Thesis-Final 03 June 2011 pdf - Jacobs University

Background Chapter 2
Once absorbed, polyphenols are subjected to human phase II metabolism (the
conjugation): this process, that mainly includes methylation, sulfation, and
glucuronidation, represents a metabolic detoxication process, common to many
xenobiotics, that restricts their potential toxic effects and facilitates their biliary and
urinary elimination by increasing their hydrophilicity. 40
Catechol-O-methyl transferase (COMPT) catalyzes the transfer of a methyl group
from S-adenosyl-L-methionine to polyphenols such as quercetin, luteolin, caffeic
acid, catechins and cyaniding. 41 COMPT activity is highest in the liver and the
kidneys, although it is present in a number of tissues. 42
Phenil-Sulfotransferases (P-PST) catalyze (the transfer of a sulphate moiety from 3’-
phosphoadenosine-5’-phosphosulfate to a hydroxyl group on various substrates,
among which are polyphenols. The sulfation occurs mainly in the liver, but the
position of sulfation for polyphenols have not been clearly identified yet. 43
UDP-glucuronosyltransferases (UDPGT) are membrane-bound enzymes located in
the endoplasmic reticulum in many tissues, which catalyze the transfer of a glucuronic
acid from UDP-glucuronic acid to polyphenols as well as to steroids, bile acids and
many dietary constituents. Glucuronidation occurs in the intestine and in the liver. 44,45
Circulating polyphenols in plasma are conjugated derivatives that are extensively
bound to albumin. Polyphenols are able to penetrate tissues, particularly those in
which they are metabolized, such as intestine and liver. 46
Functions of phenolic acids in plants
Phenolics are great importance as cell-wall support materials. They form an integral
part of the cell- wall structure, mainly in the form of polymeric materials such as
lignins. Lignins are, after cellulose, the second most abundant organic structures on
earth. A most significant function of the phenolic phytochemicals is their contribution
to flower and fruit colours. This is an important for attracting insects and birds to the
plant for pollination and seed dispersal. Phenolics may influence the competition
among plants, a phenomenon called ‘allelopathy’. Besides the well-known volatile
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