Vitamin E is a group of 4 saturated (α-, β-, γ- and δ-tocopherol) and 4 unsaturated (α-, β-, γ- and δ-tocotrienol) vitamers which share the biological activity of α-tocopherol. Of the vitamin E vitamers, α-tocopherol has been extensively studies with regard its antioxidant protection of lipid membranes, and more recently interest has increased in γ-tocopherol due to high dietary intakes. However, the other vitamers have been neglected by scientific research. Evidence suggests that the tocotrienols, because of their unique structure, play an important role in disease prevention, but estimates indicate <1% of vitamin E research in the last 30 years involves tocotrienols. The acceptance that vitamin E is nutritionally important has been slow because obvious deficiency symptoms are not present in humans with low vitamin E intakes. There are also species specific effects that have further clouded the role of vitamin E in human nutrition.
Herbert Evans discovered that vitamin E was a vitamin because rats became sterile when fed low intakes of some fat soluble factor, but that fertility was restored when the rats were fed a chlorophyll rich fraction of lettuce leaves. The factor responsible was eventually named by Barnett Sure as vitamin E, although at the time there was no indication that the substance was essential in humans. Vitamin E was eventually given the name tocopherol after the Greek for child birth (tocos), the Greek for to bring forth (pheros) and the designation of an alcohol functional group (ol). Vitamin E was not designated a vitamin in humans until the 1950’s. During this time, Denham Harman put forward the free radical theory of ageing, and the role of vitamin E as a vital part of the antioxidant defence of lipid membranes was confirmed in humans.
Vitamin E tends to be present in the fat soluble parts of plants where it protects the fatty acids from lipid peroxidation. Nuts and seeds, having high concentrations of fat, are therefore good sources of vitamin E. However, in commonly eaten foods, the tocopherols are far more numerous than the tocotrienols. Tocotrienols are present mainly in monocots such as palm oil, and oil derived from the palm plant can contain 30% tocopherols and 70% tocotrienols. In contrast corn, olive and sunflower oil contain mostly tocopherols. The position of the methyl group on the chromanol head creates the α-, β-, γ- and δ-isoforms of the vitamin E molecules. But unlike the saturated 15 carbon phytyl tail of the tocopherols, the unsaturated tail of the tocotrienols contains 3 trans double bonds in the 3’, 7’, and 11’ positions, which creates differences in chemical properties of the tocotrienols.
The unique chemical properties of the tocotrienols also gives them some unique biological properties, when compared to the tocopherols. For example, the tocotrienols may inhibit the growth of breast cancer cells, may protect against neurodegeneration and may decrease the risk of acute ischaemic stroke injury. In addition, the tocotrienols are inhibitors of HMG CoA reductase, the enzyme responsible for cholesterol synthesis. The tocotrienols may therefore be beneficial in lowering elevated cholesterol levels. Tocopherols and tocotrienols are transported in the circulation bound to the tocopherol transport protein, although the binding affinity for the various vitamin E vitamers differs between compounds. It is thought that this binding affinity is one of the key determinates of the tissue distribution and biological activity of the vitamers, with α-totcopherol having the strongest binding affinity to the tocopherol binding protein, and being the most biologically active vitamer.
Neurones are susceptible to oxidative stress because they contains high concentrations of polyunsaturated fatty acids. The double bonds contained within the hydrocarbon chains of polyunsaturated fatty acids can undergo lipid peroxidation, and the problem is compounded by the low levels of glutathione in neurones compared to glial cells. Tocotrienols may reduce the oxidative stress following brain injury because they can inhibit the enzymatic and non-enzymatic oxidative metabolism of arachidonic acid, which is released from membranes by phospholipase A2 as a natural part of the response to injury (the arachidonic acid cascade; which causes formation of prostaglandins, leukotrienes, thromboxanes and isoprostanes). Independent of its antioxidant function, α-tocotrienol is also a potent inhibitor of cytosolic phospholipase A2 and 12-lipoxygenase, that are responsible for the formation of inflammatory mediators. High tocotrienols intakes from palm oil may therefore be beneficial to brain health if concentrated in neuronal membranes.
RdB
