Phosphatidate phosphohydrolase catalyzes the hydrolysis of ceramide 1-phosphate, lysophosphatidate, and sphingosine 1-phosphate. 
A Mg2+-independent phosphatidate phosphohydrolase was purified from rat liver plasma membranes in two distinct forms, an anionic protein and a cationic protein.  Both forms of the enzyme dephosphorylated phosphatidate, ceramide 1-phosphate, lysophosphatidate, and sphingosine 1-phosphate.  When assayed at a constant molar ratio of lipid to Triton X-100 of 1:500, the apparent Km values of the anionic phosphohydrolase for the lipid substrates was 3.5, 1.9, 0.4, and 4.0 microM, respectively.  The relative catalytic efficiency of the enzyme for phosphatidate, ceramide 1-phosphate, lysophosphatidate, and sphingosine 1-phosphate was 0.16, 0.14, 0.48, and 0.04 liter (min x mg)-1, respectively.  The hydrolysis of phosphatidate was inhibited competitively by ceramide 1-phosphate, lysophosphatidate, and sphingosine 1-phosphate.  The Ki(app) values were 5.5, 5.9, and 4.0 microM, respectively.  The hydrolysis of phosphatidate by the phosphohydrolase conformed to a surface dilution kinetic model.  It is concluded that the enzyme is a lipid phosphomonoesterase that could modify the balance of phosphatidate, ceramide 1-phosphate, lysophosphatidate, and sphingosine 1-phosphate relative to diacylglycerol, ceramide, monoacylglycerol, and sphingosine, respectively.  The enzyme could thus play an important role in regulating cell activation and signal transduction.