**5. Fate of green tea catechins in the body**

#### **5.1. Gastrointestinal absorption**

iron, but also synthetic compounds are capable of chelating iron *in vivo*, thereby limiting its participation in free radical reactions. Thus, iron chelators also serve as antioxidants by suppressing iron-mediated oxidation in biological systems. Surprisingly, thiol compounds (e.g. glutathione) that are synthesised by mammals can afford significant antioxidant protec‐ tion. This protection is related to the ability of glutathione to trap radicals, reduce peroxides,

The potential of green tea to prevent or ameliorate chronic diseases is currently the subject of considerable scientific investigations. Although a number of mechanisms have been proposed for their beneficial effects, the radical scavenging and antioxidant properties of green tea catechins are frequently cited as important contributors. Emerging evidence has also shown that catechins and their metabolites possessmany additional mechanisms of action [13] by affecting numerous sites, potentiating endogenous antioxidants and elicit‐ ing dual actions during oxidative stress. Much of the evidence supporting an antioxidant function for green tea catechins is derived from assays of their antioxidant activity *in vitro*. However, the evidence that green tea catechins are acting either directly or indirectly as

Green tea catechins stoichiometrically bind ferric ion to form a redox-inactive iron-phenolic complex [15, 16] and potentially protect vital biomolecules from oxidative damage. Incredibly, the catechins could be capable of chelating excessive redox iron in iron overloaded diseases, such as thalassemia, and play important preventive or/and protective roles in this unpleasant condition [2, 17-19]. The phytochemical compounds therefore play a double role in reducing the rate of oxidation because they can participate in: i) iron chelation [19], and ii) trapping radicals [2, 20]. Catechins can protect culture cells from iron-mediated damage [21, 22], ameliorate iron accumulation [17] and inhibit hepatic iron-induced lipid oxidation [23], and also play a dual effect in decreasing labile plasma iron (LPI) in iron-loaded rats [18]. Animal studies offer a unique opportunity to assess the contribution of green tea administration to the physiological effects on different models of oxidative-related diseases. In a combination of free-radical scavenging activity with iron-chelating properties, green tea may have a capacity of chelating excess iron in iron-overloaded conditions and play important preventive and/or

Like the deferiprone (DFP) treatment, oral administrations of GTE and EGCG significantly lowered levels of plasma non-transferrin bound iron (NTBI) and LPI in wild-type (WT), heterozygous β-globin gene knockout (BKO) thalassemic and double heterozygous β-globin gene knockout carrying human βE gene (DH) mice (strain C57BL/6J) with iron overload, when compared to the DW group (Table 2) (Sakaewan Ounjaijean and Somdet Srichairatanakool, unpublished data). Elimination of these two toxic irons by green tea extract and EGCG fraction would relieve redox iron-induced oxidative stress and tissue damage in the body. GTE treatment efficiently depleted plasma malondialdehyde (MDA) concentrations in the iron-

as well asits ability to work to maintain the redox state of the cells [12].

366 Pharmacology and Nutritional Intervention in the Treatment of Disease

antioxidants *in vivo* is limited [14]

**4. Anti-oxidative and iron-chelating properties**

protective roles in this unpleasant condition.

Among these polyphenolic compounds, the hierarchy of antioxidant activity is ECG > EGCG > EGC > GA > EC ≈ C [24]. With the chelating activity of such prooxidant metals as iron (Fe2+), green tea is able to reduce dietary nonheme iron absorption [25]. The ratio of EGC, EGCG, ECG or EC to the iron was 3:2, 2:1, 2:1 and 3:1, respectively [26]. Unlike most flavonoids, tea catechins existing as aglycone are found in the blood following oral ingestion and subsequently metabolized in the liver by methylation, sulfation and glucoronidation reactions [27]. Struc‐ ture-activity studies have shown that the presence of the galloyl ring in the 3-position and trihydroxyphenyl B ring are of significant importance in terms of the antioxidant properties of the catechins [26].

**5.2. Organ metabolism**

aminotransferase (ALT) activity in mice [36].

**5.3. Excretion**

hours) [37].

