Figure 1.

Biosynthesis of prostanoids. The resting levels of free arachidonic acid (AA) in cells are low and PG synthesis is dependent on the provision of AA from phospholipid. The PLA₂ enzymes are the major providers but free AA can be generated indirectly via PLC or PLD activities. The COX protein— in either isoform —exhibits two separate enzymic activities, as shown. Inhibition of the COX activity does not inhibit peroxidase activity. All the final prostanoid products have a common precursor, PGH₂, and separate synthase enzymes. So far only PGE₂ synthase has been found to be inducible, by the same stimuli as induce COX-2. The cyclopentenone PG ligands for the nuclear receptor PPARγ, PGJ₂ and 15dPGJ₂, are derived only from PGD₂ by ill-defined pathways in vivo.