Hydroxycinnamic acids conjugates

Keywords phenylpropanoid metabolism hydroxycinnamic acid conjugates lignans coumarins furanocoumarins gallotannins eUagitarmins metabolic channelling... 

Important hydroxycinnamic acid conjugates from plants are hydroxycin-namic acid esters and hydroxycinnamic acid amides (see Petersen et al, 1999). 

For ipobscurines, hydroxycinnamic acid conjugates of serotonin (5-hydroxytryp-tamine), see Sect. 6.6.4 for withanamides, fatty acid amides of serotonin, see Sect. 8.I.2.I. 

Hydroxycinnamic acids and their derivatives (free, soluble conjugated, cell wall bound)... 

Phenolic acids are rarely present as free forms, except in processed food, but occur more frequently as soluble or insoluble esters. These esters are formed with polysaccharides or simple sugars, with quinic acid or other carboxylic acids such as tartaric or shikimic acids [Herrmann, 1989], with other phenolic acids, with lipids [Clifford, 2000], with sterols or glycerol [Clifford, 1999], or with amino acids [Clifford and Knight, 2004], To quinic acid, they can be conjugated as mono-, di-, tri-, and tetra-esters [Clifford, 2000]. The multiple esters can contain the same or different hydroxycinnamic acids. Among the hydroxycinnamic conjugates, caffeoylquinic and di-caffeoylquinic acids... 

Moreover, hydroxycinnamate-CoAs can conjugate with organic acids. In the biosynthesis of sugar derivatives of hydroxycinnamic acids, the transfer of glucose from uridine diphosphoglucose to hydroxyciimamic acid is catalyzed by glucosyl transferase [8, 16]. 

Caffeic acid 42-96 nmoI/L, entirely as glucuronide) and ferulic (not detectable-78 nmol/L, more than 50% as free acid) were detected in plasma of three volunteers post ingestion of 100 g of prunes after a 2-day low-hydroxycinnamate diet [22]. Furthermore, two of the three volunteers displayed basal plasma levels of caffeic acid (as glucuronide) and ferulic acid (more than 50% as free acid). Conjugated and free caffeic (0.146-0.496 pmol, 36-87% as free acid), ferulic acid (0.008-0.032 pmol), and chlorogenic acid (0.019-0.045 pmol as free acid) were identified post ingestion in the urine of the three volunteers, of whom two showed basal urinary levels of the compounds. Thus, there is considerable contradictory evidence for the potential absorption of chlorogenic acid per se. 

In quantitative terms, colonic metabolism and the formed colonic metabolites are more important than the absorption and conjugation of the hydroxycinnamic acids. Whether these colonic metabolites express potential protective or preventive bioactivities in vivo remains to be studied. This also applies to the effect of the dietary hydroxycinnamates and their metabolites on the composition of the colonic microflora. 

ChoiS.W., LeeS.K., Kim E.O.,Oh J.H., Yoon K.S., Parris N., Hicks K.B., Moreau R.A. Antioxidant and antimelanogenic activities of polyamine conjugates from corn bran and related hydroxycinnamic acids. Journal of Agricultural and Food Chemistry, 55 3920-3925 (2007). 

Ferulic acid in its free, conjugated, and bound forms is the most important simple phenolic compound present in cereal grains. It is a hydroxycinnamic acid associated... 

Of course, classes/groups of metabolites which occur (almost) ubiquitously or at least frequently in the plant kingdom ( general secondary metabolites ) are also constituents of both large Solanales families (i) phenolics such as simple cinnamic acid derivatives (Sect. 6.4), hydroxycoumarins (Sect. 6.6), hydroxycinnamate conjugates (Sect. 6.7), flavonoids (Sect. 6.8), lignans (Sect. 6.9), (ii) sterols (Sect. 7.6), (iii) carotenoids (Sect. 7.12), (iv) fats/oils and fatty acids (Sect. 8.1), (v) carbohydrates (Sect. 8.2) etc. 

Hydroxycinnamates as well as their conjugates may act as powerful antioxidants. In edible plants they have received much attention as protecting agents against oxidative deterioration of food. Antioxidant mechanism studies on ferulic acid and its coupling products with linoleate on the molecular level have been conducted recently. A radical scavenging reaction occurred at the 3 -position of the ferulate radical with four types of peroxyl radicals of ethyl linoleate. The produced peroxides subsequently underwent intramolecular rearrangement to afford stable tricyclic peroxides (Masuda et al. 2006). 

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