Characteristics of uptake of neutral and anionic cephalosporin antibiotics by the cloned intestinal peptide transporter PePT1

Uptake of the orally active cephalosporin antibiotics into intestinal epithelial cells is mediated by the H⁺-coupled electrogenic peptide transport system PepTL Since studies in humans show that anionic cephalosporms have a considerably lower bioavailability than the neutral aminocephalosporins, we compared the characteristics of uptake of a neutral (³H-cefadroxil) and an anionic compound (¹⁴C-cefixime) into Xenopus laavis oocytes expressing the rabbit intestinal peptide transporter PepT1. Influx of H⁺-ions as a consequence of cephalosporin transport was determined by changes in pH_in in oocytes measured by dual emission fluorometry based on the pH-dependent fluorescence changes of the dextran-coupled dye SNARF-1. A conventional two-electrode voltage clamp technique was applied to characterize responses in current and transmembrane potential to β-lactam transport. Results: Influx of both cephalosporins displayed a pronounced pH dependence with a pH optimum of 5.0 for cefixime and pH 6.5 for cefadroxii. Under these conditions cefixime and cefadroxil were transported with similar K_m and V_max values. Cross-inhibition in uptake of cefadroxil and cefixime was obtained only at pH < 5.5 indicating that anionic compounds do not interact with Peptl at pH > 6.5. Single cell pH_in recordings and electrophysiology established that both compounds are transported by PepT1 by electrogenic H⁺ coupled cotransport at pH 5.5 (cefixime) and pH 6.5 (cefadroxil) but with different flux coupling ratios. Although both compounds use the same transporter for absorption, cefixime needs a lower intestinal pH for maximal absorption that may not be realized in vivo due to an only slightly acidic mucosal surface pH in the small intestine.