Flavonoids plasma conjugates

PLASMA CONJUGATES OF FLAVONOIDS ARE NOT GLUCOSIDES BUT ARE SULFATED, GLUCURONIDATED, OR METHYLATED DERIVATIVES AND ARE RARELY SUBSTITUTED... 

A proportion of some flavonoids escapes intestinal conjugation. Evidence for this is derived from the presence of unconjugated flavonoids in the plasma. However, this has been shown only for galloylated catechins and isoflavones under certain conditions and is dose and time dependent, whereas unconjugated catechin and quercetin are not found in plasma in significant amounts. - " ... 

Hippuric acid was identified as a major metabolite associated with black tea consumption. " The compound l,3-dihydroxyphenyl-2-0-sulfate (sulfate conjugate of pyrogallol) was also identified as a black tea metabolite in humans. " Very low levels of theaflavins in plasma and urine were reported in humans after consumption of black tea. It was reported that the ingestion of either green tea or black tea results in a major increase in the excretion of hippuric acid into urine. The authors concluded that green tea and black tea consumption result in similar amounts of microbial degradation products that are absorbed by the body. They speculated that these microbial metabolites, not the native tea flavonoids, may be responsible for at least some of the health effects attributed to tea consumption. ... 

During digestion and transfer across the small intestine, and in the liver, flavanols are rapidly metabolized in phase I and phase II biotransformations to various 0-sulfated, <9-glucuronidated and (7-methylated forms. Various UGT isozymes, expressed in the intestine and in the Uver, have been identified to catalyze the glucuronidation of flavonoids. In humans consuming cocoa, plasma levels of non-methylated epicatechins such as epicatechin-7-sulfate and methylated metabolites such as 3 -0-methylepicatechin have been reported to occur in micromolar concentrations within 1 h after intake. Metabolic studies have confirmed the presence of these conjugates in the plasma and urine of rodents and humans, as well as in the bile and brain of rats. It has been reported that colonic microflora can break flavonoids flavan structure to form simple phenolics and rig-flssion metabolites that may be physiologically relevant. 

Terao, I (1999). Dietary flavonoids as antioxidants in vivo conjugated metaboHtes of (-)-epicatechin and quercetin participate in antioxidative defense in blood plasma. Journal of Medical Investigation, 46,159-168. 

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