Evidence for an Amiloride-Inhibited Mg/2H Antiporter in Lutoid (Vacuolar) Vesicles from Latex of Hevea brasiliensis.

Lutoids represent a lysosomal microvacuolar compartment of rubber-tree (Hevea brasiliensis) latex. We observed acidification of isolated vesicles after imposing an outward Mg(2+) diffusion gradient and dissipation of a preformed pH gradient in the presence of exogenous Mg(2+). These results suggest the presence of a Mg(2+)/H(+) antiporter. The maximum Mg(2+)/H(+) exchange rate was observed at pH 8.5. The K(m) values for Mg(2+) (2.6 mm) were identical for both influx and efflux experiments. When membrane potential was clamped at zero with K(+) and valinomycin, the response of the membrane potential probe oxonol VI showed that the Mg(2+)/H(+) exchange was electroneutral. Mg(2+)/H(+) exchange was inhibited by amiloride and imipramine. Both the inhibiting concentration range and the K(m) for Mg(2+) are similar to those reported for the Mg(2+)/2Na(+) antiporter in animals cell. These data are consistent with the existence of a Mg(2+)/2H(+) antiporter in a plant tonoplast.