Salt-stimulated Adenosine Triphosphatase from Smooth Microsomes of Turnip

The turnip (Brassica rapa L.) microsome fraction contains both a Mg(2+)-inhibited acid phosphatase and a salt-stimulated Mg(2+)-activated ATPase. However, as the pH optimum of the ATPase was 8.0 to 8.5, the acid phosphatase activity could be eliminated by assaying at or above pH 7.8. The ATPase was concentrated in a fraction equivalent to the smooth microsomal membranes and was not due to fragments of mitochondria. The salt-stimulated activity showed specificity for anions rather than cations. The activity was further stimulated by carbonyl cyanide m-chloro-phenylhydrazone (CCCP), 2,4-dinitrophenol, valinomycin, nigericin, and NH(4)Cl. There was a synergistic effect between CCCP and valinomycin. Activity was insensitive to oligomycin phlorizin, ouabain, and atractylate. Based on similarity to the chloroplast ATPase, it was proposed that this ATPase was situated on the outside of the vesicle.It is suggested that the ATPase is involved in the movement of ions, particularly anions, and may be related to the anion accumulation mechanism, which is known to occur across the tonoplast of such tissues.