Nucleotide exchange in membrane vesicles from the photosynthetic bacterium Rhodopseudomonas capsulata

Membrane vesicles (heavy chromatophores) prepared from the photosynthetic bacteria Rhodopseudomonas capsulata catalyze photophosphorylation of exogenous ADP and also take up [³H]ADP from the external medium. The rate of uptake depends on the concentration of external ADP reaching half-maximal velocity at 2.7 mm. The rate increases also with the increase in the concentration of internal ADP. Vesicles, preloaded with [³H]ADP release the radioactive nucleotide when ADP is included in the external medium. Regular chromatophores, which are inside-out membrane vesicles also take up [³H]ADP from the external medium when preloaded with ADP. These results are interpreted to indicate the existence of nucleotide transport across the cytoplasmic membrane of these bacteria which is catalyzed by an ADP exchange carrier.