**6. Health benefits**

**6.1. Health promotion**

EC was not glucuronidated by uridine diphosphate-glucuronosyltransferases (UGT) and sulfotransferases (SULT) in human liver and small intestinal microsomes. However, the compound was efficiently glucuronidated in rat liver microsomes with the formation of two different glucuronides, and was also sulfated in human liver cytosol, mainly through the SULT1A1 isoform, as well as in the intestine through the SULT1A1 and SULT1A3 isoforms. In comparison, the EC was much less sulfated in the rat liver than in the human liver [32]. EGCG and ECG constituents in green tea drinks (10%, *w*/*v*) almost completely, competitively inhibited the activities of the SULT1A1 and SULT1A3 enzymes that play an important role in the presystemic inactivation of β2 agonists in the liver and intestine, respectively [33]. When EGCG was glucuronidated by the liver microsomal UGT enzymes; four EGCG-glucuronides were identified as EGCG-3-glucuronide, EGCG-3'-glucuronide, EGCG-4'-glucuronide and EGCG-7-glucuronide. Under the same conditions, EGC was metabolized into two EGCglucuronides as EGC-7-glucuronide and EGC-3'-glucuronide [34]. Since rate of the glucuro‐ nidation of EGCG and ECG in liver microsomes is rather low (12.2±0.2 and 7.5±0.2%, respectively for 3 hours) due to the galloyl ring, the two potent catechins are therefore recognized to be circulating in the plasma in unconjugated forms [35]. One study claimed that catechins were toxic to rat liver cells in the order of EGCG (LD50 200±19 µM) > ECG (LD50 2,000±214 µM) > GA (LD50 3,000±298 µM), EGC (LD50 3,000±304 µM) > EC (LD50 >10,000 µM), and this was likely due to the mitochondrial membrane potential (∆ψm) collapse, and the depletion of GSH and ROS formation. The EGCG and GA dose dependently affected GSH conjugation, methylation, metabolism by NAD(P)H:quinoneoxidoreductase 1 (NQO1) in the hepatic detoxification step (Figure 2), as monitored by a significant increase of plasma alanine

Green Tea: Just a Drink or Nutraceutical http://dx.doi.org/10.5772/57519 369

Most polyphenolic catechins in green tea may not be absorbed in the small intestine. The nonabsorbed catechins will be converted by large bow bacterial flora into simpler phenol‐ ic compounds, such as hippuric acid, then absorbed into the blood and excreted in the urine (4.22±0.28 mmol/24 hours), when compared to non-consumption (1.89+0.28 mmol/24

The potential health effects of green tea catechins depend not only on the amount consumed, but also on their bioavailability, which appears to be substantially varied. Following the oral administration of tea catechins to rats [38] and mice [39], the four principal catechins have been identified in the portal vein, indicating that tea catechins are absorbed intestinally. There appear to be species-related differences in the bioavailability of EGCG compared to other tea

After green tea (25 mg/kg) and pure EGCG fractions (10 mg/kg) were intravenously adminis‐ tered into rats, a study of the pharmacokinetic (concentration-time curves) properties of the catechins in the plasma was conducted. Beta-elimination half-lives (T1/2β) were found to be 212, 45, and 41 minutes; clearances were 2.0, 7.0, and 13.9 ml•minute/kg; and apparent distribution volumes (Vd) were 1.5, 2.1, and 3.6 dl/kg for EGCG, EGC, and EC, respectively. In comparison, EGCG had a shorter T1/2β (135 minute), a larger clearance (72.5 ml•minute/kg), and a larger volume (Vd) (22.5 dl/kg) than the other two. When the green tea was intragastri‐ cally given (200 mg/kg), around 0.1, 13.7 and 31.2% of EGCG, EGC and EC were detected in the plasma compartment. The EGCG level was found to be the highest in the intestine samples and declined with a T1/2 of 173 minute. EGC and EC levels were found to be the highest in the kidneys and declined rapidly with T1/2 of 29 and 28 minute, respectively. EGCG, EGC, and EC levels in the liver and lungs were lower than those recorded in the intestine and the kidney [28]. This implies that EGCG is mainly excreted through bile, while EGC and EC are excreted through urine and bile. Inter-individual variations in the bioavailability of green tea catechins can be substantial and may be due, in part, to differences in colonic microflora and genetic polymorphisms among the enzymes involved in polyphenol metabolism [29]. The effect of green tea drinking may also differ by genotype [30].

