Examination of the mechanism of Na+/phosphate cotransport. Use of fluorophosphate and the nature of cotransporter functional asymmetry.

The effect of substrates and mono- and di-fluorophosphates on Na+/phosphate cotransport by intestinal SDS-BBM vesicles was examined. Internal Na+ inhibited Na+/phosphate cotransport and this inhibition was relieved by internal phosphate. Inhibition by internal Na+ displayed Michaelis-Menten kinetics in contrast to the sigmoidal behavior for Na+ activation of phosphate uptake. Difluorophosphate but not monofluorophosphate inhibited Na+/phosphate cotransport in the external medium. In contrast, in the absence of internal Na+ monofluorophosphate but not difluorophosphate inhibited phosphate uptake. These results suggest that the intestinal Na+/phosphate cotransporter is functionally asymmetric in its substrate specificity.