The Ecto-ATPase of the Erythrocyte Plasma Membrane in Black Sea Fishes and Factors that Affect its Activity

The effects of erythrocyte sizes and the protein–lipid, phospholipid, and fatty acid compositions of plasma membranes on the activity of the membrane ecto-ATPase were studied in two cartilaginous (Raja clavate and Dasyatis pastinaca) and eight teleost species (Scorpaena porcus, Uranoscopus scaber, Symphodus tinca, Diplodus annularis, Neogobius melanostomus, Spicara flexuosa, Trachurus mediterraneus ponticus, and Gaidropsarus mediterraneus) of Black Sea fish. The relationships between the high ecto-ATPase activity and the erythrocyte sizes were shown for both cartilaginous and teleost fish species. A higher relative protein content was found in membranes of the Black Sea rays, which probably indicates an evolutionary trend towards a decreasing proportion of the protein component and increasing lipids in teleost fish. Fish erythrocytes with a high protein content in the membrane were recorded to have a higher ecto-enzymatic activity. Plasma membrane phospholipids of fish erythrocytes belonged to five families: phosphatidylcholine (PC), phosphatidylethanolamine (PEA), phosphatidylserine, monophosphoinositide, and sphingomyelin. The high contents of phospholipids and, in particular, PC and PEA in plasma membranes of fish erythrocytes is the basis for the formation of their bilayer matrix. Plasma membranes with a higher content of saturated fatty acids exhibited a higher ecto-ATPase activity, which indicated an important role of the enzyme in the physiology of the cell membrane and the blood flow in general.