Postepy Hig Med Dosw. (online), 2011; 65: 796-803
Original Article
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Investigation of antiradical potential of different kinds of teas and extracts from these teas using antiradical activity units (TAU)
Badanie potencjału przeciwwolnorodnikowego różnych rodzajów herbat oraz wyciągów uzyskanych z tych herbat za pomocą jednostki aktywności przeciwwolnorodnikowej (TAU)
Damian Wojciechowski1  BCD, Zbigniew Sroka1  ACDEFG, Andrzej Gamian2  EG
1Department of Pharmacognosy, Wrocław Medical University, Wrocław, Poland
2Department of Medical Biochemistry, Wrocław Medical University, Wrocław, Poland
Corresponding author
Zbigniew Sroka, Ph.D., Assoc. Prof., Department of Pharmacognosy, Wrocław Medical University, pl. Nankiera 1, 50-140 Wrocław; e-mail: zbsroka@farmgn.am.wroc.pl

Authors' Contribution:
A - Study Design, B - Data Collection, C - Statistical Analysis, D - Data Interpretation, E - Manuscript Preparation, F - Literature Search, G - Funds Collection

Source of support
This work was financially supported by a grant of Wrocław Medical University

Received:  2011.08.26
Accepted:  2011.11.09
Published:  2011.11.30

Streszczenie
Wprowadzenie: Herbata zielona, czarna i pu-erh mają, w porównaniu do innych surowców roślinnych, silne wła­ściwości przeciwwolnorodnikowe. W pracy zbadano15 różnych herbat należących do trzech typów, zielonych, czarnych i pu-erh (po pięć rodzajów z każdego typu) w kierunku ich właściwości przeciwwolnorodnikowych. Aktywność przeciwwolnorodnikową wyciągów i surowców zmierzono za pomocą rodników DPPH i ABTS•+.
Materiały/Metody:
Aktywność przeciwwolnorodnikową herbat zmierzono za pomocą DPPH (rodnika 1,1-difenylo­-2-pikrylohydrazylowego) i ABTS•+ (soli diamonowej kwasu 2,2'-azyno-bis(3-ethylobenzotiazo­lino-6-sulfonowego)). Zdefiniowano jednostki TAU515 i TAU734 (jednostki aktywności przeciw­wolnorodnikowej) odpowiednio dla testów z DPPH i ABTS•+ i określono liczbę jednostek na mg wyciągów (TAU515/mg i TAU734/mg) i g surowców (TAU515/g i TAU734/g).
Wyniki/Dyskusja:
Gdy badano wyciągi, największą liczbę jednostek aktywności przeciwwolnorodnikowej TAU515/mg i TAU734/mg stwierdzono w mg wyciągu octanowego otrzymanego z liści herbaty zielonej: odpo­wiednio 57,7±0,8; 106±2,0 jednostek. Gdy liczbę jednostek aktywności przeciwwolnorodnikowej (TAU515/g) policzono na g surowca, największy potencjał wolnorodnikowy (7601±92) zaobserwowano dla liści herbaty zielonej, naj­większą wartość TAU734/g (14303±354) obliczono również dla liści herbaty zielonej. Najniższą aktywność przeciwwolnorodnikową TAU515/g (684±30) i TAU734/g (1870±180) obliczono dla liści herbaty pu-erh.
Zaobserwowano wysoką, dodatnią korelację pomiędzy aktywnością przeciwwolnorodnikową su­rowców i zawartością związków fenolowych w tych surowcach.
Słowa kluczowe: liście herbaty • związki fenolowe • właściwości przeciwwolnorodnikowe


