[The hydrogen peroxide paradox].

Since the discovery of hydrogen peroxide in 1819 it was known as a toxic agent for human organs. Due to the recent findings its role should be reevaluated. This review discusses the toxic and physiological roles of hydrogen peroxide and functions of enzyme catalase which is the main regulator of hydrogen peroxide metabolism. The concentration of hydrogen peroxide changes between 0,05 micromol/l and 117 micromol/l in exhaled breath condensate and in human fluids. Hydrogen peroxide is generated by physiological processes such as glycation, phagocytosis, cell metabolism and by pathological changes such as different tumors and side effects of some drugs. The main regulator of toxic concentration of hydrogen peroxide is the enzyme catalase while glutathione peroxidase and hemoglobin has a limited role in this process. Low concentration of hydrogen peroxide plays a role in degradation of some proteins, as a messenger in cell signaling and could contribute to apoptosis. The enzyme catalase, due to its structure and function, is very effective in destroying the toxic concentration of hydrogen peroxide without changing its low, physiologic concentration. Decrease in catalase due to anemia, decreased synthesis and especially to its inherited deficiency may be a risk factor in diabetes, cell damage due to ischemia, in uricase and ascorbic acid treatment as well as in sterilization with hydrogen peroxide. The hydrogen peroxide paradox means that its low concentration is vital for some physiological processes while its high concentration is toxic for human cells. The main regulator of hydrogen peroxide concentration is the enzyme catalase and its deficiency may be a risk factor for some pathological changes.