Prostaglandins are critical messengers in all sorts of reactions. They are part of a group called the eicosanoids and include prostacyclins (PGI), the thromboxanes (TXA), the leukotriences (LT) and lipoxins.
These compounds are all 20-carbon and are derivatives of the 20-carbon fatty acid called prostanoic acid so are also known as prostanoids. This interesting fatty acid has a cyclopentane ring which is formed by carbon atoms C8 to C12. There are two side chains with a carboxyl group on one side. The prostaglandins vary in their structure due to substituent group and double bond on the cyclopentane ring.
The most important prostaglandins are PGF2 and PGF2alpha, the prostacyclin (PGI2), thromboxane (TXA2) and various leukotrienes (LTA4). They all act within the cell where they are synthesized but are not hormones.
Synthesis Of Prostaglandins
Arachidonic acid (5,8,11,14 – eicosatetraenoic acid also known as eicosenoic acid) is the precursor for most of the prostaglandins in humans. Their synthesis occurs in the endoplasmic reticulum. Arachidonic acid is released from membrane bound phospholipids using phospholipase A2. This release is activated by certain specific stimuli from hormones epinephrine and bradykinin. The role of bradykinin indicates that these molecules will be involved in inflammation.
The key reaction in synthesis is the cyclization in the centre of the carbon chain to form a cyclopentane ring. The differences in each prostaglandin is the position of double bonds, hydroxyl and keto groups along the chain.
The prostanoids are subdivided then into prostaglandins, prostacyclins and thromboxanes. The other major group are the leukotrienes.
Biological Function
- There role is as mediators in inflammatory responses especially those relating to joints as in rheumatoid arthritis, in the skin condition psoriasis and in eye complaints.
- Mediators in fever and pain.
- Blood pressure regulation
- Contraction and relaxation of uterine muscles.
- Regulation of waking and sleeping cycles.
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