INTERACTION BETWEEN PHLORETIN AND RED BLOOD-CELL MEMBRANE

Phloretin [the aglycone of phlorizin] binding to human red blood cell components was characterized at pH 6, where binding and inhibitory potency were maximal. Binding to intact red cells and to purified Hb are nonsaturable processes approximately equal in magnitude, which strongly suggests that most of the red cell binding may be ascribed to Hb. Hb-free red cell ghosts can bind only about 10% as much phloretin as an equivalent number of red cells. The permeability of the red cell membrane to phloretin was determined by a direct measurement at the time-course of the phloretin uptake. At a 2% hematocrit, the half time for phloretin uptake was 8.7 s, corresponding to a permeability coefficient of 2 .times. 10-4 cm/s. The concentration dependence of the binding to ghosts revealed at least 2 saturable components. Phloretin bound with high affinity (Kdiss = 1.5 .mu.M) to about 2.5 .times. 106 sites/cell; it also bound with lower affinity (Kdiss = 54 .mu.M) to a 2nd (5.5 .times. 107/cell) set of sites. In sonicated total lipid extracts of red cell ghosts, phloretin binding consisted of a single, saturable component. Its affinity and total number of sites were not significantly different from those of the low affinity binding process in ghosts. No high affinity binding of phloretin was exhibited by the red cell lipid extracts. The high affinity phloretin binding sites were related to membrane proteins, and the low affinity sites resulted from phloretin binding to lipid. The identification of these 2 types of binding sites allows phloretin effects on protein-mediated transport processes to be distinguished from effects on the lipid region of the membrane.