A comparative study of antioxidant capacity of amino acids using the Fe(II)/1-nitroso-2-naphthol-3,6-disulfonic complex and the Folin–Ciocalteu reagent: application in blood serum

Abstract

The oxidation of 20 amino acids (AA; alanine, arginine, asparagine, aspartic acid, cysteine (Cys), glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine) by Fe(III) in a buffered aqueous solution (pH 8.0, Tris) containing 1-nitroso-2-naphthol-3,6-disulfonic acid (H2NRS) was spectrophotometrically evaluated. Under the employed conditions, Fe(III) was reduced to Fe(II) to afford greenish Fe(NRS)34−, the absorbance of which at λmax = 730 nm was correlated to AA reducing capacity and expressed as ascorbic acid equivalents (ASE). Comparison of the thus obtained ASE values with those determined using the Folin–Ciocalteu reagent (FCR) revealed that all analyzed AA preferentially reacted with Fe(III)–H2NRS rather than with the FCR. The obtained insights were utilized to develop a spectrophotometric procedure for the quantitation of the total antioxidant capacity (TAC) of blood serum samples in solution containing Fe(III) and H2NRS (pH 8.0, Tris). The TAC values (in mgCys mL−1 serum) of 22 serum samples determined by the Fe(III)–NRS method were well correlated with those determined using the FCR method ( r = 0.788), which suggested that the proposed procedure can be used to quantify the TAC of other protein-rich biological samples.