Biochemical characteristics of the Ca2+ pumping ATPase in the peribacteroid membrane from broad bean root nodules

Ca2+-ATPase in the peribacteroid membrane (PBM) of symbiosomes isolated from Vicia faba root nodules was characterized in terms of its hydrolytic and transport activities. Both activities were found to be pH-dependent and exhibit pH optimum at pH 7.0. Translocation of Ca2+ through the PBM by the Ca2+-ATPase was shown to be fueled by ATP and other nucleotide triphosphates in the following order: ATP > ITP a parts per thousand...aEuro parts per thousand GTP a parts per thousand...aEuro parts per thousand UTP a parts per thousand...aEuro parts per thousand CTP, the K (m) of the enzyme for MgATP being about 100 mu M. Ca-dependent ITP-hydrolytic activity of symbiosomes was investigated in the presence of the Ca-EGTA buffer system and showed the affinity of PBM Ca2+-ATPase for Ca2+ of about 0.1 mu M. The transport activity of Ca2+-ATPase was inhibited by erythrosin B as well as orthovanadate, but markedly stimulated by calmodulin from bovine brain. These results allowed us to conclude that this enzyme belongs to IIB-type Ca2+-ATPases which are present in other plant membranes.