Rate-limiting steps of glucose and sorbitol metabolism in Streptococcus mutans cells exposed to air

It has been supposed that rate of sorbitol metabolism in the air-exposed streptococcal cells could be limited by the low capacity to regenerate nicotinamide adenine dinucleotide (NAD) from reduced NAD (NADH) following inactivation of pyruvate formate-lyase by oxygen. The rate-limiting steps, however, have not been identified. The aim of this study was to examine the effect of temporary exposure of the streptococcal cells to air on the intracellular flux of glucose and sorbitol metabolism by measuring acid excretion, fluorescence dependent on cellular level of NADH, glycolytic intermediates and enzyme activities. The exposure of cells to air decreased the acid excretions during glucose and sorbitol metabolism. The analysis of the glycolytic intermediates and the fluorescence suggested that the reduced level of acid excretion in the air-exposed glucose metabolizing cells resulted from the decrease in pyruvate catabolism. In the presence of sorbitol, the decreased acid production resulted from the reduced rates of the reactions catalyzed by sorbitol-phosphoenolpyruvate phosphotransferase and sorbitol 6-phosphate dehydrogenase because of shortage of substrates for these enzymes in addition to the decrease in pyruvate catabolism.