Specific transfer of dipalmitoyl phosphatidylcholine in rabbit lung

Liposomes were used as a tool to probe the specific transfer of dipalmitoyl phosphatidylcholine in rabbit lung. After incubation of liposomes with lung slices, lamellar bodies took up 30-50% more radioactively labeled dipalmitoyl phosphatidylcholine from egg yolk phosphatidylcholine liposomes than the labeled dioleoyl phosphatidylcholine, whereas mitochondria and microsomes took up both species at relatively equal rates. Furthermore, when 50% egg yolk phosphatidylcholine was replaced by dipalmitoyl phosphatidylcholine, the uptake of [1-14C]dipalmitoyl phosphatidylcholine by lamellar bodies was greatly reduced due to the isotope-dilution effect of the unlabeled dipalmitoyl species. This competitive uptake was not observed in mitochondria and microsomes. Similar results were also observed when liposomes were incubated with the cell-free homogenate. On the other hand, distearoyl or dioleoyl phosphatidylcholine did not compete with the uptake of [1-14C]dipalmitoyl phosphatidylcholine. These results suggest an intracellular specific transfer of dipalmitoyl phosphatidylcholine from liposomes to lamellar bodies, presumably by a specific transfer protein. When liposomes were labeled with [1-14C]dioleoyl phosphatidylcholine, the addition of unlabeled dioleoyl phosphatidylcholine up to 50% of total phosphatidylcholine concentration did not compete with the uptake of [1-14C]dioleoyl phosphatidylcholine in all fractions including lamellar bodies. This indicates that the intracellular uptake of liposomal dioleoyl phosphatidylcholine by lamellar bodies and other subcellular organelles was probably dominated by the fusion mechanism.