The role of Mg2+ in proton transport by the tonoplast pyrophosphatase in Riccia fluitans vacuoles

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Abstract
The Mg2+‐dependent activity of the tonoplast pyrophosphatase (PPase) was investigated by measuring proton transport and by using the acridine orange technique on intact vacuoles of the aquatic liverwort Riccia fluitans L. In solutions with both Mg2+ and pyrophosphate present, a number of complexes are formed, which could all influence the enzymatic and hence the transport activity of the PPase. Therefore, the individual concentrations of these complexes were calculated and their contributions to proton transport across the tonoplast were tested. From these experiments we conclude that Mg2+ has three different roles: (i) Mg2+ stimulates transport activity of the PPase. (ii) Mg2PPi inhibits PPase‐mediated H+ transport, (iii) MgPPi* (= MgPPi2‐+ MgHPPi‐) is the substrate with an apparent K1/2= 5–10 μM, with no discrimination between MgPPi2‐ and MgHPPi‐.