Use of liposomes in probing the uptake of liposomal phosphatidylcholine by rabbit lung in vitro

“Hydrophobic” Surfactant Apoproteins and Augmentation of Phospholipid Recycling

A 10,000-11,000 molecular weight apoprotein was isolated from an ethanol-ether extract of rat lung surfactant and purified by silicic acid chromatography. The protein (Apo Et) significantly augmented the uptake of phospholipids in liposomal form by cultured rat granular pneumocytes by a time-dependent process that varied with protein concentration and liposome composition. The protein had no effect on cell viability and showed no phospholipase activity. The mechanism for this augmented phospholipid uptake is not known but could be due to an alteration of physical form of the phospholipids by the protein or to a receptor-mediated uptake of phospholipids. This protein may prove to be a physiologically important regulator of the recycling of lung surfactant phospholipids.

Purification, characterization and substrate specificity of rabbit lung phospholipid transfer proteins

Three phospholipid transfer proteins, namely proteins I, II and III, were purified from the rabbit lung cytosolic fraction. The molecular masses of phospholipid transfer proteins I, II and III are 32 kilodaltons (kDa), 22 kDa and 32 kDa, respectively; their isoelectric point values are 6.5, 7.0 and 6.8, respectively. Phospholipid transfer proteins I and III transferred phosphatidylcholine (PC) and phosphatidylinositol (PI) from donor unilamellar liposomes to acceptor multilamellar liposomes; protein II transferred PC but not PI. All the three phospholipid transfer proteins transferred phosphatidylethanolamine poorly and showed no tendency to transfer triolein. The transfer of [14C]PC from unilamellar liposomes to multilamellar liposomes facilitated by each protein was affected differently by the presence of acidic phospholipids in the PC unilamellar liposomes. In an equal molar ratio of acidic phospholipid and PC, phosphatidylglycerol (PG) reduced the activities of proteins I and III by 70% (P = 0.0004 and 0.0032, respectively) whereas PI and phosphatidylserine (PS) had an insignificant effect. In contrast, the protein II activity was stimulated 2-3-times more by either PG (P = 0.0024), PI (P = 0.0006) or PS (P = 0.0038). In addition, protein II transferred dioleoylPC (DOPC) about 2-times more effectively than dipalmitoylPC (DPPC) (P = 0.0002), whereas proteins I and III transferred DPPC 20-40% more effectively than DOPC but this was statistically insignificant. The markedly different substrate specificities of the three lung phospholipid transfer proteins suggest that these proteins may play an important role in sorting intracellular membrane phospholipids, possibly including lung surfactant phospholipids.

Purification and characterization of two rabbit lung Ca2(+)-dependent phospholipid-binding proteins

Two Ca2(+)-dependent phospholipid-binding proteins (PLBPs) in rabbit lung cytosolic fraction have been purified to homogeneity. The apparent molecular weights of these two proteins are 36,000 and 33,000. Both the 36,000 and 33,000 PLBPs aggregated certain negatively charged unilamellar liposomes, but not the neutral phosphatidylcholine (PC) liposomes, in the presence of Ca2+. However, both PLBPs fused PC unilamellar liposomes to membrane acceptors. The 36,000 and 33,000 PLBPs had different specificities for phospholipid head groups, effects of Ca2+ and membrane charges and amino acid compositions. Both the PLBPs aggregated the surfactant membranes (lamellar bodies or from lung lavage) and nonsurfactant membranes (microsomes or mitochondria) to a level similar to that of the synthetic acidic phospholipid vesicles, but the proteins fused [14C]PC liposomes to the surfactant membranes 13- to 16-times more than to the synthetic phospholipid vesicles. The 36,000 PLBP fused [14C]PC liposomes to microsomes or mitochondria only half that of the fusion to the surfactant; the 33,000 PLBP fused [14C]PC liposomes to the surfactant and nonsurfactant alike. The PLBP's aggregate activity was not affected by the depletion of biological membrane proteins and the disruption of the native membrane integrity, but its fusion activity was greatly reduced. These results suggest that: (1) the 36,000 and 33,000 PLBPs are two different proteins; (2) the PLBPs may possess two catalytic reactions, one for the aggregation of small vesicles due to the binding of the protein to phospholipid vesicles and one for the fusion of small vesicles to acceptors; (3) the fusion activity was probably regulated by biological membrane proteins or structures; and (4) lung PLBP(s) might play a role in lung surfactant biogenesis.

Selective use of palmitic acid over stearic acid for synthesis of phosphatidylcholine and phosphatidylglycerol in lung

The incorporation of [3H]palmitic acid and [14C]stearic acid into phospholipids in rabbit lung tissue was studied. Under equal molar concentrations of palmitate and stearate, palmitate was incorporated to the 1- and 2-positions of phosphatidylcholine (PC) and phosphatidylglycerol (PG) 2-3 times more than stearate. By contrast, palmitate was 30% less than stearate in phosphatidylethanolamine, phosphatidylinositol and phosphatidylserine. These results suggest that preferential utilization of palmitate over stearate, rather than substrate availability, determines the high content of palmitoyl at the 1- and 2-positions of PC and PG in lung.

Association of liposomes with the isolated perfused rabbit heart

Liposomes were prepared from a mixture of either phosphatidylcholine (PC) and cholesterol (Ch) (7:2) (neutral), PC, dicetylphosphate and Ch (4:1:3) (negative), or PC, stearylamine and Ch (4:1:3) (positive). In addition, as the lipid phase-liposomal marker, 3H- or 14C-Ch was added. Alternatively, 14C-labelled mannitol, glucose or inulin was used as the aqueous phase-marker in some experiments. The liposomes were purified by Sephadex gel chromatography and by using microfilter. After 4 h of sonication, 95% of the total liposomes were found to be smaller than 1.2 microns. The entrapment volume was calculated to be 0.9-1.2 microliters/mumole of lipid. The ratio of lipid radioactivity and aqueous phase-radioactivity, which were found in a non-filtrable portion of the perfusate, remained constant during a heart-perfusion period of 30 min, indicating that the liposomes were stable during the experimental period. The wash-out experiment indicated that the liposomes were distributed in a space of the perfused heart nearly as large as the mannitol space. The liposomes were rapidly taken up by the heart during perfusion in a Langendorff manner. The positive liposomes were taken up by the perfused nonischemic heart at a greater rate than were the negative liposomes. The results with perfused ischemic hearts were equivocal. The liposomal label was found to be distributed unequally in the subcellular fractions, namely, relatively greater amounts (per mg protein) of liposome-bound radioactivity of Ch or mannitol were found in the microsomal fraction than in the mitochondrial or cytosolic fractions of the perfused heart. This distribution pattern was not influenced by the electrical charge of liposomes or by the oxygenation state of the heart perfusion. The accumulation of the liposomal label in the microsomal fraction found in the heart perfusion experiment could not be observed in the experiment in which tissue slices were incubated in the presence of liposomes, or in the experiment in which free mannitol was administered in the heart perfusion.

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.