SODIUM-POTASSIUM ADENOSINE-TRIPHOSPHATASE ACTIVITY OF HUMAN-LYMPHOCYTE MEMBRANE-VESICLES - KINETIC-PARAMETERS, SUBSTRATE-SPECIFICITY, AND EFFECTS OF PHYTOHEMAGGLUTININ

We have prepared human blood lymphocyte membrane vesicles of high purity in sufficient quantity for detailed enzyme analysis. This was made possible by the use of plateletpheresis residues, which contain human lymphocytes in amounts equivalent to thousands of milliliters of blood. The substrate specificity and the kinetics of the cofactor and substrate requirements of the human lymphocyte membrane Na+, K+‐ATPase activity were characterized. The Na+, K+‐ATPase did not hydrolyze ADP, AMP, ITP, UTP, GTP or TTP. The mean ATPase stimulated by iptimal concentrations of Na+ and K+ (Na+, K+‐ATPase) was 1.5 nmol of Pi hydrolyzed, μ g protein−1, 30 min−1 (range 0.9‐2.1). This activity was completely inhibited by the cardiac glycoside, ouabain. The Km for K+ was approximately 1.0 mM and the Km for Na+ was approximately 15 mM. Active Na+ and K+ transport and ouabain‐sensitive ATP production increase when lymphocytes are stimulated by PHA. Na+, K+‐ATPase activity must increase also to transduce energy for the transport of Na+ and K+. Some studies have reported that PHA stimulates the lymphocyte membrane ATPase directly. We did not observe stimulation of the membrane Na+, K+‐ATPase when either lymphocytes or lymphocyte membranes were treated with mitogenic concentrations of PHA. Moreover, PHA did not enhance the reaction velocity of the Na+, K+‐ATPase when studied at the Km for ATP, Na+, K+ or Mg++, indicating that it does not alter the affinity of the enzyme for its substrate or cofactors. Thus, our data indicate that the increase in ATPase activity does not occur as a direct result of PHA action on the cell membrane. Copyright © 1979 Wiley‐Liss, Inc.