Is ascorbic acid an antioxidant or an oxidant
Vitamin C is an important dietary antioxidant, which can significantly reduce the adverse effects of active substances such as active oxygen and active nitrogen. These active substances will cause oxidative damage to macromolecules such as fat, DNA and protein, which are related to cardiovascular disease, stroke, cancer, neurodegenerative disease, cataract and other chronic diseases.
Ascorbic acid is an effective water-soluble antioxidant, which can remove/neutralize a series of active oxygen, namely hydroxyl, alkoxy, peroxy, superoxide anion, hydrogen peroxide radical and active nitrogen radical, such as nitrogen dioxide, nitrogen oxide, peroxynitrite at very low concentrations. In addition, ascorbic acid can α- Tocopherol, uric acid and β- Carotene free radicals regenerate from free radicals. It can be seen that ascorbic acid α- Tocopherol free radicals are converted into α- Tocopherol, as α- Co antioxidant of tocopherol, which helps prevent α- Tocopherol free radical mediated peroxidation.
These free radicals are highly active and can trigger lipid peroxidation. Therefore, the question arises. Is vitamin C used as an oxidant in vivo? The answer seems to be no, because although these reactions are easy to occur in vitro, their correlation in vivo has always been a controversial issue, because of the ready availability of free metal ions with catalytic activity in vivo. In biological systems, iron is not free, but it combines with proteins such as transferrin, hemoglobin and ferritin. Before iron catalyzes lipid peroxidation, it may be necessary to mobilize iron from these biomolecules. In addition, the concentration of free metal ions in the body is considered to be very low, because iron and other metals are isolated by various metal binding proteins. Another factor that may affect the catalytic and antioxidant properties of ascorbic acid is its concentration. In vitro experimental data show that ascorbic acid is an oxidant at low concentrations, but an antioxidant at high concentrations. In addition, a recent report showed that large doses of exogenous iron (200 mg) and ascorbic acid (75 mg) promoted the release of iron from iron binding proteins and enhanced in vitro lipid peroxidation in guinea pig serum. This finding supports the hypothesis that high intake of iron and ascorbic acid can increase lipid peroxidation of LDL in vivo, thus increasing the risk of atherosclerosis. However, another study showed that in iron overloaded plasma, ascorbic acid, as an antioxidant, prevents lipid oxidative damage in the body.
Is ascorbic acid harmful to cancer patients?
Agus et al. reported that tumor cells contain a large amount of ascorbic acid, although the role of ascorbic acid in tumors is still unclear. They have determined that vitamin C enters the cell in the form of dehydro ascorbic acid by promoting glucose transporter (glut), and then is reduced in the cell and retained in the form of ascorbic acid. It is speculated that high levels of ascorbic acid in tumor cells may interfere with chemotherapy or radiotherapy, because these therapies induce cell death through oxidative mechanisms. Therefore, supplement of ascorbic acid may reduce the effect of cancer treatment, because ascorbic acid is a strong antioxidant, which can eliminate or neutralize the oxidative stress caused by chemotherapy in cancer patients. However, more research is needed to understand the role of ascorbic acid in tumor cells and speculate the contraindication of ascorbic acid in cancer chemotherapy.
