POSSIBLE INVOLVEMENT OF THE BASE-EXCHANGE ENZYMES IN THE PHOSPHOLIPID-METABOLISM IN LAN-2 CELLS

The incorporation of [14 C] choline, [14 C] ethanolamine, and [14 C] s erine by LAN-2 cells into their corresponding phospholipids was invest igated in the presence or absence of TPA. The presence of TPA increase d the amount of radioactivity incorporated into the phospholipids with a corresponding decrease in the amount of radioactivity in the cytoso lic compartment compared to control cultures. There were no difference s between TPA-exposed and control cells in the distribution of radioac tivity in free choline, phosphorylcholine or CDP-choline of [14 C] cho line labeled cells. This indicates that the increased lipid labeling w as not accompanied by enhanced labeling of the intermediates of the de novo pathway. These results suggest that a choline base exchange enzy me was stimulated in TPA exposed cells. In addition, the enhanced inco rporation of serine by TPA into its corresponding phospholipid implies the stimulation of the serine base exchange enzyme which is responsib le for phosphatidylserine synthesis in mammals. These observations sug gest a more general involvement of the base exchange enzymes in the mo dulation of membrane phospholipids.