Stability-activity of verbascoside, a known antioxidant compound, at different pH conditions

Verbascoside (VB) is a phenolic antioxidant present in different plants, but, as reported for other polyphenols, could be unstable at different pH conditions. The aim of this work was to assess how chemical modification of VB, occurring during incubation at pH values similar to gastric (pH 3) and intestinal (pH 7) conditions, may impact antioxidant activity in two human intestinal cells lines. Quali-quantitative approaches were performed in order to evaluate the specific interactions between VB and its derivatives. HPLC analysis was performed to assess possible VB transformation into derivative products. The antioxidant activity of a mixture of VB, isoverbascoside (IsoVB) and oxidized products (obtained after incubation at pH 7), or purified VB and IsoVB, was assessed in two human intestinal cell lines (HT-29 and Caco-2) using a DCFH-DA probe. VB was stable at pH 3 with a recovery of similar to 100% after 24 h. In contrast, VB was unstable at pH 7, with a loss of 62.4%, transforming into IsoVB and other oxidative products. Cellular antioxidant assays found that the mixture of oxidized VB products resulted less active (EC50 ranging from 2.7 to 3.4 mu M) than VB (EC50 ranging from 0.24 to 0.29 mu M), IsoVB (EC50 ranging from 0.85 to 1.4 mu M), and VB + IsoVB (EC50 ranging from 0.12 to 0.21 mu M). Cellular uptake of IsoVB was found to differ between cell lines, with higher uptake by Caco-2 compared to HT-29. Both graphical and mathematical methods identified different interactions between VB and IsoVB on the two cell lines. In conclusion, the antioxidant activity of VB can be modified by the conditions present in intestinal environment. (C) 2014 Elsevier Ltd. All rights reserved.