The formation of lysophosphatidylinositol phosphate in human platelet microsomes

The formation of lysophosphatidylinositol phosphate (lysoPIP) from lysophosphatidylinositol (lysoPI) via kinase activity was studied in microsomal preparations from human platelets. For this purpose, [3H]lysoPI or [3H]phosphatidylinositol ([3H]PI) was prepared and incubated in the presence or absence of ATP, MgCl2 and Triton X-100, and the appearances of radioactivity in [3H]lysoPIP and [3H]phosphatidylinositol phosphate ([3H]PIP), respectively, were monitored using thin layer chromatography. Both lysoPI and PI phosphorylations were completely dependent upon the presence of ATP and MgCl2 in the incubation medium; Triton X-100 addition stimulated both reactions, with the stimulation of PI conversion being considerably greater than that for lysoPI can be converted to lysoPIP by phosphorylation in human platelet microsomes. The potential significance of this enzymatic reaction in stimulated cells is discussed in relation to the generation of inositol-1,4,5-trisphosphate, an important intracellular second messenger.