Mg2+-dependent (Na+ + K+)- and Na+-ATPases in the kidneys of the gilthead bream (Sparus auratus L.)

1. 1. The (Na + + K + )- and Na + -ATPases, both present in kidney microsomes of Sparus auratus L., have different activities and optimal assay conditions as, in the first of the two stocks of fish used (A), the spec. act. of the former is 51.7 μmol P i mg prot −1 hr −1 at pH 7.5, 100 mM Na + , 10 mM K + , 17.5 mM Mg 2+ , 7.5 mM ATP and that of the latter is 6.5 μmol P i mg prot −1 hr −1 at pH 6.5, 40 mM Na + , 4.0 mM Mg 2+ , 2.5 mM ATP. 2. 2. Ouabain and vanadate specifically inhibit the (Na + + K + )-ATPase but not the Na + -ATPase that is preferentially inhibited by ethacrynic acid. 3. 3. While the (Na + + K + )-ATPase is strictly specific for ATP and Na + , Na + -ATPase can be activated by various monovalent cations and, apart from ATP, hydrolyses CTP, though less efficiently. 4. 4. The second stock B, subjected to higher salinity than A, shows an acidic shifted Na + -ATPase optimal pH, opposed to the stability of that of the (Na + + K + )-ATPase, a decreased (Na + + K + )-ATPase and a strikingly depressed Na + -ATPase. 5. 5. The results are compared with literature data and discussed on the basis of the presumptive different roles as well as functional prevalence in various salinities of the two ATPases.