SPHINGOMYELINASE ACTIVITY IN HUMAN PLATELETS

The sphingolipid metabolites, ceramide, sphingosine, and sphingosine-l -phosphate, may be involved in several signalling pathways and may reg ulate cell functions such as cell growth, secretion, differentiation, and apoptosis. During activation of human platelets by thrombin, sphin gosine-1-phosphate is released from platelets and can potentiate their aggregation. Thrombin also causes an increase in platelet sphingosine levels. Since these molecules can be derived from sphingomyelin, we h ave determined whether platelets possess sphingomyelinase and whether this enzyme is regulated during platelet function. Using radioactive s phingomyelin as substrate, we assayed sphingomyelinase activity over t he range of pH 4 to 10 and observed optimal activity at pH 5.0-5.5. Li ttle activity was found at neutral or alkaline pH, and the presence of Mg++, Ca++, Zn++, or EDTA in the reaction mixture had little effect o n the pH profile. Activation of platelets by thrombin or ADP had no ef fect on sphingomyelinase activity, but thrombin caused secretion of th e acid-sphingomyelinase activity into the media, Thus, human platelets contain an acid-sphingomyelinase which is secreted during thrombin-in duced platelet activation. (C) 1998 Elsevier Science Ltd.