Free radical-dependent inhibition of prostaglandin endoperoxide H Synthase-2 by nitro-arachidonic acid.

Prostaglandin endoperoxide H synthase (PGHS) is a heme-enzyme responsible for the conversion of arachidonic acid (AA) to prostaglandin H ₂ (PGH ₂ ). PGHS have both oxygenase (COX) and peroxidase (POX) activities and is present in two isoforms (PGHS-1 and -2) expressed in different tissues and cell conditions. It has been reported that PGHS activity is inhibited by the nitrated form of AA, nitro-arachidonic acid (NO ₂ AA), which in turn could be synthesized by PGHS under nitro-oxidative conditions. Specifically, NO ₂ AA inhibits COX in PGHS-1 as well as POX in both PGHS-1 and -2, in a dose and time-dependent manner. NO ₂ AA inhibition involves lowering the binding stability and displacing the heme group from the active site. However, the complete mechanism remains to be understood. This review describes the interactions of PGHS with NO ₂ AA, focusing on mechanisms of inhibition and nitration. In addition, using a novel approach combining EPR-spin trapping and mass spectrometry, we described possible intermediates formed during PGHS-2 catalysis and inhibition. This literature revision as well as the results presented here strongly suggest a free radical-dependent inhibitory mechanism of PGHS-2 by NO ₂ AA. This is of relevance towards understanding the underlying mechanism of inhibition of PGHS by NO ₂ AA and its anti-inflammatory potential.
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