SINOPSIS

In in vivo experiments, we studied the effect of iron-containing drug (Urfuhem) on the content of hemoglobin (Hb) in the blood of old rats. To establish the biochemical mechanisms of the drug action, we determined the parameters of oxidative/nitrosative stress, hydrogen sulfide level (H₂S) in plasma and erythrocytes, the level of non-heme iron in plasma, and erythrocyte sensitivity to acid hemolysis. It has been found that in old rats the drug significantly increases the Hb content in erythrocytes and reduces their resistance to acid hemolysis. After applying the drug, the rate of superoxide anion-radical (*O₂^-) generation in erythrocytes and the content of stable hydrogen peroxide (H₂O₂) both in plasma and erythrocytes, which in old animals far exceeded the reference value, were down-regulated. The drug did not reduce the high levels of hydroxyl radical (*OH) generation and excess NO de novo synthesis by iNOS in erythrocytes, but it reduced the pools of nitrate anion (NO₃^-) and its reutilization for NO synthesis. The drug introduction resulted in up-regulation of the rate of constitutive NO synthesis by cNOS in the plasma of old rats perhaps due to cNOS coupling. The results indicate that the reason for increasing proton (H⁺) permeability in erythrocytes of old rats that causes acid hemolysis can be the change in the balance between levels of oxidative and nitrosative stress in erythrocytes in favor of the latter. Thus, toxic *OH generation in the blood of old animals occurs due to formation and decomposition of peroxynitrite (ONOO^-), not at the expense of the classical Fenton reaction in the presence of iron ions (Fe²⁺ ).