Inhibition of sugar uptake by ascorbic acid in Escherichia coli

The uptake of glucose by the glucose phosphotransferase system in Escherichia coli was inhibited greater than 90% by ascorbate. The uptake of the nonmetabolizable analog of glucose, methyl-alpha-glucoside, was also inhibited to the same extent, confirming that it was the transport process that was sensitive to ascorbate. Similarly, it was the transport function of mannose phosphotransferase for which mannose and nonmetabolizable 2-deoxyglucose were substrates that was partially inhibited by ascorbate. Other phosphotransferase systems, including those for the uptake of sorbitol, fructose and N-acetylglucosamine, but not mannitol, were also inhibited to varying degrees by ascorbate. The inhibitory effect on the phosphotransferase systems was reversible, required the active oxidation of ascorbate, was sensitive to the presence of free-radical scavengers, and was insensitive to uncouplers. Because ascorbate was not taken up by E. coli, it was concluded that the active inhibitory species was the ascorbate free radical and that it was interacting reversibly with a membrane component, possibly the different enzyme IIB components of the phosphotransferase systems. Ascorbate also inhibited other transport systems causing a slight reduction in the passive diffusion of glycerol, a 50% inhibition of the shock-sensitive uptake of maltose, and a complete inhibition of the proton-symport uptake of lactose. Radical scavengers had little or no effect on the inhibition of these systems.