Inhibition Studies on the Metallo-β-lactamase L1 from Stenotrophomonas maltophilia

In an effort to identify a competitive inhibitor that can be used in future spectroscopic and crystallographic studies and to better understand the interaction of a mercaptoacetic acid–thiolester-containing compound with metallo-β-lactamase L1 from Stenotrophomonas maltophilia, inhibition studies using two thiol-containing compounds were conducted. N-(2′-Mercaptoethyl)-2-phenylacetamide is a competitive inhibitor of L1 with a Ki of 50 ± 3 μM, and this compound is not a time-dependent inactivator of L1. N-Benzylacetyl-d-alanylthioacetic acid is a competitive inhibitor of L1 with a Ki of 1.6 ± 0.3 μM. Matrix-assisted laser desorption ionization time-of-flight mass spectrometric studies revealed that 2 mol of mercaptoacetate covalently bind to L1 upon incubation of the enzyme with N-benzylacetyl-d-alanylthioacetic acid; however, this covalently modified enzyme has the same activity as wild-type L1. Last, inhibition studies were used to demonstrate that 4-morpholinoethanesulfonic acid does not inhibit L1, even at concentrations up to 300 mM. This work identifies two possible competitive inhibitors which can be used in future structural studies and further demonstrates inhibitory heterogeneity among the metallo-β-lactamases.