Effect of intracellular pH and potassium ions on a primary transport system for glutamate/aspartate in Streptococcus mutans

We have studied the mechanism of L-glutamate/L-aspartate transport in a fermentative oral bacterium of Streptococcus mutans (strain Ingbritt). The transport rate stays virtually constant throughout the pH range 5.5-8.5 and followed Michaelis-Menten type kinetics. At high pH values from 7 to 8.5, transport was essentially insensitive to N,N'-dicyclohexyl-carbodiimide (DCCD), an inhibitor of ATPase, and to carbonyl cyanide-p-trifluoromethoxyphenyl-hydrazone (FCCP), an ionophore dissipating proton motive force indicating that S. mutans transports glutamate by a primary transport system at the expense of ATP or an alternative energized metabolite. At lower external pH (7-5.5), DCCD (100 microM) or FCCP (10 microM) significantly inhibited L-glutamate transport while the intracellular ATP level was hardly affected, indicating that the activity of the primary transport system was decreased at lower intracellular pH. The glutamate transport was stimulated in the presence of potassium ion at an external pH of 6. The stimulation can be explained partly by the regulation of intracellular pH with concomitant potassium ion movement.