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Membranes and Bioenergetics

Evidence for an Amiloride-Inhibited Mg²⁺/2H⁺ Antiporter in Lutoid (Vacuolar) Vesicles from Latex of Hevea brasiliensis¹

Laboratoire de Biotechnologie, Physiologie et Métabolisme Cellulaires; ORSTOM, 911 Av Agropolis, B.P. 5045, 34032 Montpellier cedex 1, France, Biochimie et Physiologie Végétales, E.N.S.A.-M/Institut National de la Recherche Agronomique, Centre National de la Recherche Scientifique (U.R.A. 575), 34060 Montpellier-cedex 1, France, Laboratoire de Physiologie Végétale Appliquée, Université Montpellier 2, 34095 Montpellier cedex 5, France

Lutoids represent a lysosomal microvacuolar compartment of rubber-tree (Hevea brasiliensis) latex. We observed acidification of isolated vesicles after imposing an outward Mg²⁺ diffusion gradient and dissipation of a preformed pH gradient in the presence of exogenous Mg²⁺. These results suggest the presence of a Mg²⁺/H⁺ antiporter. The maximum Mg²⁺/H⁺ exchange rate was observed at pH 8.5. The K_m values for Mg²⁺ (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 Mg²⁺/H⁺ exchange was electroneutral. Mg²⁺/H⁺ exchange was inhibited by amiloride and imipramine. Both the inhibiting concentration range and the K_m for Mg²⁺ are similar to those reported for the Mg²⁺/2Na⁺ antiporter in animals cell. These data are consistent with the existence of a Mg²⁺/2H⁺ antiporter in a plant tonoplast.

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