Prostaglandin synthesis in membrane fractions from the rabbit renal medulla

In order to investigate the subcellular location of the prostaglandin synthesizing enzyme complex (prostaglandin synthetase) the distribution of this enzyme was analysed in membrane fractions from the rabbit renal medulla using an improved radioactive assay with arachidonic acid as a substrate. 3 membrane fractions were isolated and the subcellular origin of the membranes was determined by electron microscopy and by their contents of different marker enzymes. Fraction I was a plasma membrane fraction, fraction II was a cytomembrane fraction, mainly derived from the endoplasmic reticulum and fraction III was a mitochondrial fraction. The specific activity of prostaglandin synthetase in the homogenate of rabbit renal medulla was 0-1 nmol arachidonic acid converted/min per mg protein. In fraction II, the cytomembrane fraction, the specific activity was increased 2-4 times over the homogenate, the plasma membrane fraction and the mitochondrial fraction having lower activities. The results suggest that prostaglandin synthetase is located in the endoplasmic reticulum of renal medullary cells.