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Although the importance of phospholipase D (PLD) in signal transduction in mammalian cells is well documented, the negative regulation of PLD is poorly un- derstood. This is primarily due to a lack of known specific inhibitors of PLD. We herein report that the activity of par- tially purified rat brain PLD is inhibited by certain lysophos- pholipids, such as lysophosphatidylinositol, lysophosphati- dylglycerol, and lysophosphatidylserine in a highly specific manner. Inhibition of PLD by lysophospholipids was dose- dependent: the concentration of lysophosphatidylinositol required for half-maximal inhibition was about 3 m m . An analysis of the enzyme-kinetics suggested that lysophospho- lipids act as non-competitive inhibitors of PLD activity. As expected, PLD activity was stimulated by ADP-ribosylation factor (Arf) and phosphatidylinositol 4,5-bisphosphate (PIP 2 ). The inhibition of PLD by lysophospholipids, however, was not affected by the presence or absence of Arf or by an in- crease in PIP 2 concentration. A protein-binding assay sug- gested that lysophospholipids bind directly to PLD. These results indicate that the observed inhibition of PLD by lysophospholipids is due to their direct interaction rather than to an interaction between lysophospholipids and either Arf or PIP 2 . The present study suggests that certain lyso- phospholipids are specific inhibitors of rat brain PLD in a cell-free system and may provide the new opportunities to investigate mechanisms by which PLD is regulated by lyso- phospholipids, presumably liberated by phospholipase A 2 activation, in mammalian cells. —Ryu, S. B. and J. P. Palta. Specific inhibition of rat brain phospholipase D by lyso- phospholipids. J. Lipid Res. 2000. 41: 940-944.