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 enzy me) are 0.09 muM for R-thiomandelic acid and 1.28 muM for the S-isomer. Str ucture-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 f or the design of "pro-drugs." Importantly, thiomandelic acid is a broad spe ctrum 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 a cid 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 c arboxylate binds to Arg(91). Amide chemical shift perturbations for residue s 30-40 (the beta (3)-beta (4) loop) suggest that this small inhibitor indu ces a movement of this loop of the kind seen for other larger inhibitors.