<P> The prostaglandins are a group of physiologically active lipid compounds having diverse hormone - like effects in animals . Prostaglandins have been found in almost every tissue in humans and other animals . They are enzymatically derived from arachidonic acid a 20 - carbon polyunsaturated fatty acid . Every prostaglandin therefore contains 20 carbon atoms, including a 5 - carbon ring . They are a subclass of eicosanoids and form the prostanoid class of fatty acid derivatives . </P> <P> The prostaglandins are synthesized in the cell membrane by the cleavage of arachidonate from the phospholipids that make up the membrane . This is catalyzed either by phospholipase A acting directly on a membrane phospholipid, or by a lipase acting on DAG (diacyl - glycerol). The arachidonate is then acted upon by the cyclooxygenase component of prostaglandin synthase . This forms a cyclopentane ring in roughly the middle of the fatty acid chain . The reaction also adds 4 oxygen atoms derived from two molecules of O . The resulting molecule is prostaglandin G which is converted by the hydroperoxidase component of the enzyme complex into prostaglandin H . This highly unstable compound is rapidly transformed into other prostaglandins, prostacyclin and thromboxanes . These are then released into the interstitial fluids surrounding the cells that have manufactured the eicosanoid hormone . </P> <P> If arachidonate is acted upon by a lipoxygenase instead of cyclooxygenase, Hydroxyeicosatetraenoic acids and leukotrienes are formed . They also act as local hormones . </P> <P> Prostaglandins were originally believed to leave the cells via passive diffusion because of their high lipophilicity . The discovery of the prostaglandin transporter (PGT, SLCO2A1), which mediates the cellular uptake of prostaglandin, demonstrated that diffusion alone cannot explain the penetration of prostaglandin through the cellular membrane . The release of prostaglandin has now also been shown to be mediated by a specific transporter, namely the multidrug resistance protein 4 (MRP4, ABCC4), a member of the ATP - binding cassette transporter superfamily . Whether MRP4 is the only transporter releasing prostaglandins from the cells is still unclear . </P>

When are fatty acids used as an energy source