In higher plant cells, there are some enzymes capable of utilizing pyrophosphate (PP(i)) as an energy donor. Among these, membrane-bound proton pumping pyrophosphatases (H+-PP(i)ase) have been identified. In addition to the well-known vacuolar H+-PP(i)ase (V-PP(i)ase), there is evidence for the presence of a mitochondrial H+-PP(i)ase. This enzyme is localized on the inner surface of the inner membrane and catalyzes the specific hydrolysis of PP(i), coupled to proton transport, with a H+/PP(i) stoichiometry of ca 2. This activity is Mg2+-requiring, is stimulated by monovalent cations, and is inhibited by Ca2+, F- and diphosphonates. The H+PP(i)ase contains a catalytic head which is constituted by a 35-kDa protein which is loosely bound to the inner membrane. This protein exhibits a PP(i)ase activity, stimulated by phospholipids, with characteristics very similar to the membrane-bound enzyme. The mitochondrial PP(i)ase is distinct from the V-PP(i)ase, because an antibody raised against the 35-kDa protein does not react with tonoplast membranes. The mitochondrial H+-PP(i)ase seems to have an F-type structure, similar to the F-ATP synthase and the membrane-bound PP(i)ases from mammalian and yeast mitochondria. It is suggested that, beside synthesizing PP(i), this enzyme may act as a buffer for the electrochemical proton gradient, by hydrolyzing PP(i), during conditions of oxygen deprivation.
