General Veterinary Biochemistry

EICOSANOIDS

• Eicosanoids (twenty) are derivatives of the 20 carbon polyunsaturated fatty acid, arachidonic acid.
• There are three classes of eicosanoids: Prostaglandins, thromboxanes and leukotrienes.
• They are unstable and insoluble in water. These signaling molecules generally do not move far from the tissues that produced them.
• They act primarily on cells very nearer to the point of release. Unlike hormones, they are not transported between tissues in the blood.
• These compounds are known as autocoids (Local hormones), because they act primarily in the tissues in which they are produced.
• It is abbreviated as PGX₂ where X is the letter indicating the structure ( substituents attached to the ring) and n is the number of double bonds in the hydrocarbon side chains. Similar system is adopted for thromboxanes and leukotrienes.
• The main classes of prostaglandins are PGA, PGE and PGF.
• The prostaglandins are derived from prostanoic acid, a 20 carbon fatty acid that contains a five carbon saturated ring. A side chain containing seven carbon atom is attached to the ring at carbon 8 is denoted by dashed lines indicating that it projects below the plane of the ring. The another side chain containing 8 carbon is attached at carbon 12, which projects above the plane of the ring (indicated by solid line).
• Prostaglandins with one or three double bonds have weaker inflammatory effects than with 2 double bonds.
• Thromboxane differs from prostaglandins that it has a six membered ring containing 5 carbon atoms and one oxygen atom. They were discovered from blood platelets or thrombocytes.
• Leukotriens discovered in leukocytes.

Prostaglandin Functions:

• They participate in many processes in the body. The biological actions are different in each organ.

Name of Prostaglandins	Actions
PGD2	Vasodilation, inhibition of platelet aggregation
PGF2	Myometrial contraction
PGI2 (Prostacyclin)	Vasodilation, inhibition of platelet aggregation.
PGE2	Vasodilation, Contraction of broncho and gastrointestinal smooth muscle. Inhibition of gastric secretion.
TXA2 (thromboxanes)	Vasoconstriction, platelet aggregation.
T4	Smooth muscle contraction, chemotaxis, inflammation and allergic reactions.