Characterization of OXA-9, a beta-lactamase encoded by the multiresistance transposon Tn1331

The enzyme OXA-9, an oxacillinase-carbenicillinase, is encoded by the blaOXA-9 gene which was originally found within the structure of Tn1331, a multiresistance transposon first isolated from a clinical Klebsiella pneumoniae strain. Studies to characterize OXA-9 demonstrated that it has a pI of 6.9 and the optimal pH for enzyme activity was between 7.7 and 8.2. When total soluble extracts were preincubated at 37 degrees C and at 42 degrees C, enzyme activity decayed to approximately 56% of the original value after 6 hrs. at 37 degrees C and to 50% after 30 min. at 42 degrees C. Enzymatic activity of OXA-9 was inhibited in the presence of p-chloromercuribenzoate, cloxacillin and clavulanic acid, but not by 200 mM sodium chloride. The inhibition by p-chloromercuribenzoate may indicate the presence of a cysteine residue playing a role in the catalytic activity of the enzyme. The OXA-9 enzyme was released by osmotic shock of E. coli cells harboring a recombinant clone including the blaOXA-9 gene indicating that it is located in the periplasmic space of the cells. The OXA-9 enzyme was purified from soluble protein extracts of E. coli cells carrying a recombinant clone including the blaOXA-9 by ion exchange chromatography.