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 beta-lactamase SHV-5 was studied. Mutant enzymes were constructed by site-specific mutagenesis and expressed under isogenic conditions in Escherichia coli DH5 alpha cells. Compared with SHV-5, the mutant beta-lactamases conferred lower levels of beta-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.