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

Biochemical and kinetic characteristics of the Na + ‐K + exchange were studied in Paracentrotus lividus eggs. Measurement of the 86 Rb uptake shows that ouabain‐sensitive 86 Rb 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 lC 50 is 5.10 −5 M. Egg cortices from unfertilized and fertilized eggs show comparable Na + ‐K + ATPase activity with a 50% ouabain‐sensitive fraction. V m and K m 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.