Characterization of Microsomal, Glutathione Dependent Prostaglandin E Synthase
Terminal prostanoid synthases constitute a group of several specific enzymes that catalyze the further metabolism of cyclooxygenase-derived prostaglandin H2. The efficient biosynthesis of prostaglandin E2 requires prostaglandin E synthase (E.C. 184.108.40.206). High prostaglandin E synthase (PGES) activity is found in the sheep and bovine vesicular gland. In this organ, both PGES and cyclooxygenase activities are localized to the microsomal membrane system, where the PGES activity is dependent on milli-molar concentrations of reduced glutathione. The activity of the enzyme rapidly deteriorated following solubilization, why any attempts to identify this protein by purification have not been successful1,2. A different method employed to try identifying the sheep microsomal PGES involved immunoprecipitation3. Two membrane associated PGES’s with molecular masses of 17.5 kDa and 180 kDa were reported. The smaller enzyme possessed a lower Km (40 1M) for PGH2than the larger protein (150.tM) and interestingly, the smaller enzyme was also localized to the same membrane system as cyclooxygenase3. None of these enzymes possessed any significant glutathione-S- transferase (GST) activity. In contrast, purification of the enzymes responsible for cytosolic PGES activity in human brain as well asAscaridia gallihave revealed their nature as cytosolic GST’s4,5. In addition to the glutathione dependent PGES’s, a glutathione independent, microsomal PGES has been characterized6which was recently purified from bovine heart7.
KeywordsCumene Hydroperoxide Eicosatetraenoic Acid Eicosanoid Metabolism Vesicular Gland Small Enzyme
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