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catechins are found in high levels in a variety of foods , including black tea , apples , wine , and chocolate [ 1 , 2 ] .
flavonoids including ( )-epicatechin have multiple biological functions that are thought to reduce the incidence of atherosclerosis .
epidemiological studies have suggested that the intake of catechins correlates with risk reduction for coronary heart disease [ 4 , 5 ] .
oxidation of tyrosine residues , which occurs upon the nitration of proteins , is of particular interest in the pathology of inflammation . upon oral administration
there have been a few reports to date on antioxidant activities in these metabolites [ 79 ] .
we have shown in humans and rats that , following the ingestion of cocoa powder , ( )-epicatechin is absorbed from the intestinal tract and distributed as a variety of conjugated and methylated forms in the blood before being excreted in the urine [ 10 , 11 ] .
we also isolated and identified ( )-epicatechin glucuronide metabolites from human and rat plasma . in humans , these include ( )-epicatechin-3'-o - glucuronide ( e3'g ) and 4'-o - methyl-()-epicatechin-3'-o - glucuronide ( 4'me3'g ) , and in rat , the metabolites were ( )-epicatechin-7-o - glucuronide ( e7 g ) and 3'-o - methyl-()-epicatechin-7-o - glucuronide ( 3'me7 g ) . the structures of these compounds are shown in fig .  1 .
the aim of the current study was to examine the antioxidative activities , especially that of nitrotyrosine formation , of the glucuronide metabolites of ( )-epicatechin present in human and rat plasma .
h2o2 was eliminated by passage of the peroxynitrite solution over mno2 powder with the final peroxynitrite concentration determined by its absorbance at 302  nm (   =  1620  mcm ) .
protection against peroxynitrite - mediated tyrosine nitration was performed as described by arteel et al . .
briefly , peroxynitrite ( 0.5  mm ) was added as a bolus to a 0.1  mm tyrosine solution in 0.1  m phosphate buffer ( ph  7.3 ) containing 0.1  mm dtpa with constant vortexing .
the effects of the tested compounds ( 031.5  m ) were determined by injection of 10  l of sample , supplemented with 0.1  mm 3-hydroxy-4-nitrobenzoic acid as an internal standard , onto a c-18 reverse - phase column ( ods - ug - deverosil 4.6    150 mm ; nomura chemical , aichi , japan ) interfaced with the autosampler from an hp1100 liquid chromatography system ( agilent technologies , palo alto , ca ) .
separation was performed with a 50  mm potassium phosphate buffer ( ph 7.0)/acetonitrile step - gradient delivered by the hp1100 system at a flow rate of 1.0  ml / min .
the gradient was initiated at a ratio of 95:5 buffer / acetonitrile , followed in stepwise adjustment to a final ratio of 50:50 buffer / acetonitrile over 5  min , with a return to 95:5 buffer / acetonitrile ratio at 13  min , which was then maintained for an additional 13  min .
formation of 3-nitrotyrosine was monitored with a uv / visible detector set at 430  nm . to determine the concentration of 3-nitrotyrosine in the sample , the peak area ratio of the 3-nitrotyosine to the internal standard was calculated and compared to a standard curve .
figure 2 shows the ability of the test compounds to protect free tyrosine from the nitration by peroxynitrite . the concentration of metabolites required to yield a 50% inhibition of tyrosine nitration increased in the following order : e3'g  >  ( )-epicatechin  >  e7 g  =  3'me7 g .  =  4'me3'g .
a variety of foods , particularly green tea , chocolate , cocoa , and red wine have high levels of the flavonoid ( )-epicatechin . flavonoids have multiple biological functions , including the suppression of ldl oxidation [ 3 , 15 ] , a process thought to reduce the incidence of atherosclerosis [ 16 , 17 ] . in the present study , we compared the peroxynitrite scavenging activities of ( )-epicatechin and its four major metabolites : e3'g and 4'me3'g , which are found in human plasma , and e7 g and 3'me7 g , which are found in rat plasma .
peroxynitrite may be generated in vivo under pathological conditions , especially in the inflammatory response , due to the reaction of superoxide radicals with nitric oxide .
moreover , ( )-epicatechin strongly suppresses nitration of tyrosine residues of ldl by myeloperoxidase ( mpo)/nitrite or peroxynitrite .
flavonoids protect biological molecules against peroxynitrite - mediated nitration by neutralizing peroxynitrite and its reaction intermediate , the tyrosyl radical [ 6 , 2024 ] . in this study
, we found that both ( )-epicatechin and its metabolites inhibited peroxynitrite - mediated nitration .
e3'g was more potent than ( )-epicatechin and the other metabolites at the concentrations between 2.1 and 42.8  m .
( )-epicatechin glucuronide and methylated forms of epicatechin were similar in their ability to inhibit tyrosine nitration . in the presence of 0.5  mm peroxynitrite , the ic50 values of ( )-epicatechin and e3'g were 32.1 and 21.6  m , as calculated by linear regression analyses , respectively .
reported that one equivalent of ( )-epicatechin prevents nitration by 1530 equivalents of peroxynitrite . an alternative action mechanism for epicatechin was proposed by schroeder et al .
, who hypothesized that epicatechin acts at the interference with tyrosyl radicals rather than by direct interaction with peroxynitrite . furthermore , the plasma metabolites of epicatechin may contribute to the antioxidant activity on peroxynitrite attack in blood plasma after administration of catechin rich foods .
previously , we identified four metabolites of ( )-epicatechin in blood and urine : ( )-epicatechin-3'-o - glucuronide ( e3'g ) , 4'-o - methyl-()-epicatechin-3'-o - glucuronide ( 4'me3'g ) , ( )-epicatechin-7-o - glucuronide ( e7 g ) , and 3'-o - methyl-()-epicatechin-7-o - glucuronide ( 3'me7 g ) ( natsume et al . 
 free radical biol . med . 
 34 , 840 - 849 , 2003 ) . 
 the aim of the current study was to compare the antioxidative activities of these metabolites with that of their parent compound . 
 after oral administration of ( )-epicatechin , e3'g and 4'me3'g were isolated from human urine , and e7 g and 3'me7 g isolated from rat urine . 
 we found that these compounds inhibited peroxynitrite - mediated tyrosine nitration , in the following order of potency : e3'g  >  ( )-epicatechin  >  
 e7 g  =  3'me7 g .  
 =  4'me3'g . 
 these results demonstrate that the metabolites of ( )-epicatechin retain antioxidative activity on peroxynitrite - induced oxidative damages to some extent .