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.
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