Fluoride inhibition of glucose-6-P formation in Streptococcus salivarius: Relation to glycogen synthesis and degradation

Abstract

The concentrations of glucose-6-P and ATP in intact cells of the oral microbe, Streptococcus salivarius, were analyzed during anaerobic glucose metabolism in the presence and absence of fluoride. Addition of 2.4 m m NaF to cells actively degrading glucose resulted in an immediate decrease in the cellular glucose-6-P and ATP content concomitant with the complete inhibition of glucose uptake and glycogen synthesis. A noticeable decrease in cellular glucose-6-P concentration was produced in cells metabolizing glucose by concentrations of NaF as low as 0.06 m m, regardless of whether the inhibitor was added before or after the substrate. After the initial decline in the glucose-6-P content, the concentration of this intermediate frequently increased, resulting from the degradation of endogenous glycogen. ATP, on the other hand, remained low throughout the experimental period. Experiments with crude enzyme preparations of the organism demonstrated that the glycogen synthetic enzymes, phosphoglucomutase (EC 2.7.5.1), ADPG pyrophosphorylase, and ADPG glucan transferase (ADP-glucose:glycogen glucosyl transferase), as well as phosphorylase (EC 2.4.1.1) in the degradative pathway, were fluoride-insensitive at NaF concentrations inhibiting in vivo synthesis. The inability of 2.4 m m NaF to inhibit glucose-6-P formation from glycogen in vivo in the absence of exogenous glucose confirms that phosphoglucomutase and phosphorylase in this organism are not inhibited by fluoride. The results strongly suggest that fluoride interacts, in some manner, with the sugar transport system in the organism as phosphorylation by hexokinase (EC 2.7.1.1) was not fluoride-sensitive. The “apparent” inhibition of glycogen synthesis is discussed in relation to the availability of ATP and glucose-6-P.