EVIDENCE FOR THE PRESENCE OF MULTIPLE FORMS OF SPH KINASE IN HUMAN PLATELETS

The intracellular distribution of sphingosine (Sph) kinase activity wa s examined in human platelets. A large proportion (72%) of the total a ctivity was found to be associated with the membrane fraction, and the membrane-associated fraction had higher specific activity compared wi th the cytosolic enzyme. Most of the membrane-associated activity coul d be extracted with 1 M NaCl. The cytosolic activity was unstable upon heat treatment, with 80%, Of the activity being lost during incubatio n at 45 degrees C for 1 h, whereas the NaCl-extractable fraction was s table under the same conditions. When subjected to Mono Q column chrom atography, the cytosolic fraction produced two activity peaks and the NaCl-extractable fraction gave a single peak. These three Sph kinase a ctivities showed different responses to stimulation by beta-octylgluco side and inhibition by N,N-dimethylsphingosine and L-threo-dihydrosphi ngosine, suggesting the presence of multiple enzyme forms in human pla telets.