Prostaglandin‐Synthesizing System in Rat Liver

The prostaglandin‐synthesizing system in rat liver was studied. When rat liver microsomes were incubated with radioactive arachidonic acid, two radioactive metabolites were detected which were exclusively identified with prostaglandin F 2 α and E 2 by the double isotope method and gas chromatography‐mass spectrometry (both electron impact and chemical ionization). Rat liver was found to produce always more prostaglandin F 2 α than prostaglandin E 2 . Thromboxane B 2 and other prostaglandins such as prostaglandin D 2 and 6‐ketoprostaglandia F 1 α were not detectd. When rat liver homogenates, instead of microsomes, were incubated with radioactive arachidonic acid, most of the arachidonic acid was found to be incroporated into a phospholipid fraction instead of being transformed into prostaglandin F 2 α and E 2 . When rat liver cytosol was added to the incubation mixture of microsomes prostaglandin synthesis was inhibited, while the incorporation of arachidonic acid into phospholipids was enhanced. The effect of the cytosol was lost when the cytosol was boiled beforehand. These facts suggest that some enzymes or heat labile co‐factors which facilitate the incorporation of arachidonic acid into phospholipids are present in rat liver cytosol. The prostaglandin synthetase was sensitive to small changes in pH, the optimum pH for the enzyme system being 9.7 for prostaglandin F 2 α and 8.9 for prostaglandin E 2 . Reduced glutathione, hydroquinone and EDTA were essentially inactive for the prostaglandin synthesis in liver. Under optimum conditions, rat liver microsomes were able to transform radioactive arachidonic acid into prostaglandin F 2 α and E 2 with a conversion rate as high as 20%