A H+/Ca2+ Antiporter in Membranes of Microsomal Vesicles from Maize Coleoptiles, a Secondary Energized Ca2+ Pump

Ca2+ Pump, H+-ATPase, Membrane Vesicles, Coleoptile, Zea mays Microsomal vesicles prepared from the ER and the Golgi apparatus of maize coleoptiles possess an ATP-fueled, Cl_-dependent proton pump which can cause an intravesicular acidifica­ tion of the vesicles (A. Hager and M. Heimle, Z. Naturforsch. 36 c, 997 (1981); A. Hager et al., Z. Naturforsch. 35 c, 783-793 (1980)). This acidification is very specifically inhibited by Ca2+. The inhibition is already distinct at concentrations of 30 hm. While 10-fold higher concentrations of La3+ produce similar effects, Mg2+ remains ineffective even at very high concentrations. If Ca2+ is added after acidification of the vesicles, a rapid H+-efflux proportional to the amount of Ca2+ added characterizes the first reaction phase. In the second reaction phase acidification within the vesicles commences anew, however, at a reduced rate. If Ca2+ is added to vesicles whose proton pump has been inhibited by DCCD, the first reaction phase remains unchanged, while the acidification in the second reaction phase does not set in, and only a leak out of protons is observed. These results give support for a H+/C a2+-antiporter mechanism which can function as a secondary energized Ca2+-pump and regulate the Ca2+-concentration in the cytoplasm.