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Abstract
Retinoids are present in human tissues exposed to light and under increased risk of oxidative stress, such as the retina and skin. Retinoid cation radicals can be formed as a result of the interaction between retinoids and other radicals or photoexcitation with light. It has been shown that such semi-oxidized retinoids can oxidize certain amino acids and proteins, and that α-tocopherol can scavenge the cation radicals of retinol and retinoic acid. The aim of this study was to determine (i) whether β-, γ-, and δ-tocopherols can also scavenge these radicals, and (ii) whether tocopherols can scavenge the cation radicals of another form of vitamin A—retinal. The retinoid cation radicals were generated by the pulse radiolysis of benzene or aqueous solution in the presence of a selected retinoid under oxidizing conditions, and the kinetics of retinoid cation radical decays were measured in the absence and presence of different tocopherols, Trolox or urate. The bimolecular rate constants are the highest for the scavenging of cation radicals of retinal, (7 to 8) × 109 M−1·s−1, followed by retinoic acid, (0.03 to 5.6) × 109 M−1·s−1, and retinol, (0.08 to 1.6) × 108 M−1·s−1. Delta-tocopherol is the least effective scavenger of semi-oxidized retinol and retinoic acid. The hydrophilic analogue of α-tocopherol, Trolox, is substantially less efficient at scavenging retinoid cation radicals than α-tocopherol and urate, but it is more efficient at scavenging the cation radicals of retinoic acid and retinol than δ-tocopherol. The scavenging rate constants indicate that tocopherols can effectively compete with amino acids and proteins for retinoid cation radicals, thereby protecting these important biomolecules from oxidation. Our results provide another mechanism by which tocopherols can diminish the oxidative damage to the skin and retina and thereby protect from skin photosensitivity and the development and/or progression of changes in blinding retinal diseases such as Stargardt’s disease and age-related macular degeneration (AMD).
Highlights
Tocopherols and retinoids with vitamin E and A activities, respectively, are essential for the normal growth and function of the human body [1,2,3,4,5,6]
Trolox is a synthetic hydrophilic analogue of α-tocopherol, which is soluble in water; it can be used to test whether it can scavenge retinal cation radicals formed in Triton X-100 micelles as a model of the biological lipid–water interface
The urate interacted with retinal cation radicals with a bimolecular rate constant of 6.7 × 108 M−1·s−1, which is similar to the rate of scavenging by ascorbate of 7.3 × 108 M−1·s−1 (Figure 2; Table 1) [15]
Summary
Tocopherols and retinoids with vitamin E and A activities, respectively, are essential for the normal growth and function of the human body [1,2,3,4,5,6]. Tocopherols are present in various types of natural food and as food additives. Alpha- and γ-tocopherols are the predominant forms of vitamin E in typical diets, coming mainly from vegetable oils [3,8]. All four tocopherols are incorporated into chylomicrons and secreted into the lymph and transported to the liver. There is a preferential uptake of α-tocopherol by the α-tocopherol transfer protein, which facilitates its incorporation into lipoproteins, which are secreted into the systemic circulation. Supplementation with other tocopherols can transiently increase their levels in blood plasma [9]
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