Kinetics of Ca2+/H+ Antiport in Isolated Tonoplast Vesicles from Storage Tissue of Beta vulgaris L.

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Abstract
Artificial pH gradients across tonoplast vesicles isolated from storage tissue of red beet (Beta vulgaris L.) were used to study the kinetics of a Ca2+/H+ antiport across this membrane. Ca2+-dependent H+ fluxes were measured by the pH-dependent fluorescence quenching of acridine orange. ΔpH-dependent Ca2+ influx was measured radiometrically. Both H+ efflux and Ca2+ influx displayed saturation kinetics and an identical dependence on external calcium with apparent Km values of 43.9 and 41.7 micromolar, respectively. Calcium influx was unaffected by an excess of Mg2+ but was inhibited by La3+ > Mn2+ > Cd2+. The apparent Km for external calcium was greatly affected (5-fold) by internal pH in the range of 6.0 to 6.5 and a transmembrane effect of internal proton binding on the affinity for external calcium is suggested.
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