Topographical Distribution of Base Exchange Activities in Rat Brain Subcellular Fractions

Abstract: Enrichment in the base-exchange activities was found in the micro-somal fraction of rat brain, with less activity being associated with nuclei, mitochondria and synaptosomes. The distribution of the choline base exchange in microsomal subfractions differed from that for serine and ethanolamine and these three activities seemed asymmetrically distributed in the microsomes. Choline exchange activity was trypsin-sensitive and presumably was located on the cytoplasmic side of the microsomes, while serine and ethanolamine exchange activities were trypsin-insensitive and were assumed to be located on the luminal side of the microsomes. Treatment of rat brain microsomes with phospholipases A, C and D produced significant losses of membrane-bound base exchange activities. Some activity was restored in phospholipase C-treated microsomes by exogenous phospholipid, but significant restoration was not observed in phospholipase A-treated microsomes by such additions. Exogenous phospholipid stimulated choline and ethanolamine exchange activities, but not serine exchange activity of phospholipase D-treated microsomes. The exchange activities of rat brain microsomes differed in their responses to treatment with phospholipases, choline exchange activity in general being more sensitive than either serine or ethanolamine activities.