Adenosine uptake by isolated membrane vesicles from Escherichia coli K-12

Abstract

1. 1. Three different adenosine-uptake systems were found in isolated membrane vesicles of Escherichia coli K-12 cells: the first system was strongly inhibited by adenonine nucleosides, pyrimidine nucleosides and the antibiotic showdomycin but weakly inhibited by guanine nucleosides and inosine; the second system was strongly inhibited by adenine nucleosides, guanine nucleosides and inosine but weakly inhibited by pyrimidine nucleosides and showdomycin; and the third system was strongly inhibited by adenine. In isolated membrane vesicles of a showdomycin-resistant mutant (Shm r-001), the first system was no longer detectable but the second and the third systems were found to remain active. 2. 2. The adenosine uptake by these systems was stimulated by the artificial electron-donor system, ascorbate-phenazine methosulfate. The reduced phenazine methosulfate-stimulated uptake of adenosine by these systems was inhibited by sodium azide. 3. 3. Caffeine markedly inhibited the uptake of adenosine by the second system, but stimulated that of adenosine by the first system. 4. 4. Showdomycin strongly inhibited the uptake of adenosine by the first system but only slightly inhibited that of adenosine previously loaded membrane vesicles of Escherichia coli K-12 in the presence of adenine and caffeine. The reduced phenazine methosulfate-stimulated uptake of [ 14C]-showdomycin by membrane vesicles from Escherichia coli K-12 cells was strongly inhibited by adenosine and deoxycytidine but only slightly inhibited by guanosine. These results suggest that showdomycin is taken up by the first system of adenosine uptake in the membrane vesicles from Escherichia coli K-12 cells.