PPi dependence of 32Pi incorporation
by tonoplast vesicles from seeds. The assay medium composition was 50
mm Mops-Tris buffer, pH 7.0, 0.1 mm ADP, 100
mm KCl, 5 mm MgCl2, and 0.05 mg/mL
tonoplast vesicles at 35°C, using as the substrate either 1
mm ATP (•) or 1 mm PPi (○). The reaction
was carried out in the presence of increasing Pi concentrations,
maintaining the radioactive 32Pi: nonradioactive Pi
proportions constant at 20,000 cpm/nmol of nonradioactive Pi.
Essentially the same results were obtained in four independent
experiments.