Surfactant - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 792

is synthesized in type-2 alveolar epithelial and Clara cells. A preproprotein is
synthesized and then cleaved to form a proprotein, itself splitted to produce the
mature protein. Protein SPb fluidizes lipid monolayers. It is a tubular myelin
component. It enhances lipid insertion into the air-liquid interface and respreading
during inflation, hence adjusting the surface tension. It is the most involved
surfactant protein in surface tension reduction. Protein SPb prevents lipid packing,
thereby airspace collapse. It is dissolved in the liquid-expanded rather than in
the liquid-condensed phase of artificial monolayers [ 1615 ]. 14 Protein-rich domains
form protrusions in compressed monolayers in order to cope with low surface
tension during exhalation, proteins being squeezed out of the monolayer. It then
reduces the size of the compressed monolayer.
Monomeric SPc is encoded by gene spc that belongs to chromosome 8. SPc
Protein is expressed in type-2 alveolar epithelial cells. Once it is synthesized,
SPc proprotein is cleaved to form the mature protein. Protein SPc is the lightest
and smallest surfactant protein. It is located only in the respiratory tract. This
transmembrane protein is highly hydrophobic and contributes to the organization
and maintenance of the surface active monolayer that attenuates surface tension.
Protein SPc enhances phospholipid adsorption rate to maintain a low surface tension
during inhalation and intervenes in the molecular ordering of the phospholipid
interfacial layer. It can also act on the liquid film stability.
Dodecameric SPd is the largest surfactant protein. 15 It regulates surfactant
composition, modulating surfactant lipid turnover. SPd Protein binds to phospha-
tidylinositol and glucosylceramide. It has also immunomodulatory function and
operates in mucociliary transport. Protein SPd has been identified in different
mucosal secretions, such as those of parotid, gall bladder, pancreas, digestive tract,
urinary conduits, and Eustachian tube [ 1559 ].

13.3.2 Surfactant Lipids

Surfactant proteins lower surface tension, as they combine with phospholipids to
form tubular myelin, recruit phospholipids and facilitate their spread on air-fluid
interfaces, and organize and maintain the surface active monolayer.

14 The artificial monolayer is composed of dipalmitoylphosphatidylcholine and dipalmitoylphos-
phatidylglycerol in a molar ratio of 4:1 with 0.2 mol% SPb.
15 Protein SPd is equivalent to conglutinin in blood circulation.