NOX5 protein, one of the most active generators of reactive oxygen species (ROS), plays an important role in many processes, including regulation of cell growth, death and differentiation. Because of its central role in ROS generation, it needs to be tightly regulated to guarantee cellular homeostasis. Contrary to other members of NADPH-oxidases family, NOX5 has its own regulatory calcium-binding domain and thus could be activated directly by calcium ions. While several mechanisms of activation have been described, very little is known about the mechanisms that could prevent the overproduction of ROS by NOX5. In the present study using calorimetric methods and circular dichroism we found that oxidation of cys-teine and methionine residues of NOX5 decreases binding of Ca 2+ ions and perturbs both secondary and tertiary structure of protein. Our data strongly suggest that oxidation of calcium binding domain of NOX5 could be implicated in its inactivation, serving as a possible defense mechanism against oxidative stress.