The intracellular distribution of sphingosine (Sph) kinase activity was examined in human platelets. A large proportion (72%) of the total activity was found to be associated with the membrane fraction, and the membrane-associated fraction had higher specific activity compared with the cytosolic enzyme. Most of the membrane-associated activity could be extracted with 1 M NaCl. The cytosolic activity was unstable upon heat treatment, with 80% of the activity being lost during incubation at 45 °C for 1 h, whereas the NaCl-extractable fraction was stable under the same conditions. When subjected to Mono Q column chromatography, the cytosolic fraction produced two activity peaks and the NaCl-extractable fraction gave a single peak. These three Sph kinase activities showed different responses to stimulation by β-octylglucoside and inhibition by N,N-dimethylsphingosine and l-threo-dihydrosphingosine, suggesting the presence of multiple enzyme forms in human platelets.

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