The role of free radical and active oxygen in the pathogenesis of certain human diseases, including aging, cardiovascular disease and cancer is becoming increasingly recognized. Lipid peroxidation has been regarded as one of the mechanisms of senescence of humans and the cause of atherosclerosis. Because of their very high chemical reactivity, free radicals show very short lifetimes in biological systems. However, the excessive amounts of free radicals are able to produce metabolic disturbances and to damage membrane structures in a variety of ways. Therefore, much attention has been focused on the use of antioxidants, especially natural antioxidants, to inhibit lipid peroxidation or to protect the damage of free radicals.
Many investigations indicated that intake of certain amounts of fruits and vegetables that contain a large quantity of vitamin C and vitamin E showed antioxidative activity. Tea is not only rich in vitamin C and E, but also contains an important group of polyphenols, i.e., catechins, which display obvious antioxidative activity. The polyphenols are able to act as antioxidants by virture of the hydrogen-donating capacity of their phenol groups, as well as their metal-chelating potential (). As early as in 1963, Kajimoto first reported the antioxidative activity of tea. After that, many researchers demonstrated that catechins possess more potent antioxidative activity than that of vitamin C and E as well as other synthetic antioxidants (such as BHA, BHT etc). By adding the catechins, DL-a-tocopherol (vitamin E) or BHA (butylhydroxy anisole) to lard or vegetable oils, Hara () reported that the catechins reduced the formation of peroxides more effectively than vitamin E or BHA. In an experiment using linoleic acid as the material of antioxidative activity, it was shown that among various kinds of tea, the antioxidative activity decreased in the order of semi-fermented tea>unfermented tea>fermented tea (). According to the results of rancimet method of pure lard and the lipoxygenase assay method, among the various tea components, the theaflavin (TF) compounds isolated from black tea showed the strongest antioxidative activity. The IC50 of theaflavin monogallate B and theaflavin digallate toward soybean 15-lipoxygenase enzyme was 0.4 and 0.2 µg respectively. ECG, ECG and EGCG showed moderately IC50 values ranging from 4.6-7.7 µg. By using the red cell membrane system in vitro, the peroxidation of the rabbit red blood cell membrane was induced and different compounds were introduced to this system to test their antioxidative activity.
Among the tested tea compounds, all TFs exhibited much stronger antioxidative effects than vitamin E or propyl gallate. theaflavin digallate showed the most potent antioxidative activity, inhibiting about 80% of the peroxidation, followed by theaflavin monogallate B, theaflavin monogallate A and theaflavin (). Li et al. () showed that the inhibition rate of 200 µg/ml green tea polyphenol on the lipid peroxidation of red cell membrane ranged from 32.9% to 55.3%. 1mg of green tea polyphenol showed an ability to scavenge superoxide anion radicals the same as 9 mg bovine cupro-zinc superoxide dismutase (SOD) (). Serafini et al. () evaluated the in vitro antioxidant activity of green and black tea as well as their in vivo effect on plasma oxidation potential in man. Results showed that both teas inhibited the in vitro peroxidation in a dose-dependent manner. Green tea was sixfold more potent than black tea. The addition of milk to tea did not appreciably modify their in vitro antioxidant potential. In vivo, the ingestion of tea produced a significant increase of TRAP (Human Plasma Antioxidant Capacity) (P<0.05).
