IDENTIFICATION OF LIPID INHIBITOR OF MAMMALIAN PHOSPHOLIPASE-D

Phospholipase D (PLD) is implicated in important cellular processes, s uch as hormone action, inflammation, secretion, mitogenesis, and neura l activity. Recent steadies using cell-free systems have shown that th e enzyme activity is modulated by both positive and negative regulator s. During an attempt to purify PLD from pig colon mucosa, we noted the presence of a PLD inhibitor in the tissue extract. The inhibitor was purified and identified as comprising lysophosphatidylserine, phosphat idylinositol, and lysophosphatidylinositol, of which lysophosphatidyls erine was the most potent. These lipids affected all of the PLD isofor ms examined, oleate-dependent PLD, ARF-dependent PLD (PLD1a, PLD1b), a nd phosphatidylinositol 4,5-bisphosphate-dependent PLD (PLD2), in the concentration range of the 1 or 10 mu M order. In contrast to lysophos phatidylserine, the diacyl counterpart phosphatidylserine was without effect in the same concentration range. PLD inhibition by lysophosphat idylserine could not be reversed by an increase in the concentration o f the substrate phosphatidylcholine or activator phosphatidylinositol 4,5-bisphosphate.