Thiomandelic acid, a broad spectrum inhibitor of zinc beta-lactamases: kinetic and spectroscopic studies

Resistance to Beta-lactam Antibiotics mediated by metallo-beta-lactamases is an increasingly worrying clinical problem. Candidate inhibitors include mercaptocarboxylic acids, and we report studies of a simple such compound, thiomandelic acid. A series of 35 analogues were synthesized and examined as metallo-beta-lactamase inhibitors. The K(i) values (Bacillus cereus Enzyme) are 0.09 microm for R-thiomandelic acid and 1.28 microm for the S-isomer. Structure-activity relationships show that the thiol is essential for activity and the carboxylate increases potency; the affinity is greatest when these groups are close together. Thioesters of thiomandelic acid are substrates for the Enzyme, liberating thiomandelic acid, suggesting a starting point for the design of "pro-drugs." Importantly, thiomandelic acid is a broad spectrum inhibitor of metallo-beta-lactamases, with a submicromolar K(i) value for all nine enzymes tested, except the Aeromonas hydrophila enzyme; such a wide spectrum of activity is unprecedented. The binding of thiomandelic acid to the B. cereus Enzyme was studied by NMR; the results are consistent with the idea that the inhibitor thiol binds to both zinc ions, while its carboxylate binds to Arg(91). Amide chemical shift perturbations for residues 30-40 (the beta(3)-beta(4) loop) suggest that this small inhibitor induces a movement of this loop of the kind seen for other larger inhibitors.