Abstract
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.
Highlights
Stimulation of cells with a wide variety of agonists including neurotransmitters and hormones activates phospholipases that generate glycerolipids and sphingolipids that are putative second messengers in signal transduction
Such changes in DAG are involved in the induction of DNA synthesis [1], oocyte maturation [3], and morphological changes in fibroblasts [4]
SPP is a potent stimulator of cell division and phospholipase D activation [23, 31], and it causes Ca2ϩ mobilization independently of inositol lipid hydrolysis [32]
Summary
Stimulation of cells with a wide variety of agonists including neurotransmitters and hormones activates phospholipases that generate glycerolipids and sphingolipids that are putative second messengers in signal transduction. 1 The abbreviations used are: DAG, sn-1,2-diacylglycerol; CerP, ceramide 1-phosphate; LPA, lysophosphatidate; PA, phosphatidate; PAP, phosphatidate phosphohydrolase; SPP, sphingosine 1-phosphate. SPP is a potent stimulator of cell division and phospholipase D activation [23, 31], and it causes Ca2ϩ mobilization independently of inositol lipid hydrolysis [32].
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