Non specific effects of guanine nucleotides and polyanions on cytosolic platelet phospholipase C

Phospholipase C (PLC) partially purified by electrofocusing from human platelet cytosol was incubated with platelet total lipid vesicles containing [3H] phosphatidylinositol 4,5-bis phosphate ([3H]PIP2). Under optimal assay conditions (pH 4.5), PLC activity was stimulated (1.5 to 7 fold) by GTP gamma S, ATP, ADP, GDP beta S, sodium pyrophosphate and AlF4-. With lipid dispersions containing phosphatidylethanolamine and [3H]PIP2, maximal PLC activity was detected at pH 7.4 and was stimulated by micromolar concentrations of free Ca2+. However, no effect of GTP gamma S could be observed under these conditions. This study demonstrates a possible modulation of cytosolic PLC by polyanionic compounds not involving G-proteins and indicates that in vitro data obtained with guanine nucleotides should be considered with caution.