The enzyme responsible for the metabolism of arachidonic acid to the prostanoids (including prostaglandins and thromboxanes), commonly referred to as cyclooxygenase, is also more appropriately called prostaglandin H2 synthetase (PGHS), and possesses two active sites: the COX and the peroxidase (POX) sites. The conversion from arachidonic acid to the prostanoids is in fact a two-stage process, requiring activity at the COX site to first produce the unstable intermediate hydroperoxide, prostaglandin G2 (PGG2), which is then converted to prostaglandin H2 (PGH2) via POX. The enzymatic activity of COX relies on its being in the oxidized form and it is suggested that paracetamol interferes indirectly with this by acting as a reducing co-substrate at the POX site. In intact cells, when levels of arachidonic acid are low, paracetamol is a potent inhibitor of PG synthesis, by blocking the physiological regeneration of POX. However, in broken cells, where the concentration of hydroperoxides is high, prostaglandin synthesis is only weakly inhibited. This peroxide-dependent COX inhibition explains the differential activity of paracetamol in the brain where peroxide concentrations are low, vs peripheral sites of inflammation with high peroxide levels