The antioxidant activity of aqueous extracts of green tea, black tea and Oolong tea was compared by von Gadow et al. () with the β-carotene bleaching method and α,α-diphenyl-β-picrylhydrazyl (DPPH) radical scavenging method. Results by these two methods showed the same tendency: green tea>black tea>Oolong tea. In a comparative experiment of the antioxidative activity of various kinds of tea, Xie et al. () reported that the antioxidative activity on autooxidation of lard was positively related to the contents of catechins, especially EGCG. The antioxidative activity of various teas was decreased in the order of Oolong tea>green tea and black tea (). Salah et al. () investigated the relative antioxidative activities in vitro of tea components. Results showed that those compounds with more hydroxy groups appear to exert the greatest antioxidative activity. The antioxidative potentials (Trolox Equivalent Antioxidant Activity, TEAC) of catechin and epicatechin were 2.4 mM and 2.5 mM, respectively. When the structure of catechin was modified to the catechin-gallate esters — ECG and EGCG, the TEAC was enhanced to 4.93 mM and 4.75 mM, respectively. The quercetin has an identical numbers of hydroxyl groups in the same position as catechin, but also contains the 2,3-double bond in the C ring and the 4-oxo group. This structure confers an enhancement of the TEAC value to 4.72 mM. With regards to the relationship of structure and antioxidative activity, it was noted that the conjugation between A and B rings via a planar C ring is the important structure for antioxidative activity. In addition, the contribution of the 3′,4′-dihydroxyl structure substitution in the B ring is highly significant for the antioxidative activity. And the presence of a 3-hydroxyl group in the C ring and a 5-hydroxyl group in the A ring is highly important for maximal radical scavenging potential ().
Salah et al. () used the oxidation of low-density lipoproteins (LDL) as a model for investigating the efficacy of the polyphenols as lipid chain-breaking antioxidants. The relative effectiveness of the catechins and catechin-gallate esters in inhibiting low-density cholesterol oxidation was determined. As shown, the gallic acid is the least effective, requiring about 1.2 µM for 50% inhibition of maximal oxidation. The IC50 value of EGC for inhibiting the oxidation was 0.75 µM. The IC50 values for catechin, EC, ECG and EGCG were in the ranged of 0.25-0.38 µM. Regarding the antioxidant activity of the polyphenolic components, the contribution of the components to the antioxidative effectiveness in green tea was determined as follows: EGC=EGCG>>ECG=EC. C.Zhang et al. () reported that tea polyphenols from jasmine tea and catechins showed antioxidative activity on the Cu mediated low-density cholesterol peroxidation and protected the oxidative degradation of unsaturated fatty acids in low-density cholesterol of humans. Fiala et al. () in an experiment adding peroxynitrite DNA and L-tyrosine showed that EGCG was a significantly better inhibitor of peroxynitrite-mediated oxidation of deoxyguanosine and tyrosine nitration. The 50% inhibition activity of the oxidation of the former was 4 times higher than that of vitamin C. He et al. () conducted an investigation on antioxidative effects of various tea polyphenols and catechins in a fish meat model system. Results showed that the antioxidative activity of green tea, tea polyphenols and catechin on fish lipid was higher than that of vitamin E, BHT, BHA and TBHQ.
Among the catechins, EGCG and ECG possessed the strongest activity. Yen et al. () used calf thymus DNA as the experimental material and the antioxidative effects of various tea extracts including the green tea, oolong tea and black tea were investigated. Results showed that the oxidation of deoxyribose was markedly decreased by various tea extracts in higher dosage, especially oolong tea, which inhibited 73.6% peroxidation of linoleic acid. The antioxidative activity decreased in the order of semifermented tea>nonfermented tea> fermented tea. In a similiar experiment in vitro, it was demonstrated that tea polyphenols suppressed the oxidative modification of porcine serum low-density cholesterol which is assumed to be an important step in the pathogenesis of atherosclerosis lesion. The activity was in the order of (-)-EGCG>(-)-ECG>(-)-EC>(-)-EGC. It was found that the Cu mediated cholesterol ester degradation in low-density cholesterol was almost completely inhibited by 5.0 µM EGCG (). Tomita (1997) investigated the IC50 of various antioxidants and tea catechins on the BHP (t-Butylheteroperoxide) induced peroxidative reaction in rat liver homogenerate. The IC50 of ECG, EGC, EGCG and TF, theaflavin monogallate A was 10-80 times lower than that of vitamin C and vitamin E and 10 times lower than that of BHT and BHA. The IC50 on TBARS (Thioburbituric Acid Reactive Substances) formation of low-density cholesterol was 0.95, 1.03, 1.13, 1.36, 2.74 and 3.09 µM for ECG, EGCG, EC, C, ECG, and BHT, respectively.
