Succinate dehydrogenase (complex II or succinate:ubiquinone oxidoreductase) is a tetrameric, membrane-bound enzyme that catalyzes the oxidation of succinate and the reduction of ubiquinone in the mitochondrial respiratory chain. Two electrons from succinate are transferred one at a time through a flavin cofactor and a chain of iron-sulfur clusters to reduce ubiquinone to an ubisemiquinone intermediate and to ubiquinol. Residues that form the proximal quinone-binding site (Q(P)) must recognize ubiquinone, stabilize the ubisemiquinone intermediate, and protonate the ubiquinone to ubiquinol, while minimizing the production of reactive oxygen species. We have investigated the role of the yeast Sdh4p Tyr-89, which forms a hydrogen bond with ubiquinone in the Q(P) site. This tyrosine residue is conserved in all succinate:ubiquinone oxidoreductases studied to date. In the human SDH, mutation of this tyrosine to cysteine results in paraganglioma, tumors of the parasympathetic ganglia in the head and neck. We demonstrate that Tyr-89 is essential for ubiquinone reductase activity and that mutation of Tyr-89 to other residues does not increase the production of reactive oxygen species. Our results support a role for Tyr-89 in the protonation of ubiquinone and argue that the generation of reactive oxygen species is not causative of tumor formation.

Global warming implies the risk of a changing oxygen regime in the seas and oceans of our planet. The mitochondrial complex of nuclear erythrocytes of cartilaginous fish, as the energy basis of blood cells, has repeatedly encountered such climatic fluctuations throughout their evolutionary history. In this regard, the features of the adaptive strategy of the erythrocyte mitochondrial complex in the thornback ray (Raja clavata L.) are of interest from the evolutionary and ecological points of view. The rate of oxygen consumption in resuspended (Ht = 25–30%) erythrocytes taken from the Black Sea thornback ray in saline was studied by the polarographic method. A high “basal” rate of respiration in the erythrocytes of the thornback ray was shown, which ranged from 10.5 to 21.6 pmol O2 min−1·106 cells. The addition of substrates of the mitochondrial respiration activators glutamate, maleate, and succinate to the erythrocyte suspension caused a 2–6-fold increase in the respiratory activity of thornback ray erythrocytes. In cases where the rate of respiration of erythrocytes was high, protonophore–dinitrophenol caused an inhibition of the activity of mitochondrial respiration. At low respiration rates of erythrocytes, its effect was opposite and caused a stimulation of mitochondrial respiration. Oligomycin caused a significant inhibition of the respiratory activity of the red blood cell suspension of the thornback ray. This suppression of cell respiration was enhanced under conditions of exposure to the permeabilization of erythrocytes with digitonin. This can be recommended as one of the ways to block the respiratory activity of erythrocytes in cartilaginous fish. Another way of effectively blocking the respiration of the mitochondrial complex of the thornback ray’s erythrocytes was the effect of the blockers rotenone and sodium azide. The peculiarity of the mitochondria of the erythrocytes of the thornback ray was the absence of the complete inhibition of respiration by sodium azide (NaN3), which is characteristic of the mitochondria of other fish species. Our data on the activation of the “respiration” of erythrocytes in fish indicate that the potential capabilities of cold-blooded and warm-blooded vertebrates have rather similar characteristics. This may indicate the initial “laying” of the architecture of the inner membrane to support the energy potential of the mitochondria of the cell.

An evolutionary development of the rheological characteristics of blood in the capillary blood flow is one of indicator of fishes’ adaptation for the changing temperature conditions. Our earlier studies showed that large, nucleated erythrocytes of the Black Sea thornback ray (Raja clavata L.) and scorpionfish (Scorpaena porcus L.) had high ecto-ATPase activity. This activity varied from 3.0 to 3.9 and 6.0 to 10.0 nmol Fn/min/µL RBC in rays and scorpionfish, respectively. The use of thermal energy of ATP in the hydrolysis process made by ecto-ATPases in fish erythrocytes provided a heating of the plasma cells’ membrane and the near-wall plasma layer with an aim to improve the rheological characteristics of blood in the capillary blood flow. To confirm the proposed assumption, we studied thermal phenomena in erythrocytes suspensions of fishes by continuous recording of temperature changes in the experimental cell with fishes’ erythrocytes suspension with the addition of ATP (1 mg/mL) in vitro. A higher “thermal” activity of scorpionfish erythrocytes was established in comparison with erythrocytes of the thornback ray, which had the multiplicity of activity of ecto-ATPase in the plasma membranes of erythrocytes of the studied fishes. The presence of heat-producing activity of suspensions of erythrocytes of thornback ray and scorpionfish is evidence of the possible use of thermal energy of ATP hydrolysis by fish erythrocytes, both to maintain the functional state of the plasma membrane of red blood cells and to improve the rheological characteristics of blood in the capillary section of blood flow.

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