Hydroxyl radical and its scavengers in health and disease

Abstract

It is generally believed that diseases caused by oxidative stress should be treated with antioxidants. However, clinical trials with such antioxidants as ascorbic acid and vitamin E, failed to produce the expected beneficial results. On the other hand, important biomolecules can be modified by the introduction of oxygen atoms by means of non-oxidative hydroxyl radicals. In addition, hydroxyl radicals can reduce disulfide bonds in proteins, specifically fibrinogen, resulting in their unfolding and scrambled refolding into abnormal spatial configurations. Consequences of this reaction are observed in many diseases such as atherosclerosis, cancer and neurological disorders, and can be prevented by the action of non-reducing substances. Moreover, many therapeutic substances, traditionally classified as antioxidants, accept electrons and thus are effective oxidants. It is described in this paper that hydroxyl radicals can be generated by ferric ions without any oxidizing agent. In view of the well-known damaging effect of poorly chelated iron in the human body, numerous natural products containing iron binding agents can be essential in the maintenance of human health. However, beneficial effects of the great number of phytochemicals that are endowed with hydroxyl radical scavenging and/or iron chelating activities should not be considered as a proof for oxidative stress.

Figure 1

Effect of iron (ferric chloride) on fibrinogen aggregation, as measured by optical density, and on hydroxyl radical generation expressed as fluorescence intensity according to Manevich et al. (Radiat Res 1997; 148:580-91). A stock solution of 1 mM coumarin-3-carboxylic acid was diluted 1 : 2 with PBS, pH 7.4, and stored at room temperature before the use. Fifty-microliter aliquots of ferric chloride solution at various molar concentrations were added to black plastic microplate wells containing each 100 μL of the coumarin reagent. After 2 min. incubation at RT fluorescence of the hydroxylation product was measured for one second in PerkinElmer 1420 Multilabel Counter at 350/450 nm of excitation/emission wavelength. The results presented in this figure show that the aggregation of fibrinogen is closely related to the extent of hydroxyl radicals generated in the system.