Transport of phosphoenolpyruvate through the erythrocyte membrane

Phosphoenolpyruvate was transported through the erythrocyte membrane at low pH (4.5-6.5). The influx was observed not only in an iso-osmotic sucrose medium, but also in 0.1 M-citrate solution, but it was negligible in an iso-osmotic NaC1 solution. Efflux, however, was observed in both the sucrose and NaC1 solutions. Compounds derived from phosphoenolpyruvate by replacing the methene group by similarly hydrophobic groups such as hydrogen or the methyl group were permeant but those with the hydrophilic hydroxymethyl group were impermeant. This transport was inhibited by the treatment with 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid or pyridoxal phosphate/NaBH4, which are known to be specific for the transport of anions such as C1-, SO42- and HPO42-. It showed saturation kinetics with respect to phosphoenolpyruvate concentration in the medium. These results suggest that the transport of phosphoenolpyruvate is mediated by the anion-transport system. Although phosphoenolpyruvate was transported against the concentration gradient, the transport was characterized as a passive transport, and this apparent uphill transport was interpreted by the Donnan equilibrium.