Summary
Introduction: Green, black and pu-erh teas are known to have strong antiradical properties in comparison to other plant raw materials.
In this study fifteen different teas belonging to three types, green, black, and pu-erh tea (five of each kind), were investigated for their antiradical properties. Antiradical activity of extracts and raw materials was measured using DPPH and ABTS•+ radicals.
Material/Methods: The antiradical potential of teas was measured using DPPH (1,1-diphenyl-2-picrylhydrazyl) and ABTS•+ (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt). The TAU515 and TAU734 (antiradical activity units) were defined with the tests of DPPH and ABTS•+, respec­tively, and the number of units was calculated per mg of extracts (TAU515/mg and TAU734/mg) and g of raw materials (TAU515/g and TAU734/g).
Results/Discussion: When the extracts were investigated, the highest number of antiradical units (TAU515/mg; TAU734/mg) was found per mg of ethyl acetate extract obtained from green tea leaves assayed with DPPH and ABTS•+: 57.7±0.8; 106±2.0, units respectively. When the number of antiradical units TAU515/g was calculated per g of raw material, the highest antiradical potential (7601±92) was observed for green tea leaves. The greatest TAU734/g value (14 303±354) was also calculated for green tea leaves. The lowest value of antiradical activity units TAU515/g (684±30) and TAU734/g (1870±180) was calculated for pu-erh tea leaves.
A high positive correlation was found between the antiradical activity of the raw materials and the content of phenolic compounds in these raw materials.
Key words: tea leaves • phenolic compounds • antiradical properties




