Сa2+-ATPase of the Symbiosome Membrane from Broad Bean Root Nodules: Novel Results Supporting the Mechanism of Transmembrane Translocation of Ca2+

Abstract

On the preparations of symbiosomes isolated from broad bean (Vicia faba L.) root nodules, the transport activity of symbiosome membrane (SM) Ca²⁺-ATPase expressed as ITP- or Ca²⁺-dependent alkaline pH shift inside these structures as a function of pH of their incubation medium and calcium concentration in it, respectively, was investigated. The found pH and calcium concentration dependences are considered as novel evidence for close coupling of catalytic and transport activities of Ca²⁺-ATPase and its high affinity for Ca²⁺ ions. It was also shown that Cd²⁺ and Mn²⁺ ions added to symbiosome incubation medium instead of Ca²⁺ but at the same concentration do not result in dissipation of ∆pH on the SM generated by the symbiosome H⁺-ATPase providing evidence for incapability of the calcium pump to perform their translocation through the SM. All these data are discussed in the light of the results obtained earlier by the authors and others.