The Regulation of Purine Utilization in Bacteria

Abstract Isolated membrane vesicles from Escherichia coli K-12 rapidly degrade adenosine to adenine and inosine to hypoxanthine, using membrane-localized nucleoside phosphorylase activity, without transporting them across the membrane. The free bases are then transported and accumulated intravesicularly as the nucleoside monophosphate. The kinetics of formation of the nucleoside monophosphates, AMP and IMP, respectively, shows the same response to temperature, 5-phosphoribosyl-1-pyrophosphate concentration, and nucleotide inhibitors whether the free base or ribonucleoside is the substrate for uptake. The mechanism of purine nucleoside uptake in E. coli is therefore postulated to occur by two enzymatic steps involving two membrane enzymes: (a) extramembranal cleavage of nucleoside to free base and (b) group translocation of the free bases mediated by membrane phosphoribosyltransferases resulting in the intramembranal accumulation of nucleoside monophosphate. The second reaction is the means of adenine and hypoxanthine uptake and therefore the utilization of free purine base and purine nucleoside share a common pathway in this organism. The kinetics of uptake with respect to concentration of nucleoside or free base suggests that there may be interaction or association, or both, between the two enzymes themselves on the membrane surface.