Observation of Two Modes of Inhibition of Human Microsomal Prostaglandin E Synthase 1 by the Cyclopentenone 15-Deoxy-Δ^12,14-prostaglandin J₂

Microsomal prostaglandin E synthase 1 (MPGES1) is an enzyme that produces the pro-inflammatory molecule prostaglandin E₂ (PGE₂). Effective inhibitors of MPGES1 are of considerable pharmacological interest for the selective control of pain, fever, and inflammation. The isoprostane, 15-deoxy-Δ^12,14-prostaglandin J₂ (15d-PGJ₂), a naturally occurring degradation product of prostaglandin D₂, is known to have anti-inflammatory properties. In this paper, we demonstrate that 15d-PGJ₂ can inhibit MPGES1 by covalent modification of residue C59 and by noncovalent inhibition through binding at the substrate (PGH₂) binding site. The mechanism of inhibition is dissected by analysis of the native enzyme and the MPGES1 C59A mutant in the presence of glutathione (GSH) and glutathione sulfonate. The location of inhibitor adduction and noncovalent binding was determined by triple mass spectrometry sequencing and with backbone amide H/D exchange mass spectrometry. The kinetics, regiochemistry, and stereochemistry of the spontaneous reaction of GSH with 15d-PGJ₂ were determined. The question of whether the anti-inflammatory properties of 15d-PGJ₂ are due to inhibition of MPGES1 is discussed.