Properties of Mutant SHV-5 β-Lactamases Constructed by Substitution of Isoleucine or Valine for Methionine at Position 69

The effect of replacement of Met-69 by Ile or Val on the properties of the extended-spectrum β-lactamase SHV-5 was studied. Mutant enzymes were constructed by site-specific mutagenesis and expressed under isogenic conditions in Escherichia coli DH5α cells. Compared with SHV-5, the mutant β-lactamases conferred lower levels of β-lactam resistance and were less efficient in hydrolyzing ampicillin, cephalothin, and cefotaxime. The substitutions rendered SHV-5 less susceptible to inhibition by clavulanate, sulbactam, and tazobactam; however, the MICs of penicillin-inhibitor combinations remained similar, suggesting an attenuation of penicillinase activity.