Characterization of the membrane .beta.-lactamase in Bacillus cereus 569/H/9

Abstract

The membrane-bound .beta.-lactamase from B. cereus, strain 569/H/9, was purified to apparent homogeneity. Nonionic detergent (0.5% Triton X-100) is required to keep the enzyme (traditionally called .gamma.-penicillinase and now called .beta.-lactamase III) in solution. Antibodies to .beta.-lactamase III were prepared, and the membrane-bound enzyme is immunochemically distinct from the extracellular enzymes. .beta.-Lactamase III has a MW of 31,500, in contrast to the extracellular enzymes .beta.-lactamase I and .beta.-lactamase II which have MW of 30,000 and 22,000, respectively. The isoelectric point of .beta.-lactamase III is pH 6.8; .beta.-lactamase I and .beta.-lactamase II have isoelectric points at .apprx. 8.6 and 8.3, respectively. The amino acid composition of .beta.-lactamase III differs from those of .beta.-lactamase I and .beta.-lactamase II; the difference index between the compositions of .beta.-Lactamase I and .beta.-lactamase III (52%) suggests relatedness. .beta.-lactamase III is inactivated by 6.beta.-bromopenicillanic acid and by the sulfone of 6.alpha.-chloropenicillanic acid; cephalosporins are poorer substrates than penicillins. .beta.-Lactamase III may be a membrane-bound class A .beta.-lactamase.