Surfactant protein metabolism in vivo.

The surfactant components saturated phosphatidylcholine, SP-B and SP-C, are secreted together in lamellar bodies, and at least a part of the de novo synthesized SP-A is secreted independently. The surface film forms from tubular myelin and loose lipid arrays, and it generates unilamellar vesicles that lack surfactant proteins and are thought to represent catabolic forms. The half-life values for the clearance of surfactant proteins from lungs range from 6.5 to 28 h and vary with species. There is minimal information about the associations of the surfactant proteins with lipids or with each other after film formation, although all surfactant components seem to be recycled back into lamellar bodies in type II cells. The relative importance of type II cells or macrophages to the catabolism of the protein components of surfactant remains to be characterized, as do regulators of surfactant homeostasis.