Previous page

page_134

next page >

permitting abstraction of a hydrogen atom to form a phenoxy radical. The resulting ferulate radical is resonance stabilized, making it unlikely to propagate further radical formation; its likely fate is condensation with another radical, including another ferulate radical, to yield the dimer curcumin. Inhibition of lipid peroxidation by ferulic acid has been shown in rat brain homogenates in vitro, although under the conditions employed, caffeic acid (also found in cocoa) was a lot more effective. It has also been reported that ferulic acid, unlike caffeic acid, scavenged superoxide anion radical, and also inhibited lipid peroxidation induced by superoxide, an effect similar in magnitude to that seen with superoxide dismutase (42).

Chlorogenic acid, caffeic acid and protocatechuic acid were shown by Ohnishi et al. (43) to have significant radical-scavenging activities in an in vitro test system. Chlorogenic, caffeic, ferulic and protocatechuic acids displayed activity in preventing the early stages of peroxidation of linoleic acid. These compounds were also tested to see whether they could prevent lipid peroxidation and haemolytic damage to isolated mouse erythrocytes during exposure to hydrogen peroxide. Chlorogenic and caffeic acids were both effective in this respect, showing more activity than a-tocopherol.

Ohnishi et al. (43) propose that these phenolics react with lipid peroxides (LOO ) as follows: PheOH + LOO- ® PheO- + LOOH

In addition, there is evidence that some compounds, like the chlorogenic acid radical, may react with lipid peroxides: PheO - + LOO- ® LOO-Phe

This activity is potentially important. When lipid peroxides are formed, they can react with other molecules in the vicinity in membranes, this will often be with other lipids thereby propagating a chain reaction. The above reaction demonstrates how phenolic antioxidants can act as chain terminators, breaking the cycle of peroxidation.

The activity of caffeic acid against reactive oxygen species was confirmed by Chimi et al. (44), who demonstrated that it could scavenge both hydroxyl radicals (OH) and superoxide (O2-). It was also tested in a hepatocyte cell system supplemented with iron, inducing an oxidative stress which mimics certain pathological conditions. It was shown that caffeic acid was able to inhibit iron-induced lipid peroxidation. The authors comment that although free-radical scavenging might be the explanation for this protective effect, it is possible that the mechanism might also include iron chelation, effectively preventing the iron from reacting in this iron-loaded system. Such a mechanism has been demonstrated for polyphenols, for example flavonoids.

0 0

Post a comment