Potassium and voltage dependence of the inorganic pyrophosphatase of intact vacuoles from Chenopodium rubrum.

The activity and the voltage dependence of the inorganic pyrophosphatase (PPase) was measured on intact vacuoles of Chenopodium rubrum cells using the patch-clamp technique. With K+ at the cytoplasmic side a negative current representing the forward mode of the pump was measured after addition of pyrophosphate (PPi). The pump was reversed and a positive current was detected after addition of orthophosphate (Pi) in the presence of K+ at the vacuolar side when a pH gradient across the tonoplast was applied. The PPase operates as a constant current source, because no voltage dependence was observed (-60 to 60 mV). The K+ dependence of the PPi-induced current was investigated by substitution of cytoplasmic K+ by other cations. The selectivity sequence was: K+ > or = Rb+ > NH4+ = Cs+ > Na+ > Li+ = choline+, and was independent of the membrane voltage and pHcyt. With Cs+ or Li+ in the bath and K+ inside the vacuole the PPi-induced current became voltage-dependent, and positive currents were observed even if the pump was geared to operate in the forward mode. We suggest a "tunneling' effect through a channel-like domain in the PPase molecule which, under defined electrochemical gradient conditions and in the presence of PPi, allows K+ ions to cross the energy barrier usually separating the cytoplasmic from the vacuolar face of the pump.