Abbreviations:
TAU515/mg and TAU734/mg - the number of total antiradical units calculated per mg of extract using DPPH and ABTS•+ respectively; TAU515/g and TAU734/g - the number of total antiradical units calculated per g of raw material using DPPH and ABTS•+ respectively.
Introduction
The leaves of tea (Camellia sinensis (L.) Kuntze) are known to have strong antiradical and antioxidant proper­ties [26]. Brewed tea has been consumed for many centu­ries and health benefits have been attributed to tea since the very beginning of tea history. Nowadays tea of dif­ferent kinds (especially green and black) is the beverage most widely consumed, next to water, in the whole world [18]. Some research made in vitro and using animals pro­vides evidence that the polyphenolic compounds present in teas act beneficially in several chronic diseases [4,7,11]. For example, epigallocatechin gallate (EGCG) is attribu­ted to be responsible for the ability of green tea to promo­te health [19]. Both green tea and black tea are considered to have anti-carcinogenic properties, which has been pro­ved in many animal models [15,25].
Black tea is the most consumed tea in the world. More than 70% of all tea produced is black tea [6]. Many stu­dies have demonstrated the health benefits of black tea consumption [14].
It was demonstrated that risk of myocardial infarction was lower by 11% when consumption of the beverage was three cups a day (about 700 ml) [12]. Other authors reported a considerable decrease of coronary heart disease (CHD) when more than a cup of tea is drunk daily [17]. Some au­thors did not observe any beneficial effect [9], while others observed a positive correlation between tea consumption and a lower risk of CHD mortality [10] as well as hyper­tension, plaques of the carotid artery, and increased homo­cysteine concentration in plasma [14].
Brewed tea is rich in various types of polyphenolic compo­unds [6]. Ruxton [15] investigated the content of phenols in brewed black and green tea. Black tea in decreasing or­der is rich in epigallocatechin (10.43 mg/100 ml), epica­techin (2.33), quercetin (2.07), theaflavins (1.58), catechin (1.52), kaempferol (1.34), gallocatechin (1.26) and myri­cetin (0.45). Green tea is rich in epigallocatechin (17.08 mg/100 ml), epicatechin (8.47), catechin (2.73), querce­tin (2.69), kaempferol (1.42), myricetin (1.10) and theafla­vins (0.05) [14].
Tea polyphenols possess strong antiradical and antioxidant properties, and health benefits of consumption of tea are attributed partially to the antiradical and antioxidant pro­perties of tea [6]. Tea extracts have been shown to scaven­ge singlet oxygen [8], hydroxyl radical [23], peroxyl radi­cal [16], superoxide radical [24] and many other dangerous compounds which lead, among other things, to the destruc­tion of lipids, proteins and nucleic acids [21].
The antioxidant activity of green tea extracts is always hi­gher than that of black tea [1]. Especially effective as an antioxidant is epigallocatechin gallate (EGCG), which was detected in high concentrations in green tea extracts [5]. Green and black tea, in vitro, can inhibit lipoprotein oxi­dation initiated in the presence of Cu2+.
The antioxidant and antiradical activity of teas has been me­asured by several methods. The methods can be divided into so-called hydrogen atom transfer (HAT) and electron trans­fer methods (ET). The methods called FRAP (ferric redu­cing ability of plasma, also ferric ion reducing antioxidant power) belong to ET methods and use Trolox (water soluble vitamin E analogue) as a standard. The methods using DPPA and ABTS•+ radicals belong to the HAT methods [3]. Another problem is the way of demonstration of antiradical features of substances. Some authors use standard substances known for their antiradical activity such as vitamin C or Trolox [14].
However, there is a lack of papers demonstrating antira­dical activity of raw materials, extracts or substances as a general pool of hypothetical, defined units.
In this work the antiradical activity of different types of teas was measured and presented as the number of antira­dical units TAU515 and TAU734 for the tests with DPPH and ABTS•+ radicals per mg of extracts and g of raw materials.
Material and Methods
Raw material
Green teas
The leaf of green tea from China bought from PPH Biofluid sp.j. was marked with G1, the green classic English tea leaf from India was marked with G2, natural green tea Dilmah Sri Lanka (Sri Lanka) with G3, the leaf of green tea Sir Roger (China) with G4, and the green tea Teekanne (China) with G5.
Black teas
