Cholesterol relieves the inhibitory effect of sphingomyelin on type II secretory phospholipase A(2)

Secretory type II phospholipase A(2) (sPLA(2)) is inhibited by sphingomyeli n (SPH); cholesterol either mixed with the model glycerophospholipid substr ate or added to the assay medium as separated liposomes counteracts this in hibition efficiently. The inhibition of fatty acid release assayed by quant itative gas chromatography-MS is observed when SPH is added to erythrocyte membranes as the substrate instead of a readily hydrolysable phosphatidylet hanolamine/phosphatidylser model mixture. Hydrolysis of SPH by Staphylococc us aureus sphingomyelinase suppresses its inhibitory potency. The addition of cholesterol to SPH liposomes with a 1:1 stoichiometry relieves completel y the inhibition of sPLA(2) exerted by SPH. The mechanism of inhibition sug gested by the binding assay is that sPLA(2) binds with affinity to the SPH interface, after either phase segregation at the assay temperature or on th e pure SPH liposomes added to the incubation medium. Cholesterol is shown t o suppress the binding affinity of the enzyme for the SPH interface. A mode l for inhibition is suggested in which binding of the sphingosine moiety is competitive for sPLA(2) (inhibition) or for cholesterol (release of the en zyme).