Characterization of sphingomyelinase activity released by thrombin-stimulated platelets

In this study we report that human platelets display neutral (nSMase) and a cid sphingomyelinase (aSMase) as well as acid ceramidase (aCerase) activity . Cell activation by thrombin resulted in a marked decrease of intracellula r aSMase activity, accompanied by the release of enzyme into the medium. In contrast, thrombin treatment did not affect aCerase activity. Two major pr otein bands of 73 and 70 kDa were recognized by aSMase antibodies in restin g platelet lysates and in the medium of stimulated cells. Phorbol esters to gether with the calcium ionophore A23187 fully reproduced thrombin action o n aSMase release. The secreted enzymatic activity was insensitive to digest ion with endoglycosidase H but it was stimulated by Zn2+, although to a lim ited extent compared to aSMase constitutively released by murine endothelia l cells. Taken together, these data suggest that secreted aSMase does not o riginate from the lysosomal compartment but rather from other platelet vesi cles.