Lung Surfactant

Lung surfactant is a lipid- and protein-containing substance that is secreted from lung and prevents collapse of the alveoli when air is expelled. Lung surfactant contains 50-60% dipalmitoylphosphatidylcholine, a saturated glycerophospholipid. Normally, about 90% of the fatty acids at position two for glycerophospholipids are unsaturated.

How does the palmitate arise at position 2 in lung surfactant? Current evidence rules out the possibility that an unsaturated fatty acid chain at position 2 is modified in situ to a palmitoyl chain. The two remaining possibilities are (1) direct transfer of palmitoyl chains to positions 1 and 2 of glycerphosphorylcholine and (2) hydrolytic cleavage (removal) of the unsaturated chain at position 2 followed by addition of the palmitoyl chain.

One enzyme capable of removing a fatty acid from a glycerophospholipid is phospholipase A 2, one of a class of four enzymes that hydrolyze specific bonds in phospholipids (Figure 19.6).

See also: Redistribution of Phospholipid Fatty Acids, Glycerophospholipids, Phospholipases,

INTERNET LINKS

> 1. Glycerolipid Metabolism

> 2. Phospholipid Catabolism

> 3. American Lung Association