Abstract

The sodium-dependent intestinal active transport of 3-methyl-D-glucose, a nonmetabolized glucose analog, was competitively inhibited by phenolphthalein (K_i = 0.7 x 10^-4 M) in vitro. When sugar accumulation against a concentration difference was prevented by metabolic inhibitors or by replacing Na⁺ in the bathing medium with Li⁺, phenolphthalein inhibited the specific ion-activated sugar entry process; this is interpreted as direct inhibition of the sugar carrier. Phenolphthalein was extensively bound to tissue protein, and it could not be determined whether it was itself transported across the cell membrane. Phenolphthalein inhibited the sodium-dependent intestinal active transport of α-aminoisobutyric acid as well as its entry in the presence of metabolic inhibitors or in Li⁺ medium; this suggests interaction with the amino acid carrier. It is concluded that phenolphthalein affects intestinal sugar transport by competitively inhibiting the sugar carrier. The data do not show to what extent inhibition of the sodium pump (demonstrated by others) may participate in inhibition of sugar transport.