Figure 3.

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.