Vitamin C

It was noted above (section 7.2.3.3) that vitamin C can act at the surface of cells or lipoproteins to reduce the tocopheroxyl radical back to tocopherol, forming the stable monodehydroascorbate radical. Vitamin C can also react with superoxide and hydroxyl radicals:

ascorbate + "O2- + H+ ^ H2O2 + monodehydroascorbate ascorbate + "OH + H+ ^ H2O + monodehydroascorbate

As shown in Figure 7.18, the resultant monodehydroascorbate can then undergo enzymic reduction back to ascorbate or a non-enzymic reaction between 2 mol of monodehydroascorbate to yield ascorbate and dehydroascorbate. Dehydroascorbate may then either be reduced to ascorbate or be oxidized to diketogulonate.

Although ascorbate has a protective role in the reactions shown above, it can also be a source of oxygen radicals, and hence potentially damaging:

ascorbate + O2 ^ "O2- + monodehydroascorbate ascorbate + Cu2+ ^ Cu+ + monodehydroascorbate

Figure 7.18 Metabolism of the monodehydroascorbate radical to non-radical products.

However, it is unlikely that high intakes of vitamin C will result in significant radical formation, as once intake rises above about 100—120 mg/day the vitamin is excreted quantitatively in the urine.

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