Labeling of phospholipids in the surfactant and subcellular fractions of rabbit lung.

The principal lung phospholipids were studied following pulse labeling of rabbit lung in vivo with the phospholipid precursors [3H]glycerol and [14C]palmitic acid. Alveolar wash, microsome, and lamellar body fractions subsequently were isolated. The phospholipids and the fatty acid compositions of each phospholipid in each lung fraction were quantified. The appearance of labeled phosphatidylcholine, phosphatidylglycerol, phosphatidylinositol, and phosphatidylethanolamine plus phosphatidylserine in each lung fraction was measured. Labeled precursors were incorporated into all phospholipid classes in the microsomal fraction simultaneously, and the specific activity of each phospholipid was maximal within 30 min of isotope administration. Maximal specific activities of all phospholipids were measured in the lamellar body fraction by 2 to 4 h, and in alveolar wash by about 9 h. Phosphatidylglycerol and phosphatidylcholine contained equal amounts of palmitic acid (50%) and had the same specific activities in the lung fractions when labeled with [14C]palmitic acid. The biological half-life of each phospholipid in each lung fraction also was determined for a period of 3 days following isotope administration. Phosphatidylglycerol .and phosphatidylcholine were turned over more rapidly than were the other phospholipids. Relative biological half-life values for alveolar surfactant phospholipids labeled either with palmitic acid or glycerol were phosphatidylglycerol < phosphatidylcholine < phosphatidylinositol < phosphatidylethanolamine plus phosphatidylserine. Phosphatidylglycerol is an unique phospholipid in surfactant, and thus, may be as good a biochemical marker for the study of surfactant metabolism as is disaturated phosphatidylcholine.