Lysed guard-cell protoplasts of Vicia faba L. exhibited hydrolytic act
ivity characteristic of tonoplast inorganic pyrophosphatase (V-PPase;
EC 3.6.1.1). Activity was inhibited by the specific V-PPase inhibitor
aminomethylenediphosphonate, stimulated by K+ (K-m = 51 mM) and inhibi
ted by Ca2+ (80 nM free Ca2+ was required for 50% inhibition at 0.27 m
M free Mg2+). Patch-clamp measurements of electrogenic activity confir
med enzyme localisation at the tonoplast. This is the first report of
V-PPase activity in guard cells; its possible involvement in stomatal
opening is discussed.