Phloretin keto-enol tautomerism and inhibition of glucose transport in human erythrocytes (including effects of phloretin on anion transport)

Under various pH conditions phloretin demonstrates keto-enol tautomerism with a pK value of 7.26 ± 0.06. As Wilbrandt has shown ((1950) Arch. Exp. Pathol. Pharmacol. 212, 9-29) phloretin added to erythrocytes inhibits glucose efflux, but not glucose influx. At pH 6.5 a K_i value of 0.36 and at pH 9 of 22.7 μM was measured; only the ketonic form of phloretin contributes to the inhibition of glucose efflux. This was also the case for inhibition of galactose efflux and anion exchange. The geometry optimization of a large number of conformations of the ketonic and enolic forms of phloretin demonstrates different shapes of the molecules. Only the ketonic form shows several overlapping structures with β-d-glucopyranose. Considering surplus binding of phloretin under glucose efflux conditions as being equivalent to the number of glucose transporters, a number of about 200 000 molecules was determined. By two independent methods 210 000 and 171 000 molecules per cell were calculated. This result is in close agreement with the number of glucose transporter sites of the erythrocyte.