Sodium‐potassium exchange in sea urchin egg. I. Kinetic and biochemical characterization at fertilization

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Biochemical and kinetic characteristics of the Na+-K+ exchange were studied in Paracentrotus lividus eggs. Measurement of the 86Rb uptake shows that ouabain-sensitive 86Rb uptake is dramatically stimulated within the first minute following fertilization. The Na+-K+ pump-mediated K+ entry presents a maximal rate at 8 min postfertilization and then decreases to reach a plateau within 30 min. We assess that the steep rise in cell K+ occurring at fertilization (J.P. Girard, P. Payan, C. Sardet, Exp. Cell. Res. 142:215-221, 1982) does not originate from a net entry of external K+. Measured 30 min postfertilization, the half-maximal activation by K+ of the ouabain-sensitive Na+-K+ exchange is 5-6 mM and the ouabain IC50 is 5.10(-5) M. Egg cortices from unfertilized and fertilized eggs show comparable Na+-K+ ATPase activity with a 50% ouabain-sensitive fraction. Vm and Km for Na+ and K+ of the enzyme are of the same order of magnitude in cortices of unfertilized and fertilized eggs. Cortical Na+-K+ ATPase from unfertilized eggs shows a ten fold increase of activity between pH 6.7 and pH 7.7. The results strongly suggest that the plasma membrane of unfertilized eggs contains a preexisting Na+-K+ transporting system which is obligatorily stimulated at fertilization.