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Natural SP-A is a collagen-like, hydrophilic glycoprotein and constitutes 2-3% (by weight) of total extracellular surfactant. This protein has no direct surface-lowering capability but interacts with phospholipids, carbohydrates, calcium, and cell membrane receptors . It accelerates absorption of phospholipids to an air-liquid interface, regulates secretion and re-uptake of surfactant by alveolar type Il pneumocytes as well as the extracellular transformation of lamellar bodies to tubular myelin (this latter process also requires calcium and SP-B), stimulates phagocytosis of bacteria and viruses by alveolar macraphages, and increases the resistance af surfactant to inhibition by serum proteins.
SP-D is another collagen-like hydrophilic protein present in the airspaces which stimulates the production of free oxygen radicals by alveolar macrophages, but its biophysical role in the surfactant system has not been defined.
SP-B and SP-C are two hydrophobic proteins that together constitute 1-2% (by weight) of total extracellular surfactant. Both SP-B and SP-C enhance the absorption of surfactant phospholipids to an air-liquid interface and addition of hydrophobic proteins to a protein-free artificial surfactant improves its physical and physiological properties. SP-B is an essential constituent of tubular myelin, the extracellular reservoir of surfactant generating the surface film in terminal airspaces.