Modulation of the activity and arachidonic acid selectivity of group X secretory phospholipase A2 by sphingolipids.

To investigate the role of sphingomyelin (SM) in the regulation of inflammatory reactions, we studied its effect on the activity and fatty acid specificity of group X secretory phospholipase A(2) (sPLA(2)X). Compared with other phospholipases, recombinant sPLA(2)X released more arachidonate from HDL. Pretreatment of HDL with sphingomyelinase (SMase) C activated the sPLA(2)X activity, but the release of arachidonate was stimulated less than that of linoleate. In liposomes containing synthetic phosphatidylcholines (PCs), sPLA(2)X showed no clear selectivity among the various sn-2 unsaturated fatty acids. However, when SM was incorporated into liposomes at 30 mol%, the enzyme exhibited strong preference for arachidonate, although its overall activity was inhibited. Degradation of liposomal SM by SMase C resulted in sPLA(2)X activation and loss of its arachidonate preference. Incorporation of ceramide into HDL or PC liposomes activated the enzyme activity, the release of arachidonate being stimulated more than that of linoleate. SM-deficient cells released more arachidonate than normal cells in response to exogenous sPLA(2)X. SMase pretreatment of normal cells stimulated the release of arachidonate by the exogenous sPLA(2)X. These results show that SM not only inhibits sPLA(2)X activity but also contributes to its selectivity for arachidonate, whereas ceramide stimulates the hydrolysis of arachidonate-containing PCs.