The leaf of classic English tea, Tetley from India was mar­ked with B1, Premium tea Ceylon, Dilmah (Sri Lanka) was marked with B2, Yellow Label tea, Lipton, a mixture of about 20 kinds of black teas, with B3, Yunnan Black Tea, Oskar International Trading sp. z o.o., from China, with B4, and Assam, Teekanne from India with B5.
Pu-erh teas
Pu-erh Bio-Active sp. z o.o. (China) - P1, pu-erh PPH Biofluid sp.j. (China) - P2, pu-erh Vitax Preminum Foods SA (China) - P3, pu-erh Bastek Coffee & Tea (China) - P4, pu-erh Herbapol Lublin SA (China) - P5.
Preparation of extracts
50 g of raw material was mixed with 900 ml of metha­nol and water (1:3). The raw material was heated at 70°C for 30 min, then was extracted at room temperature for 24 hours. The extract was filtered (filter discs, grade 388, Filtrak). Methanol was evaporated under reduced pressu­re and the remaining water was extracted three times with ethyl acetate (3×200 ml). The ethyl acetate extract and re­maining water was evaporated to dryness under reduced pressure to obtain the dry residues E (ethyl acetate) and A remaining water.
The obtained extracts were additionally marked with the letter of the raw material. For example, ethyl acetate extract obtained from natural green tea Dilmah Sri Lanka (Sri Lanka) G3 was marked as G3E.
Colorimetric measurement of phenolic compounds
The amount of phenolic compound was measured with the method of Singleton and Rossi [20].
Reagents
Reagent 1
5.77 g of sodium tungstate (Na2WO4 × 2 H2O) was dis­solved in 75 ml of water. Then 8 ml of 85% solution of H3PO4 was added. Then the reagent was heated at 100°C for 3 hours. After cooling the volume was adjusted to 100 ml with the water.
Reagent 2
An 18% (w/o) aqueous solution of Na2CO3 was prepared.
Measurement of phenolic compounds
0.5 ml of reagent 1 was added to 0.5 ml of the sample (extract in 50% methanol solution at the concentration 0.17 mg/ml). Then 8.5 ml of 2 was added. The absorbance at 750 nm was measured 2 min after 2 reagent addition. The absorbance was measured in a 1-cm glass cuvette against a blind sample (0.5 ml of 50% methanol was added instead of a sample with the extract). The measurement was repe­ated three times and the standard deviation was calculated.
Measurement of antiradical activity of extracts with the DPPH radical
The antiradical or antioxidant activity could be demonstra­ted in many different ways. Among others it is compared to the known antiradical standard substances such as vita­min C or Trolox. However, no authors try to demonstrate antiradical activity as a general pool of units in a determi­ned mass of raw material and/or extracts.
For that reason we define the antiradical units in a similar way as it is done for the demonstration of enzyme activi­ty [22]. We calculated these hypothetical units per mg of extracts and g of raw materials. We calculated the number of antiradical units per g of raw material as the sum of units calculated for extracts obtained from that raw material. The antiradical activity of extracts presented as the number of units per mg is compared to that of Trolox.
The obtained tea extracts contained compounds soluble in methanol as well as in water. Therefore two different tests were chosen to study the antiradical activity of these extracts: the method using the DPPH radical, which is suitable for fractions soluble in methanol, and the method with the ABTS•+ radical, more suitable for fractions soluble in water.
The method with DPPH was described by Brand-Williams et al. [2]. Antiradical properties were presented as the num­ber of antiradical units TAU515 per mg of extract (TAU515/mg) and per gram of raw material (TAU515/g).
One unit is defined as the amount of substance in 1 ml of reaction mixture which causes a decrease in absorban­ce of 1 after 1 minute at 515 nm in a glass cuvette with a 1-cm optical path.
Preparation of reagent
DPPH radical (Sigma-Aldrich) was dissolved in methanol (Merck) at the concentration of 0.094 mmol/l. The reagent was stored at 4°C for 24 h before use. Before the test the absorbance of DPPH solution was adjusted to the value equal to 1 with methanol addition.
Measurement of DPPH radical scavenging
To 1.46 ml of DPPH methanol solution, 0.04 ml of extract solution in methanol was added. Absorbance at 515 nm was measured in glass cuvettes with an optical path of 1 cm at 1 minute after extract addition to the sample. All measu­rements were repeated three times and standard deviation was calculated. The control sample was prepared by the ad­dition of 0.04 ml of methanol to 1.46 ml of DPPH radical.
The number of antiradical units TAU515/mg was calculated per mg of extract according to the equation:


