Turnover of phospholipid fatty acyl chains in cultured neuroblastoma cells: involvement of deacylation-reacylation and de novo synthesis in plasma membranes

Abstract

Cultured neuroblastoma cells (NIE-115) rapidly incorporated the essential fatty acid, linoleic acid (18:2 ( n − 6)), into membrane phospholipids. Fatty acid label appeared rapidly (2–10 min) in plasma membrane phospholipids without evidence of an initial lag. Specific activity (nmol fatty acid/μmol phospholipid) was 1.5–2-fold higher in microsomes than in plasma membrane. In these membrane fractions phosphatidylcholine had at least 2-fold higher specific activity than other phospholipids. With 32P as radioactive precursor, the specific activity of phosphatidylinositol was 2-fold higher compared to other phospholipids in both plasma membrane and microsomes. Thus a differential turnover of fatty acyl and head group moieties of both phospholipids was suggested. This was confirmed in dual-label ( 3H fatty acid and 32P), pulse-chase studies that showed a relatively rapid loss of fatty acyl chains compared to the head group of phosphatidylcholine; the opposite occurred with phosphatidylinositol. A high loss of fatty acyl chain relative to phosphorus indicated involvement of deacylation-reacylation in fatty acyl chain turnover. The patterns of label loss in pulse-chase experiments at 37 and 10°C indicated some independent synthesis and modification of plasma membrane phospholipids at the plasma membrane. Lysophosphatidylcholine acyltransferase and choline phosphotransferase activities were demonstrated in isolated plasma membrane in vitro. Thus, studies with intact cells and with isolated membrane fractions suggested that neuroblastoma plasma membranes possess enzyme activities capable of altering phospholipid fatty acyl chain composition by deacylation-reacylation and de novo synthesis at the plasma membrane itself.