Urolithins, Intestinal Microbial Metabolites of Pomegranate Ellagitannins, Exhibit Potent Antioxidant Activity in a Cell-Based Assay

Abstract

Many health benefits of pomegranate products have been attributed to the potent antioxidant action of their tannin components, mainly punicalagins and ellagic acid. While moving through the intestines, ellagitannins are metabolized by gut bacteria into urolithins that readily enter systemic circulation. In this study, the antioxidant properties of seven urolithin derivatives were evaluated in a cell-based assay. This method is biologically more relevant because it reflects bioavailability of the test compound to the cells, and the antioxidant action is determined in the cellular environment. Our results showed that the antioxidant activity of urolithins was correlated with the number of hydroxy groups as well as the lipophilicity of the molecule. The most potent antioxidants are urolithins C and D with IC(50) values of 0.16 and 0.33 microM, respectively, when compared to IC(50) values of 1.1 and 1.4 microM of the parent ellagic acid and punicalagins, respectively. The dihydroxylated urolithin A showed weaker antioxidant activity, with an IC(50) value 13.6 microM, however, the potency was within the range of urolithin A plasma concentrations. Therefore, products of the intestinal microbial transformation of pomegranate ellagitannins may account for systemic antioxidant effects.

Natural Dibenzo-α-Pyrones and Their Bioactivities

“…Four urolithins, namely urolithin A (40), isourolithin A (41), urolithin B (42) and urolithin C (43) were isolated from Trapa natans (Trapaceae) [4] and Caesalpinia sappan (Caesalpiniaceae) [53]. These urolithins ( Figure 3) were also isolated from animal feces [6,7]. Other dibenzo-α-pyrones isolated from plants included sabilactone (38) from Sabina vulgaris [54] and sarolactone (39) from Hypericum japonicum [55].…”

“…In plants, gallic acid (54), which is biosynthesized via the shikimate pathway [67], was considered as the precursor of ellagitannins [68]. The ellagitannins would be transformed into ellagic acid (55), and then into a series of urolithins (Scheme 1) [6,7,63]. However, in microorganisms, dibenzo-α-pyrones are biosynthesized via the polyketide pathway [3,25].…”

“…After administration of [1][2][3][4][5][6][7][8][9][10][11][12][13] C]acetate and [1,[2][3][4][5][6][7][8][9][10][11][12][13] C]acetate to cultured lichen mycobionts of Graphis spp., acetate units were incorporated into the 6H-dibenzo[b,d]pyran-6-one derivatives including alternariol (10), AME (11) and graphislactones A-F (18-23) [3]. Alternariol (10) could suffer oxidative demethylation at C-1, hydroxylation at C-4, and O-methylation at C-9 to lead formation of graphislactones E (22) and F (23).…”

“…Ellagitannins are hydrolyzed to ellagic acid (55) in the acidic environment of the stomach by the action of the intestinal bacteria. The proposed transformation from ellagic acid to urolithins by the intestinal bacteria [6,7,63] is shown in Scheme 1.…”

“…They are usually isolated from fungi [1], mycobionts [2,3], plants [4,5], and animal feces containing the transformed products of plant-derived ellagitannins and ellagic acid by intestinal bacteria [6,7]. Many of them possess a wide spectrum of biological activities, spanning from toxicity on human and animals [8], cytotoxic activity [9], phytoxicity [10], antioxidant [6],…”