where TAU515/mg = number of antiradical units calculated per mg of extract; A0 = absorbance of sample at the be­ginning of the reaction; A1 = absorbance of sample after 1 minute of reaction; m = weight of extract [mg] in 1 ml of measured sample (in cuvette).
The number of antiradical units TAU515/g was calculated per 1 g of raw material according to the equation:


where TAU515/g is the number of antiradical units calculated per g of raw material; TAU515A/mg is the number of antiradi­cal units calculated per 1 mg of aqueous extract; ma is the whole mass of aqueous extract [mg]; TAU515E/mg is the num­ber of antiradical units calculated per mg of ethyl acetate extract; me is the whole mass of ethyl acetate extract [mg]; and 50 is the weight of raw material taken for extraction [g].
The maximal error (ΔTAU515/mg and ΔTAU515/g) was calcu­lated according to the total differential method.
Measurement of antiradical activity of extracts with the ABTS•+ radical
Antiradical activity of extracts was measured with the me­thod of Re et al. [13].
Reagents
ABTS solution at 7 mmol/l in water and aqueous solution of K2S2O8 at 2.45 mmol/l were prepared. The two solutions were mixed in the volume ratio 1:1 and stored for 16 ho­urs in a dark place at room temperature. During that time ABTS•+ radical was formed. The final solution was diluted with water to the value of absorbance at 734 nm equal to 1.
Measurement of ABTS•+ scavenging
0.015 ml of aqueous solution of extract was added to 1.5 ml of aqueous solution of ABTS•+. The absorbance at 734 nm was measured at the beginning and after 1 minute of reaction. The measurement was performed three times and standard deviation was calculated.
The number of antiradical units TAU734 per mg of extract (TAU734/mg) and g of raw material (TAU734/g) was calculated in the same way as shown above for the test with the DPPH radical. The maximal error (ΔTAU734/mg and ΔTAU734/g) was calculated on the basis of the total differential method.
Results and Discussion
The weight of extracts [mg], number of antiradical units per mg of extracts (TAU515/mg), distribution of the entire number of antiradical units between ethyl acetate and aqu­eous extracts [%], amount of phenolic compounds [%] cal­culated per dry weight of raw material, and number of an­tiradical units per g of raw material (TAU515/g) are shown in Table 1. The diagram demonstrating the number of an­tiradical units per g of raw material (TAU515/g) is shown in Figure 1.
Table 1. Weight of extracts, number of antiradical units (TAU515/mg) per mg of extract, number of antiradical units (TAU515/g) per g of raw material, amount of phenolic compounds expressed in percentage per dry weight of raw material, the antiradical unit distribution between ethyl acetate and aqueous extracts. Value of TAU515/mg for Trolox was 51.5±0.9

