Reactivity of Various Compound Classes Towards the Folin-Ciocalteu Reagent

The Folin-Ciocalteu assay has been used for over 80 years for the detection and quantitation of phenols. A modification of it, called the Lowry assay, is used for the quantitation of proteins. It has been commonly reported that the Folin-Ciocalteu reagent, which is a complex mixture containing sodium molybdate and sodium tungstate, is reactive towards other antioxidants besides phenols. However, until now, no one has done experiments to test this hypothesis. In our study, we tested the reactivity of the reagent towards over 70 compounds. Compound classes included phenols, thiols, vitamins, amino acids, proteins, nucleotide bases, unsaturated fatty acids, carbohydrates, organic acids, inorganic ions, aldehydes and ketones. All phenols, proteins and thiols tested were reactive towards the reagent. Other compounds which showed reactivity included guanine, glyceraldehyde, dihydroxyacetone, tyrosine, tryptophan, cysteine, ascorbic acid, Trolox, retinoic acid, pyridoxine, Fe^+2, Mn⁺², I^- and SO3-2. In summary, our study showed that the Folin-Ciocalteu reagent is significantly reactive towards other compounds besides phenols. Therefore, it should be seen as a measure of total antioxidant capacity rather than phenolic content. It would be useful as a general antioxidant assay for measuring antioxidant capacities of compounds of biomedical interest.