Properties of a Partially Purified Nucleoside Triphosphatase (NTPase) from the Chloroplast Envelope of Pea

The Mg-nucleoside triphosphatase activity associated with the inner envelope membrane of the pea chloroplast is comprised of at least two components, a major activity that is sensitive to vanadate and sodium fluoride and a minor insensitive activity. The vanadate/fluoride sensitive activity has been partially purified (about 35-fold) from Triton X-100 solubilized membranes by DEAE-Sephadex chromatography and sucrose density gradient centrifugation. The partially purified enzyme resembles the membrane-bound activity in requiring either Mg²⁺ or Mn²⁺, having a broad specificity for nucleoside triphosphates, having a K_m for ATP of 0.18 millimolar, and being inhibited by N-ethylmaleimide, but insensitive to sodium azide and dicyclohexylcarbodiimide. The partially purified enzyme obtained after sucrose gradient centrifugation has a markedly increased sensitivity to inhibition by inorganic pyrophosphate compared with the less pure enzyme. Pyrophosphate is not a substrate of either the membrane-bound or partially purified enzyme.