Three mechanisms of antioxidant activity

1. Quenching of non-radical singlet oxygen ( ¹O₂ )
Reactions between singlet oxygen and an antioxidant produces a stable oxygen species and an excited antioxidant molecule, which can dissipate excess energy through rotational and vibrational interactions with nearby molecules, regenerating itself for further quenchings. Thousands of ( ¹O₂ ) can be quenched before the antioxidant degrades to an inactive form.

2. Resonance stabilisation during reactions between antioxidants and free radicals.

3. Upregulation of antioxidant response element, stimulating production of cellular enzymes.

Antioxidant/pro-oxidant dependence on cellular oxygen partial pressure

Almost 25 years ago, it was established that the cellular oxygen partial pressure (OPP) determined the nature of a constituent's activity. Below the OPP in normal air (150 torr), i.e., the physiological conditions prevailing in most tissues, in vivo beta carotene acts as an antioxidant, while at OPPs at, or above atmospheric, it becomes a pro-oxidant, especially at high concentrations.¹ Around the same time, it was shown that compounds that inhibited cellular antioxidants left organisms susceptible to oxygen toxicity. ² In 1997, all-trans retinol was found to be more effective as an antioxidant at low OPPs (15 torr) than at higher OPPs (160 torr).³ The subtle balance (switch) of state provides a simple example of the complexity surrounding the in vivo plasma antioxidant capacity.

Phenolic epitope sites

The location of radical-trapping sites on simple polyphenol structures is illustrated on the Phytochemistry page. Using four different assaying techniques, in vitro activities of 34 phenolic antioxidants are ranked, and the various classes tabulated against the proposed mechanisms leading to their therapeutic activity.⁴

Physiological concentrations

The importance of working at in vivo physiological concentrations in in vitro experiments is emphasised in cell culture to study the effect of quercetin and its in vivo conjugates on markers for atherosclerosis. ⁵

Cell types determine antioxidant/pro-oxidant properties of vitamin C - by single cell ultramicroelectrode amperometry.

Ascorbic acid acts as an antioxidant in phagocytes (PLB - 985) and as a prooxidant in macrocytes (RAW - 264.7). These findings are discussed in relation to recent epidemiological statements about the ambiguity of vitamin C and oxidative stress leading to increased human mortality rates. ⁶ Antioxidant badlist .

A new antioxidant from wild nutmeg - Argenteane

It is characterised as a lipophillic dilignan and descibed as having more powerful antioxidant properties than vitamin E. The paper discusses a mechanism for its action, likening it to vitamin E as strongly lipophillic and therefore able to be distributed thinly across a membrane or near to it, giving it an increased "virtual" concentration. Thus, a relatively low concentration in the diet could still provide a powerful protection against oxidative stress. ⁷



References
1. G.W. Burton and K.U. Ingold, Science, 224, (4649) 569-573, 1984.
2. C.D. Puglia and S.R. Powell, Env. Health Perspectives, 57, 307-311, 1984.
3. L. Tesoriere et al., Arch. Biochem. Biophys. 343 (1), 13-18, 1997.
4. M.A. Soobrattee et al., Mutation Res./Fundamental Mol. Mechan. Mutagenesis, 579, (1-2), 200-213, 2005.
5. S. Tribolo et al., Atherosclerosis, 197, 50-56, 2008.
6. C. Amatore et al., J. Electroanalyt. Chem., 615, 34-44, 2008.
7. C.A. Calliste et al., Food Chem., 118 (3) 489-496, 2010