This review article by Robin F. Irvine focuses on the control of free arachidonic acid levels in mammalian cells, a critical step in the synthesis of eicosanoids. The article discusses the involvement of lipases and acyltransferases in the liberation and reesterification of arachidonic acid, respectively. Key points include: 1. **Lipases and Arachidonic Acid Liberation**: The article explores the role of lipases in hydrolyzing phospholipids to release arachidonic acid. It highlights the specificity of certain lipases for arachidonic acid and the mechanisms by which these enzymes are regulated. 2. **Acyltransferases and Reesterification**: The role of acyltransferases in reesterifying arachidonic acid back into phospholipids is discussed, emphasizing the balance between liberation and reesterification processes. 3. **Subcellular Location of Arachidonic Acid Liberation**: The article examines the subcellular distribution of phospholipases and acyltransferases, noting that different enzymes may be localized in distinct cellular compartments. 4. **Control of Phospholipase Activity**: The regulation of phospholipases by various factors such as calcium, cyclic AMP, and hormones is reviewed, along with the challenges in interpreting the effects of phospholipase inhibitors. 5. **Corticosteroids and Arachidonic Acid Release**: The impact of corticosteroids on arachidonic acid availability is discussed, including the induction of proteins that inhibit phospholipase activity and the long-term effects on arachidonic acid synthesis. 6. **Contribution of Various Phospholipid Esters**: The article addresses the relative contributions of different phospholipid esters to arachidonic acid release, noting the complexity of arachidonic acid pools and the challenges in quantifying liberation accurately. 7. **Deacylation of Phosphatidylinositol**: The article also discusses the deacylation of phosphatidylinositol, a process that can release arachidonic acid, and the evidence for this mechanism in various tissues. Overall, the review provides a comprehensive overview of the complex mechanisms involved in the control of free arachidonic acid levels, highlighting the interplay between lipases, acyltransferases, and regulatory factors.This review article by Robin F. Irvine focuses on the control of free arachidonic acid levels in mammalian cells, a critical step in the synthesis of eicosanoids. The article discusses the involvement of lipases and acyltransferases in the liberation and reesterification of arachidonic acid, respectively. Key points include: 1. **Lipases and Arachidonic Acid Liberation**: The article explores the role of lipases in hydrolyzing phospholipids to release arachidonic acid. It highlights the specificity of certain lipases for arachidonic acid and the mechanisms by which these enzymes are regulated. 2. **Acyltransferases and Reesterification**: The role of acyltransferases in reesterifying arachidonic acid back into phospholipids is discussed, emphasizing the balance between liberation and reesterification processes. 3. **Subcellular Location of Arachidonic Acid Liberation**: The article examines the subcellular distribution of phospholipases and acyltransferases, noting that different enzymes may be localized in distinct cellular compartments. 4. **Control of Phospholipase Activity**: The regulation of phospholipases by various factors such as calcium, cyclic AMP, and hormones is reviewed, along with the challenges in interpreting the effects of phospholipase inhibitors. 5. **Corticosteroids and Arachidonic Acid Release**: The impact of corticosteroids on arachidonic acid availability is discussed, including the induction of proteins that inhibit phospholipase activity and the long-term effects on arachidonic acid synthesis. 6. **Contribution of Various Phospholipid Esters**: The article addresses the relative contributions of different phospholipid esters to arachidonic acid release, noting the complexity of arachidonic acid pools and the challenges in quantifying liberation accurately. 7. **Deacylation of Phosphatidylinositol**: The article also discusses the deacylation of phosphatidylinositol, a process that can release arachidonic acid, and the evidence for this mechanism in various tissues. Overall, the review provides a comprehensive overview of the complex mechanisms involved in the control of free arachidonic acid levels, highlighting the interplay between lipases, acyltransferases, and regulatory factors.