Confirmation of Carrier-Mediated Intestinal Transport of β-Lactam Antiboitics by Using Gene Expression in <I>Xenopus laevis</I> Oocytes

Abstract

In the present study, to prove the carrier-mediated transport in the intestinal absorption for orally active β-lactam antibiotics, Xenopus laevis oocytes were employed as exogenous gene expression system. It was demostrated that the uptake of zwitterionic cefadroxil (CDX) was increased by Xenopus laevis oocytes injected with poly(A)+RNA derived from rat, rabbit or human small intestinal mucosa. The uptake of CDX was inhibited by other orally active β-lactam antibiotics and dipeptides significantly, but not by amino acid. Furthermore at acidic extracellular pH, the uptakes of amino-β -lactam antibiotics, CDX, cephalexin and cephradine increased, suggesting that H+ gradient is a driving force for transports of these antibiotics. By size-fractionation of poly(A)+RNA on a sucrose density gradient centrifugation, poly(A)+RNA of which size is approximately 3 kb was identified to encode dipeptide transporter. Photoaffinity labeling of membrane protein with [3H]benzylpenicillin showed an expression of the specific binding protein with a molecular mass of 130 kDa. Stereospecific transport of ceftibuten with cis and trans-isomers was also exhibited in oocytes injectedwith poly(A)+RNA. From these results it was clarified that β-lactam antibiotics are absorbed in the small intestine by a H+ gradient dependent carrier-mediated mechanism which is common to dipeptide transporter.