Lysosomes from rabbit type II cells catabolize surfactant lipids

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Abstract
The role of a lysosome fraction from rabbit type II cells in surfactant dipalmitoylphosphatidylcholine (DPPC) catabolism was investigated in vivo using radiolabeled DPPC and dihexadecylphosphatidylcholine (1,2-dihexadecyl- sn-glycero-3-phosphocholine; DEPC), a phospholipase A1- and A2-resistant analog of DPPC. Freshly isolated type II cells were gently disrupted by shearing, and lysosomes were isolated with Percoll density gradients (density range 1.0591–1.1457 g/ml). The lysosome fractions were relatively free of contaminating organelles as determined by electron microscopy and organelle marker enzymes. After intratracheal injection of rabbits with [3H]DPPC and [14C]DEPC associated with a trace amount of natural rabbit surfactant, the degradation-resistant DEPC accumulated 16-fold compared with DPPC in lysosome fractions at 15 h. Lysosomes can be isolated from freshly isolated type II cells, and lysosomes from type II cells are the primary catabolic organelle for alveolar surfactant DPPC following reuptake by type II cells in vivo.