His-tagged and non-tagged forms of Anterior Gradient 2 (AGR2), oncoprotein and potential cancer biomarker, were studied for the first time using voltammetry at carbon electrodes. In addition to oxidation peak of Tyr and Trp, N-terminal His-tagged AGR2 form yielded characteristic electro-oxidation peak of histidine. Qualitatively similar results were obtained with other N-terminal His-tagged proteins, such as, glutathione-S-transferase, alpha-synuclein and cytochrome b5. Our results suggested that His-tag modification of proteins (commonly used in recombinant protein preparation) may change adsorption and orientation of the proteins at electrode surfaces. In absence of the His-tag in His-containing proteins, appearance of His peak was influenced by accessibility of His residues, and depended on the carbon electrode type. Oxidation His peak, in combination with Tyr and Trp oxidation responses may find use in label-free analysis of numerous proteins, including those important in biomedicine. (C) 2014 Elsevier Ltd. All rights reserved.