Further evidence for an ATP-driven sodium pump in the marine alga Tetraselmis (Platymonas) viridis

Abstract

The ATP-dependent 22Na+ accumulation by inside/out plasma membrane vesicles isolated from the marine green microalga Tetraselmis (Platymonas) viridis Rouch. has been studied. The 22Na+ uptake was observed only within a narrow region of weakly alkaline pH with a maxium at pH 7.8–8.0. When permeant anions such as NO3−, were absent from the reaction medium, ATP-dependent 22Na+ uptake was low but could be stimulated by 6 μmol/L ClCCP. In the presence of NO3−, however, the rate of ATP-dependent 22Na+ uptake was much higher than in the absence of nitrate and not affected by the uncoupler ClCCP. The pH optimum of ATP-driven 22Na+ uptake differed from that of ATP-dependent ΔpH formation across the vesicle membranes (optimum pH 6.0–7.0). These data indicate that a ΔμH+-dependent Na+/H+ antiporter does not contribute to the observed uptake under the experimental conditions; 100 μmol/L orthovanadate completely inhibited ATP-dependent 22Na+ uptake, whereas the uptake was not affected by 50 μmol/L amiloride and only slightly reduced by 200 μmol/L of this inhibitor. It is concluded that the ATP-supported 22Na+ uptake by Tetraselmis (Platymonas) viridis plasma membrane vesicles is carried out by a mechanism independent of the proton-motive force. For example it could be catalysed by a primary Na+-pump. It is thought that this pump is an orthovanadate-sensitive electrogenic Na+-ATPase.