Figure 1. The number of antiradical units TAU515/g calculated per g of raw material. G1-G5 - green tea, B1-B5 - black tea, P1-P5 - pu-erh

The highest number of antiradical activity units per mg of extract (TAU515/mg) was noted for green tea extract G4E (57.7±0.8), the lowest for pu-erh tea extract P4A (3.0±0.1). The number of units (TAU515/mg) per mg of Trolox was equ­al to 51.5±0.9. The highest number of antiradical units cal­culated per g of raw material (TAU515/g) was calculated for green tea classic English leaf from India G2 (7601±92), the lowest for pu-erh Bastek Coffee & Tea (China) tea P4 (684±30).
The green teas exhibited the strongest antiradical proper­ties, black teas lower and pu-erh the lowest.
The correlation coefficient „r" between the number of TAU515/g units and phenolic concentration expressed as a percentage is 0.89 (Figure 2).
Figure 2. The correlation between number of antiradical units TAU515/g and amounts of phenolic compounds C [%]

The number of antiradical units calculated per mg of extract (TAU734/mg), per g of raw material (TAU734/g), and the distri­bution of units between ethyl acetate and aqueous extracts, for the test with ABTS•+ radical, are shown in Table 2. A diagram showing the number of antiradical units per g of raw material (TAU734/g) is presented in Figure 3.
Table 2. The number of antiradical units per mg of extract (TAU734/mg), number of antiradical units per g of dry raw material (TAU734/g), antiradical units distribution in percentage between ethyl acetate and aqueous extracts. Value of TAU734/mg for Trolox was 81.2±1.5

Figure 3. The number of antiradical units TAU734/g per g of raw material. G1-G5 - green tea, B1-B5 - black tea, P1-P5 - pu-erh

The highest number of antiradical units (TAU734/mg) was cal­culated for extract G4E (106±2) and the lowest for extract P3A (11.3±1.3). The value TAU734/mg for Trolox is 81.2±1.5. The TAU734/g value was highest for G4 (14 303±354) raw material, and the lowest number of TAU734/g units was cal­culated for P3 (1870±180). The correlation coefficient „r" between the number of TAU734/g units and the amount of phenolic compounds in the raw material is 0.93 (Figure 4).
Figure 4. The correlation between number of antiradical units TAU734/g and amounts of phenolic compounds C [%]

