Phosphatidyl choline or Lecithin is a phosphoglyceride, ester of a diacylglycerol with a phosphate linked to another alcohol. The phosphoglyceride group includes, among others, phosphatidyl choline, phosphatidyl serine, phosphatidyl ethanolamine, and phospathidyl inositol.
Phosphoglycerides and sphingomyelin form the group of phospholipids. Phospholipids are amphipatic molecules: they have a hydrophobic part and a hydrophilic part. It makes them very suitable for the structure of membranes and lipoproteins.
Amphipatic molecules have surfactant properties: they decrease surface tension.
Normal lung functions depend on a constant supply of surfactants. Since in the case of a sphere, surface tension is inversely proportional to its radius (Laplace Law), the decrease of the radius of the alveolus during expiration increases the surface tension.
In absence of a surfactant substance, small alveolus will collapse easily, and the newborn should make great efforts to continue breathing. It explains that some premature babies who show a deficiency of Pulmonary Surfactant develop Respiratory Distress Syndrome (RDS).
Phospatidyl choline is the most abundant of the phospholipids in the extracellular fluid layer that lines alveoli of normal lungs, and is the main responsible of decreasing the surface tension of the fluid layer of the lung (surfactant properties), preventing atelectasia at the end of the expiration phase of breathing.
Different lecithins (phosphatidyl cholines) are remodeled by enzymes in pneumocytes in such a way that the resultant product is dipalmitoyl phosphatidyl choline.
Dipalmitoyl phosphatidyl choline is the most abundant phospholipid in adult surfactant.
Composition of adult surfactant
• Lipids: 80-90 % by weight
– Phospholipids (80-90 % of the lipids)
• Dipalmitoyl-phosphatidylcholine (about 50 %)
• Phosphatidyl glycerol (about 10 %)
• Sphingomyelin (about 3 %)
• Other lipids (10-20 %)
Surfactant Proteins: 10-20 % by weight
Once produced in the pneumocyte, surfactant migrates as “lamellar bodies” from the nucleus to the apical cell surface where the surfactant is released by exocytosis into the alveolus.
Usually between 32 and 36 weeks of pregnancy occurs a notable increase in the synthesis of phosphatidyl choline, while the synthesis of sphingomyelin does not experiment significant changes. That is why the ratio L/S (Lecithine/Sphingmyelin) in amniotic fluid is used as a marker of lung maturation when this information becomes necessary, for example, in the schedule of a medical elective cesarean section. Since the volume of amniotic fluid is very variable, it makes sense, for eliminating the influence of the dilution factor, to use the ratio of L/S instead of measuring just Lecithin concentration.
About 30 000 infants present Respiratory Distress Syndrome (former called Hyaline Membrane), in the United States each year. Around 10 % of them die.
Corticosteroid treatment is used to increase lung maturity in cases of risk of premature delivery. The effects of corticosteroids in enhancing the production of surfactant have been related to the induction of palmitate synthase expression in the pneumocytes. It makes more palmitate available for the synthesis a remodeling of lecithins to dipalmitoyl phosphatidyl choline.
The treatment of premature babies with RDS includes the administration of surfactant substances into the tracheobronchial tree. During this process, the infant is turned from side to side to facilitate uniform acinar distribution of surfactant throughout both lungs. Natural and synthetic surfactant preparations have currently been approved by the U.S. Food and Drug Administration, including colfosceril palmitate, a synthetic formulation, and natural surfactants composed of modified bovine and porcine lung extracts.
More information about surfactants and RDS can be found in:
Feng, A.; Stelle, D: Pediatrics, Respiratory Distress Syndrome
Pramanik, A: Respiratory Distress Syndrome