An experiment conducted in China showed that the green tea polyphenols had an inhibitory effect on iron-induced lipid peroxidation in synaptosomes. Among the various components of green tea polyphenols, the inhibitory effect decreased in the order of EGCG>ECG>EGC>EC, similar to the results from Miura et al. (). However, the free radical-scavenging activity was decreased in the order of ECG> EGCG>EC>EGC. It was regarded that the preventive activity of these catechins on lipid peroxidative damage induced by Fe++/Fe+++ is not only depended on the complexing ability with iron and free radical-scavenging ability, but also the stability of formed semiquinone free radicals (). Shen et al. () reported the threshold values for catechins to protect red cell membrane from injury caused by free radicals in the presence of iron ions. The values were 0.02 mM/L for EGCG, 0.025 mM/L for ECG, 0.028 mM/L for EGC and 0.05 mM/L for EC, respectively, with the same order reported by Guo et al. ().
By using mice as the experimental animal, Chang et al. () indicated that tea inhibited the formation of peroxidative lipid in the mice liver-brain tissue homogenate in vitro. By oral administration of tea infusion, the peroxidative lipid contents in heart, liver and brain tissue of young and adult mice decreased. The decreasing rates in old and adult BALB/c mice ranged from 23.7-41.95% and 12.7-23.2% in the low dosage group (3 g tea/kg/d) and 24.2-44.0% and 12.5-31.3% in the high dosage group (9 g tea/kg/d), respectively. Hara () administered excessive lipid to rats and observed the antioxidative activity of tea catechins on peroxidated lipid in the plasma of the rat. Results showed that the TBARS (Thiobarbituric Acid Reactive Substance) value of plasma was decreased (P<0.05) in the perilla oil+1% catechin diet. At the same time, the plasma α-tocopherol content was increased significantly (P<0.001). Yoshino et al. () examined the effects of tea polyphenols on the contents of lipids and lipid peroxidation in rat plasma, kidney and liver in vivo. The supplementation of tea polyphenols (0.5% and 1%) in the diet was performed from weanling (3 weeks of age) to 19 months old. The TBARS (Thiobarbituric Acid Reactive Substances) in the plasma of the 1.0% tea polyphenol group was significantly lower than that of the in vivo control group (P<0.01). The content of plasma lipids in the 1.0% tea polyphenols group of 19 months old rats were significantly lower than the control group (P<0.05), indicating the hypolipidemic activity and antioxidative effect of tea polyphenols. Sano et al. () reported that feeding 3% tea leaf powder to rats for 50 days protected the BHP-induced and tert-butyl hydroperoxide-induced lipid peroxidation in rat liver and kidney slices. Feeding with black tea resulted in an excellent antioxidant effect against lipid peroxidation, which was similar to that observed after feeding with green tea.
A recent experiment on the antioxidant effect of green tea and black tea in man conducted in Italy showed that both teas possessed a potent antioxidative activity in vitro and in vivo (). The ingestion of tea causes a significant increase of human plasma antioxidant capacity that represents the /(moles of peroxyl radicals trapped by one liter of plasma. Both green and black teas show the peak increase at 30–50 min; however, the antioxidative ability of green tea was about five times more potent than that of black tea. Moreover, the pro-oxidant property for some antioxidants including the tea polyphenols has been increasingly studied in recent years. The pro-oxidant activity is a result of the ability to reduce metals, such as Fe3+, to forms that react with O2 or H2O2 to form initiators of oxidation. The tea extracts also showed pro-oxidant effects at lower dosages (). So, in evaluating antioxidant activity of tea polyphenols, it must be assessed for pro-oxidant properties in vivo.
The above experiments clearly demonstrate that tea drinking possesses an antioxidative ability on lipid, which is believed to be exerted by the EGCG, ECG and related catechins in green tea as well as the TFs and thearubigins (TRs) in black tea. Even the ingestion of one large cup of tea could produce an appreciable increase in plasma TRAP values in man (). It can be deduced that the inhibition of lipid peroxidation might be one of the mechanisms in preventing cardiovascular disease in humans.
Selections from the book: “Tea: Bioactivity and Therapeutic Potential”. Edited by Yong-su Zhen, Zong-mao Chen, Shu-jun Cheng, and Miao-lan Chen. Series “Medicinal and aromatic plants – industrial profiles”. 2002.