Lipocortin I and lipocortin II inhibit phosphoinositide‐ and polyphosphoinositide‐specific phospholipase C The effect results from interaction with the substrates

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Abstract
Lipocortins I and II, known to inhibit phospholipase A2, have been purified from bovine lung and tested with respect to their ability to affect the enzymatic activities of phosphoinositide- and polyphosphoinositide-specific phospholipase C from human platelets, rat liver cytosol or rat brain membranes. At 0.67,μM, both lipocortins led to complete inhibition of phospholipase C activity with either phosphatidylinositol or phosphatidylinositol 4,5-bisphophate as substrate. The inhibition could be overcome by increasing the substrate concentration. Ultracentrifugation studies with lipocortin II showed a direct interaction between phosphatidylinositol and the lipocortin, indicating that the lipocortins inhibit phospholipase C not directly but by interacting with the substrate. In experiments with plasma membranes from [3H]inositol-labeled HL-60 cells, lipocortin II did not affect PI-specific phospholipase C activity in the absence or presence of calcium plus or minus GTP-γ-S.