Ceramide induces structural defects into phosphatidylcholine bilayers and activates phospholipase A2.

We studied the effects of bovine brain ceramide on the structure of dipalmitoylphosphatidylcholine (DPPC) bilayers and cobra venom phospholipase A2, (PL-A2) activity using 2H NMR and specific enzymatic assays. Addition of ceramide to DPPC at 45 degrees C induces lateral phase separation of the bilayers into regions of gel and liquid crystalline phases. The order parameters of the DPPC acyl chains in the liquid crystalline phase are only slightly affected by the presence of ceramide, indicating that the latter is largely partitioned in the gel phase of DPPC, whereas at 60 degrees C the presence of ceramide induced a large increase of the order parameters of DPPC side chains. The observed structural effects of ceramide correlated with ceramide-induced activation of cobra venom phospholipase A2 (PL-A2). Ceramide activated PL-A2 in a concentration-dependent manner, with a significant effect observed at 5 mol% ceramide, which caused an approximately 3-fold increase in PL-A2 activity. The results showing activation of PL-A2 by ceramide illustrate an additional feature of the biological effects of this second messenger and suggest the possibility of cross-talk between the sphingomyelinase and PL-A2 signal transduction pathways.