ADP‐ or pyrophosphate‐dependent proton pumping of pea stem tonoplast‐enriched vesicles

Pea stem tonoplast‐enriched vesicles have an ADPase and PP i ase activity capable of generating a transmembrane proton gradient (ΔpH) and an electric potential (Δψ). Both proton translocating activities have a pH optimum around 6.5, are Mg 2+ ‐dependent, require the presence of a monovalent cation (K + , Rb + or Cs + ) and of a permeant anion, such as NO − 3 , Cl − or Br − . They are almost completely inhibited by 50 μM DIDS, DES and DCCD, 50% inhibited by 100 μM molybdate and unaffected by Na 3 VO 4 or KNO 3 . Hexokinase and ATP do not prevent H + ‐ADPase and H + ‐PP i ase activity, thus indicating that these functions are not caused by an ATP‐dependent proton pumping and that they have catalytic sites different from those of H + ‐ATPase, respectively. On the basis of these characteristics, ADP‐ and PP i ‐dependent proton translocating activities seem carried out by a similar enzyme complex which appears different from the NO − 3 ‐inhibited, VO 3− 4 ‐insensitive H + ‐ATPase.