Synergistically Stimulated (Na+,K+)-Adenosine Triphosphatase from Plasma Membrane of a Marine Diatom
Abstract
An ATP-hydrolyzing activity with the properties of a Mg2+-dependent (Na+,K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) from a 20-fold purified plasma membrane fraction of the marine diatom, Nitzschia alba is described.
The basal activity requires Mg2+ and further stimulation by Na+ or Na+ plus K+ is dependent on the presence of Mg2+; Mn2+ or Co2+ can partially substitute for the divalent cation requirement but Ca2+ equimolar with Mg2+ inhibits the activity by 54%. ATP is the preferred substrate for the Na+ plus K+ stimulated activity, while CTP, UTP, and ADP are only slightly hydrolyzed. The apparent Km is 8 × 10-4 M ATP.
The ATP hydrolysis-rate is dependent on the relative concentrations of Na+ and K+; the K 0.5 for Na+ and K+ are 2 mM and 50 mM, respectively. Basal activity is synergistically stimulated by Na+ plus K+ only at certain ion concentrations and shows a strong specificity for both cations.
In the presence of Na+ at 5 mM and K+ at 350 mM, the ATPase is completely inhibited by p-chloromercuric benzoic acid 10-4 M, N-ethyl maleimide 10-3 M, and iodoacetamide 10-2 M, but is insensitive to ouabain at 10-7 to 10-3 M.
This study demonstrates for the first time that algal plasma membrane contains an ATPase that is synergistically stimulated by Na+ and K+.
