AOP1, a New Live Cell Assay for the Direct and Quantitative Measure of Intracellular Antioxidant Effects

Taking advantage of Light Up Cell System (LUCS) technology, which allows for fine monitoring of reactive oxygen species (ROS) production inside live cells, a new assay called Anti Oxidant Power 1 (AOP1) was developed to specifically measure ROS and/or free-radical scavenging effects inside living cells. This method is quantitative and EC₅₀s obtained from AOP1 dose-response experiments were determined in order to classify the intracellular antioxidant efficacy of 15 well known antioxidant compounds with different hydrophilic properties. Six of them (epigallocatechin gallate, quercetin, butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), ethoxyquin, resveratrol) gave EC₅₀s in the range of 7–64 μM, four (Trolox, catechin, epicatechin, EUK134) in the range of 0.14 to 1 mM, and 5 (sulforaphane, astaxanthin, α- and γ-tocopherols, vitamin E acetate) showed only partial or no effect. Interestingly, effects with measurable EC₅₀s were observed for compounds with hydrophilic properties (LogP ≤ 5.3), while all antioxidants known to act at the plasma membrane level (LogP ≥ 10.3) had partial or no effect. Sulforaphane, a hydrophilic but strict Keap1/Nrf2 pathway enhancer, did not show any effect either. Importantly, AOP1 assay captures both antioxidant and prooxidant effects. Taken together, these results led us to the conclusion that AOP1 assay measures antioxidant effect of compounds that selectively enter the cell, and act as free radical scavengers in the cytosol and/or nucleus level.