Following oral administration of green tea catechin solutions (0.6%, *w*/*v*) to the rats, plasma levels of the catechins measured at 6:00 AM., 9:00 AM., 0:00 PM and 6:00 PM on the same day were found to be 983.9±114.2, 372.89±56.7, 186.89±34.5 and 548.1±221.6 ng/ml EGC; 1,527.3±163.7, 449.2±82.7, 224.6±58.0 and 845.6±374.0 ng/ml ECG; and 105.0±12.6, 85.5±15.8, 95.7±18.3 and 114.8±45.6 ng/ml EGCG, respectively, for which the plasma EGCG concentra‐ tions were found to be even lower than those of EGC and EC. There wasa gradual increase in plasma concentrations of EGC, EC and EGCG during Days 1-4. Levels of EGC and EC in the plasma on Day 14 were approximately three times higher than those on Day 1. Plasma levels of these three catechins decreased after Day 14, and by Day 28, plasma levels of the EGC and EC returned to the levels recorded on Day 1. On Days 4, 14 and 28, the catechins were mostly present in glucuronic acid (MW=194)/sulfuric acid (MW=98) conjugated-EGC (91.4, 95.6 and 86.4 ng/ml, respectively) and-EC (92.4, 95.5 and 89.4 ng/ml, respectively), while a much lower proportion of EGCG was found in the conjugated form (21.2, 60.7 and 39.3 ng/ml, respectively). The highest EGC concentration was found in the bladder; the highest EGCG concentration was found in the large intestine; very high concentrations of EGC and EGCG were found in the kidneys, prostate gland and lungs; and low levels of these three catechins were present in the liver, spleen, heart, and thyroid glands [31].

#### **5.2. Organ metabolism**

green tea is able to reduce dietary nonheme iron absorption [25]. The ratio of EGC, EGCG, ECG or EC to the iron was 3:2, 2:1, 2:1 and 3:1, respectively [26]. Unlike most flavonoids, tea catechins existing as aglycone are found in the blood following oral ingestion and subsequently metabolized in the liver by methylation, sulfation and glucoronidation reactions [27]. Struc‐ ture-activity studies have shown that the presence of the galloyl ring in the 3-position and trihydroxyphenyl B ring are of significant importance in terms of the antioxidant properties

After green tea (25 mg/kg) and pure EGCG fractions (10 mg/kg) were intravenously adminis‐ tered into rats, a study of the pharmacokinetic (concentration-time curves) properties of the catechins in the plasma was conducted. Beta-elimination half-lives (T1/2β) were found to be 212, 45, and 41 minutes; clearances were 2.0, 7.0, and 13.9 ml•minute/kg; and apparent distribution volumes (Vd) were 1.5, 2.1, and 3.6 dl/kg for EGCG, EGC, and EC, respectively. In comparison, EGCG had a shorter T1/2β (135 minute), a larger clearance (72.5 ml•minute/kg), and a larger volume (Vd) (22.5 dl/kg) than the other two. When the green tea was intragastri‐ cally given (200 mg/kg), around 0.1, 13.7 and 31.2% of EGCG, EGC and EC were detected in the plasma compartment. The EGCG level was found to be the highest in the intestine samples and declined with a T1/2 of 173 minute. EGC and EC levels were found to be the highest in the kidneys and declined rapidly with T1/2 of 29 and 28 minute, respectively. EGCG, EGC, and EC levels in the liver and lungs were lower than those recorded in the intestine and the kidney [28]. This implies that EGCG is mainly excreted through bile, while EGC and EC are excreted through urine and bile. Inter-individual variations in the bioavailability of green tea catechins can be substantial and may be due, in part, to differences in colonic microflora and genetic polymorphisms among the enzymes involved in polyphenol metabolism [29]. The effect of

