The metallo-␤-lactamase determinant of Acinetobacter baumannii AC-54/97, a clinical isolate from Italy that was previously shown to produce an enzyme related to IMP-1, was isolated by means of a PCR methodology which targets amplification of gene cassette arrays inserted into class 1 integrons. Sequencing revealed that this determinant was an allelic variant (named bla IMP-2 ) of bla IMP found in Japanese isolates and that it was divergent from the latter by 12% of its nucleotide sequence, which evidently had been acquired independently. Similar to bla IMP , bla IMP-2 was also carried by an integron-borne gene cassette. However, the 59-base element of the bla IMP-2 cassette was unrelated to those of the bla IMP cassettes found in Japanese isolates, indicating a different phylogeny for the gene cassettes carrying the two allelic variants. Expression of the integron-borne bla IMP-2 gene in Escherichia coli resulted in a significant decrease in susceptibility to a broad array of ␤-lactams (ampicillin, carbenicillin, cephalothin, cefoxitin, ceftazidime, cefepime, and carbapenems). The IMP-2 enzyme was purified from an Escherichia coli strain carrying the cloned determinant, and kinetic parameters were determined with several ␤-lactam substrates. Compared to IMP-1, the kinetic parameters of IMP-2 were similar overall with some ␤-lactam substrates (cefoxitin, ceftazidime, cefepime, and imipenem) but remarkably different with others (ampicillin, carbenicillin, cephaloridine, and meropenem), revealing a functional significance of at least some of the mutations that differentiate the two IMP variants. Present findings suggest that the environmental reservoir of bla IMP alleles could be widespread and raise a question about the global risk of their transfer to clinically relevant species.

The crystal structure of the class-B ␤-lactamase, BlaB, from the pathogenic bacterium, Chryseobacterium meningosepticum, in complex with the inhibitor, D-captopril, has been solved at 1.5-Å resolution. The enzyme has the typical ␣␤/␤␣ metallo-␤-lactamase fold and the characteristic two metal binding sites of members of the subclass B1, in which two Zn 2؉ ions were identified. D-Captopril, a diastereoisomer of the commercial drug, captopril, acts as an inhibitor by displacing the catalytic hydroxyl ion required for antibiotic hydrolysis and intercalating its sulfhydryl group between the two Zn 2؉ ions. Interestingly, D-captopril is located on one side of the active site cleft. The x-ray structure of the complex of the closely related enzyme, IMP-1, with a mercaptocarboxylate inhibitor, which also contains a sulfhydryl group bound to the two Zn 2؉ ions, shows the ligand to be located on the opposite side of the active site cleft. A molecule generated by fusion of these two inhibitors would cover the entire cleft, suggesting an interesting approach to the design of highly specific inhibitors.

The bla IMP gene coding for the IMP-1 metallo-β-lactamase produced by a Pseudomonas aeruginosaclinical isolate (isolate 101/1477) was overexpressed via a T7 expression system in Escherichia coli BL21(DE3), and its product was purified to homogeneity with a final yield of 35 mg/liter of culture. The structural and functional properties of the enzyme purified from E. coli were identical to those of the enzyme produced by P. aeruginosa. The IMP-1 metallo-β-lactamase exhibits a broad-spectrum activity profile that includes activity against penicillins, cephalosporins, cephamycins, oxacephamycins, and carbapenems. Only monobactams escape its action. The enzyme activity was inhibited by metal chelators, of which 1,10-o-phenanthroline and dipicolinic acid were the most efficient. Two zinc-binding sites were found. The zinc content of the P. aeruginosa 101/1477 metallo-β-lactamase was not pH dependent.

The metallo-β-lactamase produced by Chryseobacterium(formerly Flavobacterium) meningosepticum,which is the flavobacterial species of greatest clinical relevance, was purified and characterized. The enzyme, named BlaB, contains a polypeptide with an apparent Mr of 26000, and has a pI of 8.5. It hydrolyses penicillins, cephalosporins (including cefoxitin), carbapenems and 6-β-iodopenicillanate, a mechanism-based inactivator of active-site serine β-lactamases. The enzyme was inhibited by EDTA, 1-10 phenanthroline and pyridine-2,6-dicarboxylic acid, with different inactivation parameters for each chelating agent. The C. meningosepticum blaBgene was cloned and sequenced. According to the G+C content and codon usage, the blaBgene appeared to be endogenous to the species. The BlaB enzyme showed significant sequence similarity to other class B β-lactamases, being overall more similar to members of subclass B1, which includes the metallo-enzymes of Bacillus cereus(Bc-II) and Bacteroides fragilis(CcrA) and the IMP-1 enzyme found in various microbial species, and more distantly related to the metallo-β-lactamases of Aeromonasspp. (CphA, CphA2 and ImiS) and of Stenotrophomonas maltophilia(L1).

Vesicles shed by cancer cells are known to mediate several tumor-host interactions. Tumor microenvironment may, in turn, influence the release and the activity of tumor-shed microvesicles. In this study, we investigated the molecular mediators of the pH-dependent proinvasive activity of tumor-shed vesicles. Gelatinase zymography showed increased microvesicle activity of matrix metalloproteinases 9 and 2 as a result of acid exposure (pH 5.6) compared to pH 7.4. Thus, we reasoned that the cysteine protease cathepsin B might play a role in mediating the pH-dependent activation of gelatinases. Cathepsin B expression in tumor-shed microvesicles was confirmed by Western blot analysis and zymography. The activity of vesicle-associated cathepsin B measured using Z-Arg-Arg-pNA as substrate was significantly increased at acidic pH values. Inhibition of protease activity by the cysteine protease inhibitor, E-64, and treatment of ovarian cancer cells with small interfering RNA against cathepsin B suppressed the ability of tumor-shed microvesicles to stimulate both gelatinase activation and the invasiveness of endothelial cells observed at low pH values. We conclude that microvesicle shedding is a major secretory pathway for cathepsin B release from tumor cells. Hence, the acidic microenvironment found in most solid tumors may contribute to cathepsin B-mediated proinvasive capabilities of tumor-shed vesicles.