α‐Tocopherol Stereoisomers
Introduction
Vitamin E is the exception to the paradigm that natural and synthetic vitamins are equivalent because their molecular structures are identical. Natural α‐tocopherol (RRR‐α‐tocopherol) is a single stereoisomer. Plants and other oxygenic, photosynthetic organisms are the only organisms able to synthesize tocopherols (DellaPenna, 2005), and since this synthesis is facilitated by stereo‐specific enzymes, the resulting tocopherols always posses the same stereochemical structure, namely the RRR‐structure (Fig. 1).
Synthetic α‐tocopherol (all‐rac‐α‐tocopherol) is produced commercially by a chemical reaction of tetramethylhydroquinone (TMHQ) with racemic isophytol (VERIS Research summary, 1999). Racemic isophytol is synthesized from isoprenoid units and since isophytol has three chiral centres the resulting α‐tocopherol has 23 possible conformations and thus yields a racemic mixture of all eight possible stereoisomers.
Section snippets
Presence in Food/Feed Ingredients
Commercial vitamin E supplements can be classified into several distinct categories: fully synthetic vitamin E (all‐rac‐α‐tocopherol), the most inexpensive, most commonly sold supplement forms usually as the acetate ester.
Most natural vitamin E is derived during refining of vegetable oils, mainly soybean oil, sunflower oil, and canola/rapeseed oil.
The natural sources of vitamin E can be divided into truly natural RRR‐ α‐tocopherol, where RRR‐α‐tocopherol is extracted and isolated directly from
Analytical Methods for Separation of α‐Tocopherol Stereoisomers
Analysis of the individual stereoisomers of α‐tocopherol is an important tool in order to quantify relative bioavailability of the individual stereoisomers (Jensen et al., 2006). Separations of the eight stereoisomers of α‐tocopherol are a great challenge and until now no single method allow the separation of all eight stereoisomers in one single chromatographic run (Nelis et al., 2000). The use of deuterium labeled α‐tocopherol in conjunction with GC‐MS (Ingold et al., 1987) or HPLC‐MS (
Bioavailability and Secretion into Milk
As discussed above, bioavailability is an important part of the term bioactivity and is quantitative measurable in blood, tissue, and excreta. In addition, the lack of good biological markers for bioactivities, bioavailability is often used as one of the surrogate markers for bioactivities with those limitations this must give (Blatt 2004, Jensen 2006). Therefore, results of α‐tocopherol stereoisomers in different animal species and humans are presented with the focus on bioavailability rather
α‐Tocopherol‐Binding Protein (α‐TTP)
In contrast to the relatively well‐investigated binding proteins for vitamins A and D, proteins that bind and transport vitamin E have only been identified in the past decade and many of their specific biological roles remain elusive. The term “tocopherol‐associated proteins” has been used to distinguish a molecularly defined family of proteins that are capable of binding α‐tocopherol (Zimmer et al., 2000) with a higher affinity than other tocopherols (Yamauchi et al., 2001) and are also
Conclusions
The discussion on the bioavailability of RRR‐ and all‐rac‐α‐tocopheryl acetate has primarily been based on human and animal studies using deuterium‐labeled forms, whereby a higher biopotency of 2:1 (of RRR: all‐rac) has been demonstrated, differing from the accepted biopotency ratio of 1.36:1. However, the quantitative separation of the individual stereoisomers of the 2R‐forms allow us to get a more detailed picture of the bioavailability of natural and synthetic vitamin E forms.
In agreement
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