Transport of β-lactam antibiotics in kidney brush border membrane - Determinants of their affinity for the oligopeptide/H⁺ symporter

This study was designed to determine whether β-lactam antibiotics (cephalosporins and penicillins) are all substrates for the renal oligopeptide /H⁺ symporter and, if so, whether the transport system discriminates among the numerous β-lactam antibiotics. We used [³H]glycylglutamine, [³H]cephalexin, and [³H]-ampicillin as probes for the transport of oligopeptides, cephalosporins, and penicillins in kidney brush border membrane vesicles, respectively. Among the β-lactam antibiotics, only those with an α-amino group in the phenylacetamido moiety were found to interact with the oligopeptide / H⁺ symporter. Aminocephalosporins displayed high affinities (K_is generally < 250 μM), whereas aminopenicillins displayed low affinities (K_i 0.78-3.03 mM). These differences in affinities appeared to be a consequence of conformational features of the substrates, especially the sterical location of the carboxy group. The affinities of aminolactams for the oligopeptide / H⁺ symporter were, furthermore, related to the hydrophobicity of the phenylglycyl chains and the substituents attached to the thiazolidine and dihydrothiazine ring. In sharp contrast to the uptake of [³H]-glycylglutamine and [³H]cephalexin, the uptake of [³H]-ampicillin was not dependent on a pH gradient and was inhibited by various β-lactam antibiotics, whether or not they contained an α-amino group. Our data suggest that: (a) the transport of aminocephalosporins is largely mediated by the oligopeptide/H⁺ symporter, which is highly influenced by the substrate structure; and (b) penicillins are transported by another system, which is less discriminative with respect to substrate structure.