The largest amount of phenolic compounds expressed in percentage per mass of raw material was found in G2 (5.84±0.36) and the lowest in P3 (1.26±0.12).
The distribution of antiradical units between ethyl acetate and aqueous extract for each of the raw materials is shown in Table 1 for tests with the DPPH radical and Table 2 for tests with the ABTS+• radical. For green tea leaves, 60-78% of TAU515/mg and 58-74% of TAU734/mg units were calculated for ethyl acetate extracts. For black tea, the respective va­lues were 32-53% of TAU515/mg and 26-37% of TAU734/mg units. For pu-erh tea, the amount of TAU515/mg units in ethyl acetate extracts was 30-44%, and the amount of TAU734/mg units was 20-26%.
Our results show that the strongest antiradical properties are exhibited by green tea leaves and, in decreasing order, black tea and pu-erh tea. Similar results were described in the literature [1]. The ethyl acetate extracts of green tea leaves showed stronger antiradical activity than extracts obtained from leaves of black or pu-erh teas. The antira­dical activity correlated well with the amount of pheno­lic compounds determined with the colorimetric method.
Analysis of the distribution among extracts of antiradical activity units TAU515/mg and TAU734/mg led to the following observations. For green tea leaf extracts a greater number of antiradical units was present in ethyl acetate extracts (Tables 1 and 2). When black and pu-erh teas were inve­stigated, a greater number of units was found in the aqu­eous extracts (Tables 1 and 2).
An explanation for this observation is that phenolic com­pounds present in green tea leaves such as epicatechin gal­late and epigallocatechin gallate (strong antioxidants) are more soluble in ethyl acetate than water.
During the fermentation process, green tea phenols are oxidized and turn into more polymerized structures, the­arubigins and theaflavins. These macromolecular substan­ces have a better affinity for water than for ethyl acetate.
We can conclude that if we take the green tea leaves for the preparation of extracts with antioxidant activity, ethyl acetate is a better solvent, while in the case of black tea and for pu-erh the better solvent is water.
One can draw the following conclusions from the investigation:
a) The leaves of green tea appeared to have the strongest antioxidant properties but those of pu-erh tea the lowest.
b) For green tea, a higher total number of antiradical units TAU515 and TAU734 was distributed to ethyl acetate extracts than to aqueous extract, but for black and pu-erh teas, more units were calculated for aqueous extracts.
c) There is a strong positive correlation between the con­tents of phenolic compounds in raw materials and the­ir antiradical activity.
REFERENCES
[1] Bancirova M.: Comparison of the antioxidant capacity and the antimicrobial activity of black and green tea. Food Res. Int., 2010; 43: 1379-1382
[Abstract]  
[2] Brand-Williams W., Cuvelier M.E., Berset C.: Use of a free radical method to evaluate antioxidant activity. LWT - Food Sci. Tech., 1995; 28: 25-30
[Abstract]  
[3] Folch-Cano C., Jullian C., Speisky H., Olea-Azar C.: Antioxidant activity of inclusion complexes of tea catechins with β-cyclodextrins by ORAC assays. Food Res. Int., 2010; 43: 2039-2044
[4] Hodgson J.M., Croft K.D.: Tea flavonoids and cardiovascular health. Mol. Aspects Med., 2010; 31: 495-502
[PubMed]  
[5] Hu J., Zhou D., Chen Y.: Preparation and antioxidant activity of green tea extract enriched in epigallocatechin (EGC) and epigallocatechin gallate (EGCG). J. Agric. Food Chem., 2009; 57: 1349-1353
[PubMed]  
[6] Khan N., Mukhtar H.: Tea polyphenols for health promotion. Life Sci., 2007; 81: 519-533
[PubMed]  
[7] Lee A.H., Fraser M.L., Binns C.W.: Tea, coffee and prostate cancer. Mol. Nutr. Food Res., 2009; 53: 256-265
[PubMed]  
[8] Mukai K., Nagai S., Ohara S.: Kinetic study of the quenching reaction of singlet oxygen by tea catechins in ethanol solution. Free Radic. Biol. Med., 2005; 39: 752-761
[PubMed]  
[9] Mukamal K.J., MacDermott K., Vinson J.A., Oyama N., Manning W.J., Mittleman M.A.: A 6-month randomized pilot study of black tea and cardiovascular risk factors. Am. Heart J., 2007; 154: 724.e1-724.e6
[PubMed]  
[10] Mukamal K.J., Maclure M., Muller J.E., Sherwood J.B., Mittleman M.A.: Tea consumption and mortality after acute myocardial infarction. Circulation, 2002; 105: 2476-2481
[PubMed]  [Full Text HTML]  [Full Text PDF]  
[11] Naito Y., Yoshikawa T.: Green tea and heart health. J. Cardiovasc. Pharmacol., 2009; 54: 385-390
[PubMed]  
[12] Peters U., Poole C., Arab L.: Does tea affect cardiovascular disease? A meta-analysis. Am. J. Epidemiol., 2001; 154: 495-503
[PubMed]  [Full Text HTML]  [Full Text PDF]  
[13] Re R., Pellegrini N., Proteggente A., Pannala A., Yang M., Rice-Evans C.: Antioxidant activity applying an improved ABTS radical cation decolorization assay. Free Radic. Biol. Med., 1999; 26: 1231-1237
[PubMed]  
[14] Rice-Evans C.A., Miller N.J., Paganga G.: Structure-antioxidant activity relationships of flavonoids and phenolic acids. Free Radic. Biol. Med., 1996; 20: 933-956
[PubMed]  
[15] Ruxton C.H.: Black tea and health. Nutr. Bull., 2008; 33: 91-101
[Abstract]  
[16] Sang S., Tian S., Wang H., Stark R.E., Rosen R.T., Yang C.S., Ho C.T.: Chemical studies of the antioxidant mechanism of tea catechins: radical reaction products of epicatechin with peroxyl radicals. Bioorg. Med. Chem. 2003; 11: 3371-3378
[PubMed]  
[17] Sesso H.D., Gaziano J.M., Buring J.E., Hennekens C.H.: Coffee and tea intake and the risk of myocardial infarction. Am. J. Epidemiol., 1999; 149: 162-167
[PubMed]  [Full Text PDF]  
[18] Sharangi A.B.: Medicinal and therapeutic potentialities of tea (Camellia sinensis L.). Food Res. Int., 2009; 42: 529-535
[Abstract]  
[19] Singh R., Akhtar N., Haqqi T.M.: Green tea polyphenol epigallocatechin-3-gallate: inflammation and arthritis. Life Sci., 2010; 86: 907-918
[PubMed]  [Full Text HTML]  [Full Text PDF]  
[20] Singleton V.L., Rossi J.A.Jr.: Colorimetry of total phenolics with phosphomolybdic-phosphotungstic acid reagents. Am. J. Enol. Viticult., 1965; 16: 144-158
[Abstract]  
[21] Skrzydlewska E., Ostrowska J., Farbiszewski R., Michalak K.: Protective effect of green tea against lipid peroxidation in the rat liver, blood serum and the brain. Phytomedicine, 2002; 9: 232-238
[PubMed]  
[22] Sroka Z., Bełz J.: Antioxidant activity of hydrolyzed and non-hydrolyzed extracts of the inflorescence of linden (Tiliae inflorescentia). Adv. Clin. Exp. Med., 2009; 18: 329-335
[Abstract]  
[23] Takeuchi Y., Okuno K., Yoshioka H., Yoshioka H.: Characteristics of the OH radical scavenging activity of tea catechins. J. Radioanal. Nucl. Chem., 2007; 272: 455-459
[Abstract]  [Full Text PDF]  
[24] Unno T., Sugimoto A., Kakuda T.: Scavenging effect of tea catechins and their epimers on superoxide anion radicals generated by a hypoxanthine and xanthine oxidase system. J. Sci. Food Agric., 2000; 80: 601-606
[Abstract]  
[25] Yang C.S., Wang X.: Green tea and cancer prevention. Nutr. Cancer, 2010; 62: 931-937
[PubMed]  [Full Text HTML]  [Full Text PDF]  
[26] Yokozawa T., Dong E., Nakagawa T., Kashiwagi H., Nakagawa H., Takeuchi S., Chang H.Y.: In vitro and in vivo studies on the radical-scavenging activity of tea. J. Agric. Food Chem., 1998; 46: 2143-2150
[Abstract]  
The authors have no potential conflicts of interest to declare.