ules were used to investigate the ATP-dependent generation of membrane potential (
) and pH gradient (pH) across the peribacteroid membrane (PBM). Oxonol VI and acridine orange were employed as probes sensitive
to
and pH, respectively. Based on the study of electrogenic and proton-translocation activities of PBM, the ability of some metabolites mediating the interaction between symbiotic partners to cross PBM was par-
tially characterized. The peribacteroid membrane ATPase was shown to operate as an electrogenic proton
pump; it created
in the absence of permeable ions and pH in their presence. Vanadate was an effective inhibitor of both
and pH formation, whereas nitrate converted into pH. These effects of inhibitors suggest that PBM of lupine nodules contains the P-type H+-ATPase, which is similar in its characteristics to the
plasma membrane H+-ATPase of higher plants. Dicarboxylic acids, i.e., malate, succinate, and 2-oxoglutarate,
eliminated
on PBM of symbiosomes that were preenergized by ATP or artificial means. Similar elimination of
by dicarboxylates was observed in vesicle preparations of PBM. Dicarboxylic acids also induced rapid dissipation of the transmembrane pH gradient across PBM, no matter by which means the
pH was generated. Glutamate evoked rapid dissipation of the transmembrane pH gradient but had no effect on
. It is concluded that PBM of Lupinus luteus nodules is capable of transporting not only dicarboxylates but also glutamate.