(Na + + K +)- and Na +-stimulated Mg 2+-dependent ATPase activities in kidney of sea bass ( Dicentrarchus labrax L.)

Abstract

1. 1. Sea bass kidney microsomal preparations contain two Mg 2+ dependent ATPase activities: the ouabain-sensitive (Na + + K +)-ATPase and an ouabain-insensitive Na +-ATPase, requiring different assay conditions. The (Na + + K +)-ATPase under the optimal conditions of pH 7.0, 100 mM Na +, 25 mM K +, 10 mM Mg 2+, 5 mM ATP exhibits an average specific activity (S.A.) of 59 μmol P i/mg protein per hr whereas the Na +-ATPase under the conditions of pH 6.0, 40 mM Na +, 1.5 mM MgATP, 1 mM ouabain has a maximal S.A. of 13.9 μmol P i/mg protein per hr. 2. 2. The (Na + + K +)-ATPase is specifically inhibited by ouabain and vanadate; the Na +-ATPase specifically by ethacrynic acid and preferentially by frusemide; both activities are similarly inhibited by Ca 2+. 3. 3. The (Na + + K +)-ATPase is specific for ATP and Na +, whereas the Na +-ATPase hydrolyzes other substrates in the efficiency order ATP > GTP > CTP > UTP and can be activated also by K +, NH 4 + or Li +. 4. 4. Minor differences between the two activities lie in the affinity for Na 1, Mg 2+, ATP and in the thermosensitivity. 5. 5. The comparison between the two activities and with what has been reported in the literature only partly agree with our findings. It tentatively suggests that on the one hand two separate enzymes exist which are related to Na + transport and, on the other, a distinct modulation in vitro in different tissues.