The type II epithelial cells of the lung. II. Chemical composition and phospholipid synthesis

A method to isolate 93 per cent viable (trypan blue), 95 plus or minus 2 per cent pure type II alveolar epithelial cells (Kikkawa, Y., and Yoneda, K. Lab. Invest. 30: 76, 1974) was slightly modified and was used to isolate type II cells of similar purity and viability from New Zealand White male rabbits. The modification as described in this report now permits the regular harvest of 20 to 30 times 10-6 cells from a rabbit weighing 2 to 4 kg. Lipid composition of type II cell pools thus collected was compared with that of macrophages collected by alveolar washing. Per milligram of protein, type II cells contained 19 mug. of lipid phosphorus, compared with 11 mug. for macrophages. Per 1 times 10-8 cells, type II cells contained 110 mug. of lipid phosphorus compared with 155 mug. for macrophages; Phosphatidylcholine comprised 48 per cent of total phospholipid in type II cells and 34 per cent in macrophages. Forty-nine per cent of the phosphatidylcholine in type II cells was disaturated, compared with 29 per cent in macrophages. These data indicate that, per cell, type II cells contain almost twice as much disaturated phosphatidycholine as do alveolar macrophages. Radiolabeled precursor studies suggest that isolated type II cells synthesize disaturated lecithin predominantly through the cytidine diphosphocholine pathway. -14C-choline incorporation into disaturated lecithin was 3 times as active in type II cells as in macrophages. No evidence was found for a significant contribution to synthesis of disaturated lecithin by the transmethylation pathway in isolated type II cell preparations. These results offer the most direct evidence to date of the significant role of the type II alveolar epithelial cell in the synthesis of disaturated lecithin, a major constituent of pulmonary surfactant.