Following oral administration of green tea catechin solutions (0.6%, *w*/*v*) to the rats, plasma levels of the catechins measured at 6:00 AM., 9:00 AM., 0:00 PM and 6:00 PM on the same day were found to be 983.9±114.2, 372.89±56.7, 186.89±34.5 and 548.1±221.6 ng/ml EGC; 1,527.3±163.7, 449.2±82.7, 224.6±58.0 and 845.6±374.0 ng/ml ECG; and 105.0±12.6, 85.5±15.8, 95.7±18.3 and 114.8±45.6 ng/ml EGCG, respectively, for which the plasma EGCG concentra‐ tions were found to be even lower than those of EGC and EC. There wasa gradual increase in plasma concentrations of EGC, EC and EGCG during Days 1-4. Levels of EGC and EC in the plasma on Day 14 were approximately three times higher than those on Day 1. Plasma levels of these three catechins decreased after Day 14, and by Day 28, plasma levels of the EGC and EC returned to the levels recorded on Day 1. On Days 4, 14 and 28, the catechins were mostly present in glucuronic acid (MW=194)/sulfuric acid (MW=98) conjugated-EGC (91.4, 95.6 and 86.4 ng/ml, respectively) and-EC (92.4, 95.5 and 89.4 ng/ml, respectively), while a much lower proportion of EGCG was found in the conjugated form (21.2, 60.7 and 39.3 ng/ml, respectively). The highest EGC concentration was found in the bladder; the highest EGCG concentration was found in the large intestine; very high concentrations of EGC and EGCG were found in the kidneys, prostate gland and lungs; and low levels of these three catechins were present in the

of the catechins [26].

green tea drinking may also differ by genotype [30].

368 Pharmacology and Nutritional Intervention in the Treatment of Disease

liver, spleen, heart, and thyroid glands [31].

EC was not glucuronidated by uridine diphosphate-glucuronosyltransferases (UGT) and sulfotransferases (SULT) in human liver and small intestinal microsomes. However, the compound was efficiently glucuronidated in rat liver microsomes with the formation of two different glucuronides, and was also sulfated in human liver cytosol, mainly through the SULT1A1 isoform, as well as in the intestine through the SULT1A1 and SULT1A3 isoforms. In comparison, the EC was much less sulfated in the rat liver than in the human liver [32]. EGCG and ECG constituents in green tea drinks (10%, *w*/*v*) almost completely, competitively inhibited the activities of the SULT1A1 and SULT1A3 enzymes that play an important role in the presystemic inactivation of β2 agonists in the liver and intestine, respectively [33]. When EGCG was glucuronidated by the liver microsomal UGT enzymes; four EGCG-glucuronides were identified as EGCG-3-glucuronide, EGCG-3'-glucuronide, EGCG-4'-glucuronide and EGCG-7-glucuronide. Under the same conditions, EGC was metabolized into two EGCglucuronides as EGC-7-glucuronide and EGC-3'-glucuronide [34]. Since rate of the glucuro‐ nidation of EGCG and ECG in liver microsomes is rather low (12.2±0.2 and 7.5±0.2%, respectively for 3 hours) due to the galloyl ring, the two potent catechins are therefore recognized to be circulating in the plasma in unconjugated forms [35]. One study claimed that catechins were toxic to rat liver cells in the order of EGCG (LD50 200±19 µM) > ECG (LD50 2,000±214 µM) > GA (LD50 3,000±298 µM), EGC (LD50 3,000±304 µM) > EC (LD50 >10,000 µM), and this was likely due to the mitochondrial membrane potential (∆ψm) collapse, and the depletion of GSH and ROS formation. The EGCG and GA dose dependently affected GSH conjugation, methylation, metabolism by NAD(P)H:quinoneoxidoreductase 1 (NQO1) in the hepatic detoxification step (Figure 2), as monitored by a significant increase of plasma alanine aminotransferase (ALT) activity in mice [36].

#### **5.3. Excretion**

Most polyphenolic catechins in green tea may not be absorbed in the small intestine. The nonabsorbed catechins will be converted by large bow bacterial flora into simpler phenol‐ ic compounds, such as hippuric acid, then absorbed into the blood and excreted in the urine (4.22±0.28 mmol/24 hours), when compared to non-consumption (1.89+0.28 mmol/24 hours) [37].
