Content of Methemoglobin in the Blood of Teleost Fish: Effect of Environmental Factors and Natural States of the Organism (Review)

This paper presents summarized information on the factors determining an increase in content of methemoglobin (MtHb) in the blood of teleost fish. It is shown that the transition of hemoglobin to the ferric form in fish can be caused not only by cases of toxic methemoglobinemia. A significant increase in the concentration of MtHb in fish blood is also observed under the conditions of external hypoxia and hypothermia. This may be due to an increase in the content of the deoxy-form of the pigment (hypoxia) and a decrease in the activity of NADH-diaphorase (hypothermia). In fish, there is also a periodic increase in the content of MtHb in the blood during the annual cycle. This state coincides with the prespawning and spawning periods. It is associated with the monocyclic functioning of the hematopoietic tissue. Active erythropoiesis in fish occurs only in the postspawning period for 2–3 months. At all other times, the destructive processes dominate in the red blood system, which affects both the number of circulating red blood cells and the level of oxidative processes in them. In comparison with mammals, fish hemoglobins prove to be less resistant to oxidative stress, which is probably due to their instability and low efficiency of NADH diaphorase in red blood cells of this systematic group of organisms. At the same time, fish red blood cells are characterized by a high content of glutathione (GSH), especially in terms of hemoglobin (GSH/Hb index). This is due to the high efficiency of the pentose shunt reactions, which allows maintaining a high level of NADPH in the cell. This feature probably compensates for the low activity of NADH diaphorase. A protective effect of Cl– and epinephrine on hemoglobin is recorded under nitrite intoxication. They significantly reduce the toxic effect of N[Formula: see text]. We further discuss the mechanisms responsible for the transition of fish hemoglobin to the oxidized state.