Effects of pH on beta-hydroxybutyrate exchange kinetics rat erythrocytes

The influx of beta-hydroxybutyrate into rat erythrocytes at 0 degrees C was measured in the presence of various substrate concentrations and pH values at equilibrium. Under these circumstances, influx measures equilibrium exchange. Acidification (pH 6.3 compared to pH 7.3 and pH 8.3) increased exchange V markedly, increased exchange Km and increased exchange V/Km moderately. The data indicate that the base form of the carrier (either lacking H+ or bearing OH-) binds beta-hydroxybutyrate more tightly than the acid form (either bearing H+ or lacking OH-) and that the beta-hydroxybutyrate-bearing base form is relatively or completely immobile. If these pH effects reflect titration of the transport-partner site, then the findings favor an A-/H+-symport mechanism; for, with this mechanism, a beta-hydroxybutyrate-bearing base form of the carrier is expected. The findings do not favor an A-/OH--antiport mechanism; for, with this mechanism, a beta-hydroxybutyrate-bearing base form is not expected, and OH- should compete with beta-hydroxybutyrate for binding.