Ginger (
Zingiber officinale Roscoe) is one of the world’s best-known spices, and it is universally used throughout history for its health benefits. The dried extract of ginger contains monoterpenes and sesquiterpenes. The main antioxidant principles in ginger are the gingerols, shogaols and some related phenolic ketone derivatives. Ginger extract possesses antioxidative characteristics, since it can scavenge superoxide anion and hydroxyl radicals. Ginger alone contains up to 12 compounds providing as much as 40-times higher antioxidant activity than vitamin E. The National Cancer Institute has identified ginger as one of the top-10 anti-cancer foods. As with other botanicals and herbs, ginger also offers numerous other benefits and could easily be placed in the cardiovascular category. Dehydrogingerdione (
1), one of the constituents of ginger, possesses a partially methylated catechol moiety, a 1,3-diketo functionality and an extended conjugation (Fig.
3
).


Fig. 3Chemical structure of ginger compounds (
1–
4)
In principle, compounds possessing multiple double bonds and especially with active hydrogen can act as radical scavengers via addition to double bonds and/or abstraction of hydrogen atom from the allylic position as is the case with lipids. The positive role of the β-diketone moiety in the antioxidant action of four ginger-derived phenolic diketones (
1–4) (Fig.
3
) against lipid peroxidation were reported [
31] using biologically relevant model systems and pulse radiolysis. In iron-independent peroxidation, however,
1 showed activity comparable to that of curcumin. This was ascribed due to its higher affinity for the lipid peroxyl radical due to higher hydrophobicity. The antioxidant/prooxidant activity of a test compound in iron-mediated lipid peroxidation in a biological system is critically governed by factors such as the concentration of endogenous reducing systems and/or redox couples. Thus, the true antioxidant activity of
1 was assessed by use of a biologically relevant liposomal system, devoid of reductants such as ascorbate, reduced form of nicotinamide adenine dinucleotide phosphate (NADPH), etc., and by carrying out the Fe2+-mediated peroxidation at pH 5.0, at which value Fe2+ remains unoxidized, for at least 3 h, during which considerable peroxidation took place. This study not only ensured minimization of the prooxidant effect of ascorbate, but also assisted in the quantitative comparison of the antioxidant activities of
1 and ascorbate, individually and in combination. The protection offered by vitamin C (15 µM) alone was considerably less than that offered by
1 even at lower concentrations (5 and 10 µM). More interestingly, the antioxidant activity of
1 increased drastically in the presence of vitamin C at increasing concentrations. These results strongly suggest that
1 scavenges the LOO· radical more quickly than vitamin C does, but the radical generated from
1 then reacts with vitamin C to regenerate
1. This established a synergistic effect of vitamin C on the antioxidant activity of
1. The above results were also confirmed by pulse radiolytic studies of the reaction between
1 and the trichloromethylperoxyl radical (CCl3O2·). In spite of its higher reduction potential value (1.5 V), compared to the physiologically relevant peroxides, the CCl3O2· radical is extensively used as a representative peroxyl radical because of the inherent simplicity in performing the experiments, and it was indeed used earlier to study a very important biophysical phenomenon, namely free radical interaction between vitamin E and vitamin C [
32]. The radical can be generated in aerated water/isopropanol/acetone mixtures containing carbon tetrachloride. A very high rate constant (2 × 109 M−1s−1) for the reaction between
1 and CCl3O2· established the superior antioxidant efficacy of
1 in comparison to vitamins C and E. This study also established a synergistic effect of vitamin C on the antioxidant activity of
1. On the basis of pulse radiolysis study, liposomal peroxidation study and HPLC analysis of the products, a mechanism for the antioxidant action of
1 has been proposed, suggesting the contribution of the phenolic group as well as the active methylene group of the 1,3-diketones [
31].
The pulse radiolysis studies revealed its high reactivity with the hydroxyl and glutathiyl radicals. With the hydroxyl radicals, it initially formed three species, an adduct, a phenoxyl and a methylenic radical. The bimolecular rate constants for the formation of these species were the same (6.8 × 109 M−1s−1). At a later stage a phenoxyl radical is produced through an intramolecular transformation from the initial radicals [
33]. In the case of glutathiyl-radical-induced oxidation, a carbon-centered radical was inferred from the result of the oxygen effect on the decay of the radical. A suitable mechanism for all the reactions mentioned for the oxidation of
1 has been proposed
31. Patro B.S., Rele S., Chintalwar G.J., Chattopadhyay S., Adhikari S., Mukherjee T. Lipid protective activities of dehydrogingerdiones and probable mode of action. Chem. Bio. Chem. 2002;3:364–370.
32. Packer J.E., Slater T.F., Willson R.L. Direct observation of a free radical interaction between vitamin E and C. Nature. 1979;278:737–738. [
PubMed]
33. Patro B.S., Adhikari S., Chintalwar G.J., Chattopadhyay S., Mukherjee T. The radioprotection and antioxidant properties of dehydrogingerdione. Res. Chem. Interm. 2005;31:667–678.
34. Ng T.B., Liu F., Wang Z.T. Antioxidant activity of natural products from plants. Life Sci. 2000;66:709–723. [
PubMed]