Purification and biochemical properties of Pitton's type 2 beta-lactamase (SHV-1).

A five-step procedure has been developed for purifying Pitton's type 2 plasmid-mediated beta-lactamase (PIT-2, also called SHV-1) from cultures of a hyperproducing variant of an Escherichia coli K12 strain carrying the plasmid p453. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis revealed a single protein band with a molecular weight of about 27,500. The amino acid composition of the protein and the amino acid sequence of the NH2-terminal region have been determined. PIT-2 enzyme contains 272 amino acid residues with 2 cysteines. Studies of the S-carboxymethylated protein (previously reduced or unreduced) suggest that these two residues are presumably in the form of free sulphydryl groups in the native protein. Conversely, TEM-2 beta-lactamase contains 2 cysteine residues which are in the form of a disulphide bond. Comparison of PIT-2 with other beta-lactamases was made using the difference index (DI) of Metzger et al. The PIT-2 enzyme appeared more closely related to the TEM-type penicillinases (DI of 6.5).