Sphingomyelinase ofBacillus Cereusas a Bacterial Hemolysin

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Abstract
The enzymatic, toxic and molecular properties of sphingomyelinase from Bacillus cereuswere described in relation to other bacterial sphingomyelinases such as β-toxin of Staphylococcus aureus. The complete amino acid sequences of B. cereussphingomyelinase, determined from its chromosmal DNA, were examined in relation to other phospholipases C from B. cereusand Clostridium perfringens. The gene of B. cereussphingomyelinase codes 27 amino acid residues of signal peptide and 306 residues of mature protein. Both the genes of sphingomyelinase and phosphatidylcholine-hydrolyzing phospholipase C of B. cereusformed a gene cluster. According to the prediction of secondary structure of sphingomyelinase, almost half of the total amino acid residues might participate in loop or turn structure, while one-fifth and one-fourth of amino acid residues might be involved in α-helix and β-structure, respectively. The helical content determined by circular dichroism (CD) spectra indicated 0–5%. The enzyme specifically hydrolyzes sphingomyelin in the intact erythrocyte membranes as well as in the micelles and liposomes, causing the hemolysis of erythrocytes. The enzyme is specifically adsorbed onto the surface of membranes of erythrocytes and liposomes. The hydrolysis of sphingomyelin and the adsorptive properties were influenced by the divalent metal ions such as Mg2+, Ca2+and Mn2+. Selective modfication of amino acid residues in the molecule of sphingomyelinase suggested that acidic amino acid residues such as Asp and Glu would be involved in the catalytic center and adsorptive sites in the sphingomyelinase molecule. Also, the effects of membrane-perturbing agents were described.

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