Prostaglandin H2

Prostaglandin H₂ (PGH₂), or prostaglandin H2 (PGH2), is a type of prostaglandin and a precursor for many other biologically significant molecules. It is synthesized from arachidonic acid in a reaction catalyzed by a cyclooxygenase enzyme. The conversion from arachidonic acid to prostaglandin H₂ is a two-step process. First, COX-1 catalyzes the addition of two free oxygens to form the 1,2-dioxane bridge and a peroxide functional group to form prostaglandin G₂ (PGG₂). Second, COX-2 reduces the peroxide functional group to a secondary alcohol, forming prostaglandin H₂. Other peroxidases like hydroquinone have been observed to reduce PGG₂ to PGH₂. PGH₂ is unstable at room temperature, with a half life of 90–100 seconds, so it is often converted into a different prostaglandin.

It is acted upon by:

• prostacyclin synthase to create prostacyclin
• thromboxane-A synthase to create thromboxane A2 and 12-(S)-hydroxy-5Z,8E,10E-heptadecatrienoic acid (HHT) (see 12-Hydroxyheptadecatrienoic acid)
• prostaglandin D₂ synthase to create prostaglandin D₂
• prostaglandin E synthase to create prostaglandin E₂

It rearranges non-enzymatically to:

• A mixture of 12-(S)-hydroxy-5Z,8E,10E-heptadecatrienoic acid (HHT) and 12-(S)-hydroxy-5Z,8Z,10E-heptadecatrienoic acid (see 12-hydroxyheptadecatrienoic acid)

Functions of prostaglandin H₂:

• regulating the constriction and dilation of blood vessels
• stimulating platelet aggregation
  • binds to thromboxane receptor on platelets' cell membranes to trigger platelet migration and adhesion to other platelets.

Effects of aspirin on prostaglandin H₂:

• Aspirin has been hypothesized to block the conversion of arachidonic acid to prostaglandin