction of&nbsp;<a href="https://en.wikipedia.org/wiki/Coenzyme_Q" title="Coenzyme Q">coenzyme Q</a>&nbsp;in&nbsp;<a href="https://en.wikipedia.org/wiki/Complex_III" title="Complex III">complex III</a>, since a highly reactive free radical is formed as an intermediate (Q<strong>&middot;</strong>^&minus;). This unstable intermediate can lead to electron &quot;leakage&quot;, when electrons jump directly to oxygen and form the superoxide anion, instead of moving through the normal series of well-controlled reactions of the electron transport chain.^{<a href="https://en.wikipedia.org/wiki/Antioxidant#cite_note-59">[59]</a>}&nbsp;Peroxide is also produced from the oxidation of reduced&nbsp;<a href="https://en.wikipedia.org/wiki/Flavoprotein" title="Flavoprotein">flavoproteins</a>, such as&nbsp;<a href="https://en.wikipedia.org/wiki/Complex_I" title="Complex I">complex I</a>.^{<a href="https://en.wikipedia.org/wiki/Antioxidant#cite_note-60">[60]</a>}However, although these enzymes can produce oxidants, the relative importance of the electron transfer chain to other processes that generate peroxide is unclear.^{<a href="https://en.wikipedia.org/wiki/Antioxidant#cite_note-61">[61]</a>}^{<a href="https://en.wikipedia.org/wiki/Antioxidant#cite_note-Pathways_Ofoxidativedamage-62">[62]</a>}&nbsp;In plants,&nbsp;<a href="https://en.wikipedia.org/wiki/Algae" title="Algae">algae</a>, and&nbsp;<a href="https://en.wikipedia.org/wiki/Cyanobacteria" title="Cyanobacteria">cyanobacteria</a>, reactive oxygen species are also produced during&nbsp;<a href="https://en.wikipedia.org/wiki/Photosynthesis" title="Photosynthesis">photosynthesis</a>,^{<a href="https://en.wikipedia.org/wiki/Antioxidant#cite_note-63">[63]</a>}&nbsp;particularly under conditions of high&nbsp;<a href="https://en.wikipedia.org/wiki/Irradiance" title="Irradiance">light intensity</a>.^{<a href="https://en.wikipedia.org/wiki/Antioxidant#cite_note-64">[64]</a>}&nbsp;This effect is partly offset by the involvement of&nbsp;<a href="https://en.wikipedia.org/wiki/Carotenoid" title="Carotenoid">carotenoids</a>&nbsp;in&nbsp;<a href="https://en.wikipedia.org/wiki/Photoinhibition" title="Photoinhibition">photoinhibition</a>, and in algae and cyanobacteria, by large amount of&nbsp;<a href="https://en.wikipedia.org/wiki/Iodide" title="Iodide">iodide</a>&nbsp;and&nbsp;<a href="https://en.wikipedia.org/wiki/Selenium" title="Selenium">selenium</a>,^{<a href="https://en.wikipedia.org/wiki/Antioxidant#cite_note-65">[65]</a>}&nbsp;which involves these antioxidants reacting with over-reduced forms of the&nbsp;<a href="https://en.wikipedia.org/wiki/Photosynthetic_reaction_centre" title="Photosynthetic reaction centre">photosynthetic reaction centres</a>&nbsp;to prevent the production of reactive oxygen species.^{<a href="https://en.wikipedia.org/wiki/Antioxidant#cite_note-66">[66]</a>}^{<a href="https://en.wikipedia.org/wiki/Antioxidant#cite_note-67">[67]</a>}</p>

<h2>Oxidative stress in disease</h2>