2-Fluoroadenine (F-Ade) is a metabolite of 9-beta-D-arabinofuranosyl-2-fluoroadenine (F-ara-A) that may be involved in the development of toxic side effects from this anticancer drug. The liberation of F-Ade from F-ara-A has been examined in different biological systems. Extracts of Escherichia coli but not mammalian cells or tissues catalyzed the conversion of F-ara-A to F-Ade with apparent Km and Vmax values of 1350 microM and 7.7 nmol/min/mg protein respectively. This reaction depended on the presence of phosphate and was inhibited by purine nucleosides in a competitive manner, indicating that the enzyme responsible for the conversion is purine nucleoside phosphorylase. After incubation of intact bacteria with 100 microM [3H]F-ara-A, [3H]F-Ade was the same percentage of cellular radioactivity as in the medium, but it was only one-tenth the concentration of F-ara-A in the cells. In contrast, the cellular concentration of 2-fluoro-ATP was 20-fold greater than that of F-ara-A-5'-triphosphate. These results suggest that F-ara-A entered the bacteria intact and was phosphorolytically cleaved to liberate F-Ade, which would have been either anabolized to the toxic triphosphate or excreted. The latter pathway would provide a route by which F-Ade might be absorbed into the host circulation.