Proteins are not considered as true antioxidants but are known to protect antioxidants from oxidation in various antioxidant activity assays. This study aims to investigate the contribution of proteins, especially thiol-containing proteins, to the observed overall antioxidant capacity measured by known methods. To determine the antioxidant properties of thiol-containing proteins, the CUPRAC method of antioxidant assay using the oxidizing reagent Cu(II)-neocuproine previously used for simultaneous analysis of cystine and cysteine was adopted. While the CUPRAC method is capable of determining all antioxidant compounds including thiols in complex sample matrices, the Ellman method of thiol quantitation basically does not respond to other antioxidants. The antioxidant quantities in the selected samples were assayed with the ABTS and FRAP methods as well as with the CUPRAC method. In all applied methods, the dilutions were made with a standard pH 8 buffer used in the Ellman method by substituting the Na(2)EDTA component of the buffer with sodium citrate. On the other hand, the standard CUPRAC protocol was modified by substituting the pH 7 ammonium acetate buffer(at 1 M concentration) with 8 M urea buffer adjusted to pH 7 by neutralizing with 6 M HCl. Urea helps to partly solubilize and denaturate proteins so that their buried thiols be oxidized more easily. All methods used in the estimation of antioxidant properties of proteins (i.e., CUPRAC, Ellman, ABTS, and FRAP) were first standardized with a simple thiol compound, cysteine, by constructing the calibration curves. The molar absorptivities of these methods for cysteine were: epsilon(CUPRAC) = 7.71 x 10(3), epsilon(Ellman) = 1.37 x 10(4), epsilon(ABTS) = 2.06 x 10(4), and epsilon(FRAP) = 2.98 x 10(3) L mol(-1) cm(-1). Then these methods were applied to various samples containing thiols, such as glutathione (reduced form:GSH), egg white, whey proteins, and gelatin. Additionally, known quantities of selected antioxidants were added to these samples to show the additivity of responses. (C) 2009 Elsevier B.V. All rights reserved.