Synergistically stimulated (Na+,K+)-adenosine triphosphatase from plasma membrane of a marine diatom.

An ATP-hydrolyzing activity with the properties of a Mg(2+)-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 Mg(2+) and further stimulation by Na(+) or Na(+) plus K(+) is dependent on the presence of Mg(2+); Mn(2+) or Co(2+) can partially substitute for the divalent cation requirement but Ca(2+) equimolar with Mg(2+) 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 K(m) is 8 x 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(+).