Although there is a complexity of structures under the heading of PGs and their relatives, it will be helpful to describe briefly the differences between the major classes. All of these compounds are derived from fatty acids, usually arachidonic acid, with an open chain, 20-carbon structure. The PGs, including TX and PGL, all arise by mediation through a cyclic endoperoxide. On the other hand, LTs utilizing cy clooxygenase are formed directly from a fatty acid metabolite via lipoxygenases and do not involve a cyclic endoperoxide intermediate.
PGs resemble hairpins with a five-membered ring and two chains extending from the ring. Substituents on the five-membered ring determine the subclass and activity of the PGs. TXs have a six-membered ring with two chains extending from the ring. The six-membered ring has one or more oxygens associated with it. PGI2 has two adjacent rings, one of which contains an oxygen with one chain extending from each ring. Often TX and PGI2 are found to have opposing biological activities. LTs are essentially modified open chain fatty acids that may be conjugated to glutathione or glutathione degradation products. Lipoxins are open chain fatty acids. There is a wide array of related structures with characteristic activities or inactivity if such structures result from metabolism. The characteristics of the
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