Oxidative stress plays an important role in the modulation of several important physiological functions. On the other side, oxidative stress is accountable for development of many unphysiological changes, which can be deleterious for cells. Consequently, at the present time there is increased interest about study mechanisms and changes evocated by oxidative stress. Despite the highly oxidizing environment (21% oxygen, at sea level), at normal conditions, the cell cytoplasm of all aerobic organisms is reduced and proteins contain free sulfhydryl groups. In the cytoplasm, two major systems were identificated responsible for maintaining a reduced state: thioredoxin and glutathione/glutaredoxin system. Thioredoxin in bacteria, thanks to the low redox potential is the major dithiol reductant in the cytosol, or an advanced equivalent to dithiothreitol of cells (Holmgren 1985). Thioredoxin system acts the dominant role in many physiological processes (see below) and it is also a cell antioxidant.