Eicosanoids Exert Their Action Locally

The diversity of eicosanoid structures is paralleled by a diversity of biological effects. They are generally considered to be hormones that exert their main effect locally. Their 

Specific binding of PGE2 to a variety of cells correlates with an activation of adenylate cyclase and the accumulation of cAMP. In human adipocytes, PGE2 causes a 15-fold increase in the concentration of cAMP. In platelets, PGI2 rather than PGE2 appears to mediate the increase in cAMP. 

The eicosanoids have been implicated as mediators of tissue inflammation since aspirin s anti-inflammatory effect was shown to be the result of its inactivation of cyclooxygenase. How eicosanoids cause tissue inflammation is the focus of much current research. 

Fatty acids are components of a rich variety of complex lipid molecules that play critical structural roles in membranes. Some of these lipids are also the precursors of compounds with hormone or second-messenger activities. In this chapter we focused on the metabolism of these compounds with some mention of their functions. The following points are the highlights of our discussion. 

Lipid synthesis is unique in that it is almost exclusively localized to the surface of membrane structures. The reason for this restriction is the amphipathic nature of the lipid molecules. Phospholipids are biosynthesized by acylation of either glycerol-3-phosphate or dihydroxyacetone phosphate to form phosphatidic acid. This central intermediate can be converted into phospholipids by two different pathways. In one of these, phosphatidic acid reacts with CTP to yield CDP-diacylglycerol, which in bacteria is converted to phosphatidylserine, phosphatidylglycerol, or diphos- 

---

Eicosanoid Biosynthesis Eicosanoids Exert Their Action Locally... 

---