Antioxidant Micronutrients Vitamins E C carotene and Selenium

A deficiency in these important antioxidant nutrients will cause chromosomal damage and oxidative lesions to membrane structures.

Outside of its crucial role as a coenzyme for copper-dependent hydroxylases and a-ketoglutarate-linked iron-containing hydroxylases, vitamin C has generalized antioxidant properties, including salvage of the spent tocopheroxyl radical back to a -tocopherol (vitamin

E). Vitamin C can neutralize reactive oxygen species (ROS) and, hence, protect nuclear and membrane structures:

It is worth noting that at high levels, vitamin C, as with other antioxidants such as j-carotene, may actually potentiate radical formation and compromise the integrity of genomic mechanisms (see carotenoid Figure 3.4 above):

Vitamin C as a protective antioxidant:

ascorbate + +H+ ^ H2O2 + monodehydroascorbate (stable, and salvageable vitamin C radical)

Vitamin C as a prooxidant source of free radicals: ascorbate + O2 ^ + monodehydroascorbate

Monodehydroascorbate can be enzymatically returned to ascorbate via monodehy-droascorbate reductase or undergo dismutation to ascorbate and dehydroascorbate (see Figure 3.5).

Vitamin Scheme Antioxidant
Figure 3.5. Scheme showing antioxidant effect and salvage of vitamin C.

Haptoglobin polymorphisms are associated with inflammatory and immune diseases possibly due to a phenotype-dependent modulation of oxidative stress and prostaglandin synthesis. Strong genetic selection favors the HP2-2 phenotype indicating an important role of haptoglobin in human pathology.

Haptoglobin is an acute-phase plasma protein that binds free hemoglobin and thus prevents catalysis of reactive oxygen species. The HP2-2 phenotype of the haptoglobin gene leads to an accumulation of iron, making individuals more susceptible to disease by lowering vitamin C levels. In other words, a higher rate of L-ascorbic acid oxidation in Hp2-2 carriers occurs because they have less protection against hemoglobin-iron-driven peroxidation (80, 81). This vitamin C-related gene-nutrient interaction might well be a significant one in determining evolutionary fitness. It is interesting to consider that lower levels of potentially prooxidant vitamin C at the onset of infection may actually protect tissue from excessive free radical damage via mechanisms such as the macrophage respiratory burst.

Vitamin E protects polyunsaturated fatty acids within the cell membrane and plasma lipoproteins from lipid peroxidation. It reduces lipid peroxide radicals to unreactive fatty acids. As a consequence of peroxyl-radical scavenging, vitamin E is converted into the stable tocopheroxyl radical. Both vitamin C and the selenium enzyme glutathione peroxidase can salvage this compound back into «-tocopherol. It is this role that classifies selenium as an antioxidant (Figures 2.7, 3.5, and 3.6 show the salvage mechanisms for important native antioxidant micronutrients). See earlier for significance in gametogenesis.

Reduced vitamin E (a-tocopherol)

Reduced vitamin E (a-tocopherol)

Alpha Tocopherol Metabolism

Stable vitamin E radical (resonance stabilized tocopheroxyl radical)

Figure 3.6. Vitamin E (a-tocopherol) can reduce lipid peroxides but in the process becomes oxidized. Vitamin C reduces the relatively stable tocopheroxyl radical back to a-tocopherol.

Stable vitamin E radical (resonance stabilized tocopheroxyl radical)

Figure 3.6. Vitamin E (a-tocopherol) can reduce lipid peroxides but in the process becomes oxidized. Vitamin C reduces the relatively stable tocopheroxyl radical back to a-tocopherol.

a-Tocopherol also modulates two important signal transduction pathways that are centered on phospatidylinositol 3-kinase and protein kinase C. These pathways alter cell proliferation, platelet aggregation, and NADPH-oxidase activation. a-Tocopherol also regulates genes independent of these kinase pathways. y-Tocopherol also has some gene regulatory properties (82).

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