Measuring Polyphenols in Evening Primrose

The total polyphenol content (TPC) was calculated using the Folin–Ciocalteu method [2 (link)] and expressed as gallic acid equivalents (GAE) [28 ]. The EPE concentration range 0.01–1000 µg/mL was equivalent to 0.00208–208.24 µg GAE/mL, that means 0.01224–1224 µM GAE. GAE is a commonly used determinant in assessing TPC reported in previous studies [29 (link),30 (link)]. To calculate TPC in O. paradoxa extract, the gallic acid standard curve was determined for the concentration range from 12.5 to 200 µg/mL of gallic acid. Briefly, 2.5 mL of 10 times diluted Folin–Ciocalteu reagent was added to 0.5 mL of evening primrose extract solution in the concentrations: 100 µg/mL and 50 µg/mL and incubated with 2 mL of 7.5% (w/v) sodium carbonate solution for 2 h at room temperature. The absorbance was measured at 765 nm on a LAMBDA 25 UV Spectrophotometer (Perkin Elmer, UK). All determination was performed from three independent experiments. In this study, the concentration of extract used for in vitro MPM cells treatment was presented in µg/mL of extract as well as µM GAE to enable further discussion of the results.