Vitamin E and Similar Compounds

HPLC has been used for measuring various compounds, for example, carbohydrates, vitamins, additives, mycotoxins, amino acids, proteins, triglycerides in fats and oils, lipids, chiral compounds, and pigments. Several sensitive and selective detectors such as ultraviolet-visible, FL, electrochemical, and diffractometric are available to utilize with HPLC depending on the compound to be analyzed. Various HPLC methods based on these detectors have been published for the measurement of vitamin E in biological and pharmaceutical samples and food products. Excellent literature reviews of HPLC based on various detectors in the analysis of vitamin E content in various matrices have been reported (Abidi, 2000 Aust et al., 2001 Ruperez et al., 2001 Lai and Franke, 2013). Table 19.5 reports several recent HPLC methods for the analysis of vitamin E and similar compounds in various matrices. 

TABLE 19.5 Summarized Characteristics of HPLC Methods Used for the Analy sis of Vitamin E and Similar Compounds in Diverse Samples 00 00 0... 

Further work on the structural requirements for the antioxidant effect has been performed with micelles and liposomes [ 104]. This group concluded that the antioxidant properties of vitamin E and its model compound without the phytyl side-chain were similar within micelles, liposomes and homogeneous solution but that the phytyl side-chain enhances the retention of vitamin E in liposomes and suppresses the transfer of the vitamin between liposomal membranes. 

Many different compounds can be used as biopolymer additives, most of them are quite similar to those used in traditional polymer formulations. The use of various compounds as plasticizers, lubricants, and antioxidants has been recently reported.Antioxidants are normally used to avoid, or at least minimize, oxidation reactions, which normally lead to degradation and general loss of desirable properties. Phenol derivatives are mostly used in polymers, but vitamin E and a-tocophe-rols are those most commonly found in biopolymer formulation. 

In plants, coenrymes Q are accompanied by other lipophilic compounds of similar structure, plastoquinones (5-138), vitamin E and vitamin Kj. Similarly to tocopherols and tocotrienols, plastoquinones are produced from homogentisic acid by C-alkylation... 

No unequivocal unique function for vitamin E has been defined. However, it does act as a hpid-soluble antioxidant in cell membranes, where many of its functions can be provided by synthetic antioxidants. Vitamin E is the generic descriptor for two famihes of compounds, the tocopherols and the tocotrienols (Figure 45—5). The different vitamers (compounds having similar vitamin activity) have different biologic potencies the most active is D-a-tocopherol, and it is usual to express vitamin E intake in milhgrams of D-a-tocoph-erol equivalents. Synthetic DL-a-tocopherol does not have the same biologic potency as the namrally occurring compound. 

Dedicated plants predominate in the bulk chemicals industry. They suit the manufacture of well-defined products using a determined technology. Any change of the product or the production process usually produces problems, which illustrates the inflexibility of a dedicated plant. A batch plant may also be operated as a dedicated plant to produce a single chemical. Some fermentation plants (with reactors of up to 200 m volume) are examples of dedicated batch plants for the production of a family of similar products. So-called bulk fine chemicals, i.e. compounds that are produced in larger quantities, are also manufactured in dedicated plants, e.g. vitamin C and aspirin (see Fig. 7.1-1). The va.st majority of batch plants, however, produce several chemicals. 

This method is also used to measure ex vivo low-density lipoprotein (LDL) oxidation. LDL is isolated fresh from blood samples, oxidation is initiated by Cu(II) or AAPH, and peroxidation of the lipid components is followed at 234 nm for conjugated dienes (Prior and others 2005). In this specific case the procedure can be used to assess the interaction of certain antioxidant compounds, such as vitamin E, carotenoids, and retinyl stearate, exerting a protective effect on LDL (Esterbauer and others 1989). Hence, Viana and others (1996) studied the in vitro antioxidative effects of an extract rich in flavonoids. Similarly, Pearson and others (1999) assessed the ability of compounds in apple juices and extracts from fresh apple to protect LDL. Wang and Goodman (1999) examined the antioxidant properties of 26 common dietary phenolic agents in an ex vivo LDL oxidation model. Salleh and others (2002) screened 12 edible plant extracts rich in polyphenols for their potential to inhibit oxidation of LDL in vitro. Gongalves and others (2004) observed that phenolic extracts from cherry inhibited LDL oxidation in vitro in a dose-dependent manner. Yildirin and others (2007) demonstrated that grapes inhibited oxidation of human LDL at a level comparable to wine. Coinu and others (2007) studied the antioxidant properties of extracts obtained from artichoke leaves and outer bracts measured on human oxidized LDL. Milde and others (2007) showed that many phenolics, as well as carotenoids, enhance resistance to LDL oxidation. 

The fat-soluble vitamins comprise vitamins A, D, E, and K, whose biological activities are attributed to a number of structurally related compounds known as vitamers. Also included are those carotenoids that are precursors of vitamin A. Recommended dietary allowances (RDAs) based on human epidemiological and experimental animal studies have been published in the United States for vitamins A, D, E, and K (1). Other countries and international bodies have compiled similar recommendations. In the United States and Canada, fluid milk is supplemented by law with vitamin D to a level of 400 international units per quart (10 /zg/0.95 L) to meet the RDA of 10 p%. Other commodities, such as margarine, milk products, ready-to-eat breakfast cereals, and dietetic foods, are commonly supplemented with vitamins A, D, and E. Except for infant formulas, vitamin K is not added to foods. The addition of vitamins to a particular processed food is intended to provide a specific proportion of the RDA. 

Thompson and Hatina (135) showed that the sensitivity of a fluorescence detector toward unesterified vitamin E compounds under normal-phase conditions was at least 10 times greater than that of a variable-wavelength absorbance detector. The relative fluorescence responses of the tocopherols at 290 nm (excitation) and 330 nm (emission), as measured by HPLC peak area, were a-T, 100 /3-T, 129 y-T, 110 and 5-T, 122. The fluorescence responses of the corresponding to-cotrienols were very similar to those of the tocopherols, and therefore tocotrienol standards were not needed for calibration purposes. The fluorescence detector also allows the simultaneous monitoring of ubiquinone derivatives for example ubiquinone-10 has been detected in tomato (136). 

The pioneering work of Evans, Emerson and Emerson led to the isolation of a-tocopherol, a phenol exhibiting vitamin-E-like activity in biological systems. Now, eight compounds, usually oils, have been identified from plant sources, with similar vitamin-E-like actions. The tocopherols differ only in the number of methyl groups in the aromatic ring and therefore can be regarded as derivatives of 2-methyl-6-chromanol, onto which a 16-carbon isoprenoid chain is attached at C-2 and which is methylated at positions C-5, C-7 and C-8, respectively. 

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