Human plasma dynamically quenches the fluorescein at the initial point of oxygen radical absorption capacity (ORAC) assay

OBJECTIVE: Oxygen radical absorbance capacity (ORAC) assay measures the quenching of fluorescent probe by peroxyl radicals. Antioxidants present in biological systems block the quenching of fluorescence probe. We experienced the dynamic quenching of fluorescein, the fluorescence probe used in ORAC assay by the human plasma while plasma ORAC assay was optimized. Therefore, for the first time, we report the quenching of fluorescein by human plasma at the initial point of ORAC assay. RESULTS: Aqueous whole and non-protein fractions of plasma were used in the analysis. Since the both fractions showed a similar pattern of quenching at the initial stage, quenched percentage of fluorescein was calculated and added to each sample in subsequent analysis. Addition of extra 20% fluorescein allowed plasma samples to quench the required amount of fluorescein and follow the normal decay curves afterwards. Further, change of fluorescein quenching (ΔF/F(0)) disclosed a dose dependent linear relationship with plasma (R(2) = 0.8). It can be speculated that dynamic quenching exhibited by human plasma biomolecule/s at the initial stage would be of non-protein aqueous phase molecule/s. We suggest initiating further studies to detect, identify and quantify the fluorescein quenching biomolecules present in human plasma for further improvements in plasma ORAC assay.