Calcium Transport in Sealed Vesicles from Red Beet (Beta vulgaris L.) Storage Tissue 1: II. Characterization of 45Ca2+ Uptake into Plasma Membrane Vesicles

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Calcium uptake was examined in sealed plasma membrane vesicles isolated from red beet (Beta vulgaris L.) storage tissue using 45Ca2+. Uptake of 45Ca2+ by the vesicles was ATP-dependent and radiotracer accumulated by the vesicles could be released by the addition of the calcium ionophore A23187. The uptake was stimulated by gramicidin D but slightly inhibited by carbonylcyanide m-chlorophenylhydrazone. Although the latter result might suggest some degree of indirect coupling of 45Ca2+ uptake to ATP utilization via δμH+, no evidence for a secondary H+/Ca2+ antiport in this vesicle system could be found. Following the imposition of an acid-interior pH gradient, proton efflux from the vesicle was not enhanced by the addition of Ca2+ and an imposed pH gradient could not drive 45Ca2+ uptake. Optimal uptake of 45Ca2+ occurred broadly between pH 7.0 and 7.5 and the transport was inhibited by orthovanadate, N,N′-dicyclohexylcarbodiimide, and diethylstilbestrol but insensitive to nitrate and azide. The dependence of 45Ca2+ uptake on both calcium and Mg:ATP concentration demonstrated saturation kinetics with Km values of 6 micromolar and 0.37 millimolar, respectively. While ATP was the preferred substrate for driving 45Ca2+ uptake, GTP could drive transport at about 50% of the level observed for ATP. The results of this study demonstrate the presence of a unique primary calcium transport system associated with the plasma membrane which could drive calcium efflux from the plant cell.

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