α-Tocopherol (alpha-tocopherol) is a type of vitamin E. Its E number is "E307". Vitamin E exists in eight different forms, four tocopherols and four tocotrienols. All feature a chromane ring, with a hydroxyl group that can donate a hydrogen atom to reduce free radicals and a hydrophobic side chain, along with an aromatic ring is situated near the carbonyls in the fatty acyl chains of the phospholipid bilayer, allows for penetration into biological membranes.[2] It is found most in the membrane's non-raft domains, associated with omega-3 and 6 fatty acids, to partially prevent oxidation.[3] The most prevalent form, α-tocopherol, is involved in molecular, cellular, biochemical processes closely related to overall lipoprotein and lipidhomeostasis. Compared to the others, α-tocopherol is preferentially absorbed and accumulated in humans.
Vitamin E is found in a variety of tissues, being lipid-soluble, and taken up by the body in a wide variety of ways. Ongoing research is believed to be "critical for manipulation of vitamin E homeostasis in a variety of oxidative stress-related disease conditions in humans."[4] One of these disease conditions is the α-tocopherol role in the use by malaria parasites to protect themselves from the highly oxidative environment in erythrocytes.[5] A second of these disease conditions is the α-tocopherol antioxidant properties' role cardiovascular heart disease. In preventing LDL (low-density lipoprotein) oxidation, it is able to decrease chances of atherosclerosis and arterial build-up.[6]
Synthesis
To synthesize the ⍺-diastereomer selectively, tocol acetate is transformed to the naturally occurring, kinetically favored α-Tocopherol after being catalyzed by the lipase enzyme. This reaction occurs under biological conditions, commonly in the digestive system.[7]
Stereoisomers
α-Tocopherol has three stereocenters, so it is a chiral molecule.[8] The eight stereoisomers of α-tocopherol differ in the configuration of these stereocenters. RRR-α-tocopherol is the natural one.[9] The older name of RRR-α-tocopherol is d-α-tocopherol, but this d/l naming should no longer be used, because whether l-α-tocopherol should mean SSS enantiomer or the SRR diastereomer is not clear, from historical reasons. The SRR may be named 2-epi-α-tocopherol, the diastereomeric mixture of RRR-α-tocopherol and 2-epi-α-tocopherol may be called 2-ambo-α-tocopherol (formerly named dl-α-tocopherol). The mixture of all eight diastereomers is called all-rac-α-tocopherol.[10] The α-Tocopherol is the most active diastereomer biologically, while being maintained at a high level in plasma and tissues of many different animal species.[11]
One IU of tocopherol is defined as 2⁄3 milligram of RRR-α-tocopherol (formerly named d-α-tocopherol). 1 IU is also defined as 0.9 mg of an equal mix of the eight stereoisomers, which is a racemic mixture, all-rac-α-tocopheryl acetate. This mix of stereoisomers is often called dl-α-tocopheryl acetate.[12] Starting with May 2016, the IU unit is made obsolete, such that 1 mg of "Vitamin E" is 1 mg of d-alpha-tocopherol or 2 mg of dl-alpha-tocopherol.[13]
^Burton, G. W.; Ingold, K. U. (1 June 1986). "Vitamin E: application of the principles of physical organic chemistry to the exploration of its structure and function". Accounts of Chemical Research. 19 (7): 194–201. doi:10.1021/ar00127a001.
^Atkinson, Jeffrey; Harroun, Thad; Wassall, Stephen R.; Stillwell, William; Katsaras, John (May 2010). "The location and behavior of α‐tocopherol in membranes". Molecular Nutrition & Food Research. 54 (5): 641–651. doi:10.1002/mnfr.200900439.
^Rigotti A (2007). "Absorption, transport, and tissue delivery of vitamin E". Molecular Aspects of Medicine. 28 (5–6): 423–36. doi:10.1016/j.mam.2007.01.002. PMID17320165.
^Singh, U.; Devaraj, S.; Jialal, I. (21 August 2005). "VITAMIN E, OXIDATIVE STRESS, AND INFLAMMATION". Annual Review of Nutrition. 25 (1): 151–174. doi:10.1146/annurev.nutr.24.012003.132446.
