Mounting evidence suggests that hydrolysable polyphenols in
PoP, specifically ellagitannins, are the most active antioxidants
amongst the tannins contained therein. These compounds (ellagic
acid, punicalagin, punicalin and gallagic acid) have been shown
to hold heightened antioxidant and pleiotropic biological activities
and notably, to act synergistically together (Seeram & Heber,
2011). Nevertheless in vivo studies suggest that the antioxidant
properties of dietary absorbed polyphenols are tied to their metabolised
compounds, e.g. urolithins (Johanningsmeier & Harris,
2010).
High molecular weight ellagitannins are water soluble plant
phenolics that yield different biologically relevant by-products
upon hydrolysis. Under normal physiological conditions, orally
ingested ellagitannins undergo microbial hydrolysis by gut microflora
to relatively smaller compound, i.e. ellagic acid and, upon further
bacterial metabolism, urolithins. Ellagitannins’ hydrolysis,
either through acid, base or microbial activity yields ellagic acid.
Punicalagin is unique to pomegranate and is part of a family of
ellagitannins which include the minor tannins called punicalin
and gallagic acid, which are characterised by good water solubility.
Hydrolysable tannins are reported to be the first plant polyphenols
subjected to analytical research around 200 years ago
(Arapitsas, 2012): nonetheless data are still scant to interpret the
nutraceutical and food features of a substantial number of PoP
polyphenols. Amongst a wide array of PoP isolated fractions of
phytochemicals (Fig. 1A and B) only a few have been thoroughly
investigated to date for their efficacy against certain disorders
and their potential to be technologically exploited as food additives.
Since the major and most studied phenolics of PoP are punicalagin
and its metabolites, it would be advisable to study more in
depth other PoP compounds to establish their potential role as
nutraceutical and food additives.
3. Structure activity relationship of PoP