Comparison of the β-Toxins fromStaphylococcus aureusandStaphylococcus intermedius

The β-toxins produced byStaphylococcus aureusandStaphylococcus intermediuswere purified to homogeneity from culture supernatants. Although the toxin fromS. aureushas been throughly studied, less is known about its unique counterpart fromS. intermedius.This is the first reported purification and analysis of theS. intermediusβ-toxin. Both toxins have similar enzymatic properties, belong to the class of neutral sphingomyelinases C, and have a high specificity for sphingomyelin. They also hydrolyze lysophosphatidylcholine at a much slower rate, but have no activity toward phosphatidylcholine, phosphatidylethanolamine, or phosphatidylserine. The kinetic parameters determined for both proteins (apparentKm1.4 mM,Vmax100 mmol/min/μg protein) are identical. Despite these similarities, the size and amino acid composition of the two β-toxins differ. Molecular mass values, determined by electrophoresis and gel filtration, indicate that the both enzymes are single polypeptides. The decrease in sphingomyelinase activity ofS. aureusβ-toxin upon pretreatment with dithiothreitol (DTT) indicates the presence of a disulfide bond in the protein. In contrast, DTT has no effect on the enzymatic activity ofS. intermediusβ-toxin. This observation is consistent with the absence of detectable cysteine residue in the protein. N-terminal amino acid sequences determined for the first 19 residues of both β-toxins also differ, only nine of the first 19 residues are identical. Further evidence that the two proteins differ was obtained by immunological analysis which demonstrated crossreactivity but a lack of identity.