Functional characterization of pH-sensitive organic anion transporting polypeptide OATP-B in human.
The pH-sensitive activity of human organic anion transporting polypeptide OATP-B, which is expressed at the apical membrane of human small intestinal epithelial cells, was functionally characterized. When initial uptake of estrone-3-sulfate, a typical substrate of OATP, was studied kinetically, we observed an increase in V(max) with decrease of pH from 7.4 to 5.0, whereas the change in K(m) was negligible. OATP-B-mediated uptake of estrone-3-sulfate was independent of sodium, chloride, bicarbonate, or glutathione, whereas the proton ionophore carbonylcyanide p-trifluoromethoxyphenylhydrazone exhibited a pH-dependent inhibitory effect, suggesting that a proton gradient is a driving force for OATP-B. When OATP-B was expressed in human embryonic kidney 293 cells, uptake activities for anionic compounds showed various kinds of pH sensitivity. Dehydroepiandrosterone-sulfate, estrone-3-sulfate, and fexofenadine were transported by OATP-B at both neutral and acidic pH, whereas estradiol-17beta-glucuronide, acetic acid, and lactic acid were not transported at all. Transport of taurocholic acid and pravastatin by OATP-B was observed only at acidic pH, demonstrating a pH-sensitive substrate specificity of OATP-B. Because the physiological pH close to the surface of intestinal epithelial cells is acidic, the roles of OATP-B in the small intestine might be different from those in other tissues, such as liver basolateral membrane. Although the driving force for OATP-B has not been fully established, the clarification of factors, such as pH, that affect the OATP-B-activity is essential for an understanding of the physiological and pharmacological relevance of the transporter in the small intestine.
Nozawa
Takashi
T
Department of Molecular Biopharmaceutics, Faculty of Pharmaceutical Sciences, Tokyo University of Science, Noda, Chiba, Japan.
Imai
Kozue
K
Nezu
Jun-Ichi
J
Tsuji
Akira
A
Tamai
Ikumi
I
D001692:Biological Transport; D002478:Cells, Cultured; D004970:Estrone / Q000031:analogs & derivatives* / Q000493:pharmacokinetics; D006801:Humans; D006863:Hydrogen-Ion Concentration; D027361:Organic Anion Transporters / Q000502:physiology*
D027361:Organic Anion Transporters; C450859:SLCO2B1 protein, human; D004970:Estrone; C017296:estrone sulfate