Cyclo-oxygenase pathway

Once arachidonic acid has been liberated cyclo-oxygenase catalyzes a step common to most body cells:

• arachidonic acid is converted to the cyclic endoperoxidase, PGG2, via an intermediate radical
• PGG2 is then converted to PGH2

The next prostanoid produced from PGH2 is largely dependent on the enzymes produced by the individual cell. For example:

• platelets have the enzyme thromboxane synthetase which can produce thromboxane A2 - TXA2
• vascular endothelium has the enzyme prostacyclin synthetase for the production of prostaglandin I2 - PGI2
• many cells have enzymes capable of producing prostaglandins D2, E2 and F2alpha: PD2, PE2, PF2alpha

Generally, the prostanoids have a short half-life. Most are rapidly taken up and degraded by intracellular enzymes. PGI2 is degraded by the kidney, also the route by which most prostanoid metabolites are excreted.