Binding of 125I-labeled beta-lactam antibiotics to the penicillin binding proteins of Escherichia coli.

125I-Labeled derivatives of the beta-lactam antibiotics cephalexin, cephradine, cefaclor and 6-alpha-aminopenicillanic acid have been obtained by reacting these compounds with (125I)-Bolton-Hunter reagent. The following target proteins were found in Escherichia coli: (1) The derivatives of cephalexin, cefaclor and cephradine preferentially interact with the high molecular weight penicillin binding proteins ( PBP1a and PBP1b ); (2) The 125I- derivative of 6-alpha-aminopenicillanic acid is preferentially bound by the low molecular weight penicillin binding proteins 4 and 5/6. The iodinated derivatives showed a very high affinity of binding to their target proteins with apparent half-saturating concentrations in the nano -molar range.