INTERACTION OF THE ORALLY-ACTIVE DIANIONIC CEPHALOSPORIN CEFIXIME WITH THE UPTAKE SYSTEM FOR OLIGOPEPTIDES AND ALPHA-AMINO-BETA-LACTAM ANTIBIOTICS IN RABBIT SMALL-INTESTINE

The uptake of two orally active beta-lactam antibiotics of different c hemical structure, the zwitterionic alpha-aminocephalosporin cephalexi n and the dianionic carboxymethoxyimino-cephalosporin cefixime, by bru sh border membrane vesicles obtained from rabbit small intestine and t heir molecular interaction with the H+/oligopeptide transport system w ere investigated. The uptake of both compounds was stimulated by an in wardly directed H+-gradient with a profound pH-maximum for cephalexin at pH 6outside, and pH 7.4inside whereas cefixime uptake was maximal b elow pH 5outside. Modification of histidyl residues of membrane protei ns led to a complete loss of pH dependence of transport of both cephal osporins. The uptake of cephalexin was competitively inhibited by cefi xime and dipeptides and vice versa that of cefixime by cephalexin and dipeptides. The uptake of cefixime was trans-stimulated by cephalexin and glycyl-L-proline whereas cephalexin uptake could only be trans-sti mulated by glycyl-L-proline, not by cefixime. Photoaffinity labeling w ith [H-3]benzylpenicillin as a direct photoaffinity probe of the H+/ol igopeptide transport system demonstrated a direct molecular interactio n of both cephalexin and cefixime with this transporter in the pH rang e of 5-8. Thermal pretreatment of membrane vesicles inhibited the ceph alexin transport system temperature-dependently, whereas cefixime upta ke was not inhibited, but stimulated. Taken together we conclude that dianionic cephalosporins like cefixime bind to the transport system sh ared by oligopeptides and alpha-amino-beta-lactam antibiotics. Their t ransport across the enterocyte brush border membrane, however, may occ ur to a significant extent by a different transport system.