Prostaglandin E synthase -PGES) catalyzes the isomerization of PGH2 to PGE2. Cytosolic and microsomal PGES enzymes have been cloned and characterized. Cytosolic PGES -cPGES) is a glutathione-dependent enzyme with a predicted size of 18.6 kDa. The enzyme is expressed in a variety of tissues and cells, the levels of which are unaffected by treatment with interleukin-1.beta. and TNF-.alpha.. However, enzyme expression increases approximately 3-fold in rat brain following lipopolysaccharide treatment.