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

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Abstract
Pea stem tonoplast‐enriched vesicles have an ADPase and PPiase 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 Mg2+‐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 Na3VO4 or KNO3. Hexokinase and ATP do not prevent H+‐ADPase and H+‐PPiase 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 PPi‐dependent proton translocating activities seem carried out by a similar enzyme complex which appears different from the NO− 3‐inhibited, VO3− 4‐insensitive H+‐ATPase.