text: Prostaglandin H (PGH), 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 H:
  * 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 H:
  * Aspirin has been hypothesized to block the conversion of arachidonic acid to prostaglandin