Pathway of significance for the synthesis of pulmonary surfactant.

The most important component of pulmonary surfactant is dipalmitoyl phosphatidylcholine, or DPPC. At the end of an expiration, when the surfactant film is maximally compressed, DPPC is probably almost the sole component in the monomolecular film, but, during inspiration, as the film expands, other elements move in. They may be phospholipids with different fatty acids, saturated or unsaturated, or with the alcohol choline replaced with ethanolamine, inositol, glycerol or serine. The elements moving into the expanding film may also be free fatty acids, di- or triglycerides, etc. The synthesis of DPPC, similar to that of other phospholipids, has been thoroughly studied and is reviewed. The building blocks, glycerol, palmitic acid, choline and phosphate, are generally produced in the lung itself, but choline is generously supplied with the bloodstream. The slowest and, therefore, rate-limiting reaction is conversion of choline phosphate to cytidine diphosphate choline (CDP choline). The enzyme required, choline phosphate cytidyltransferase, is, therefore, crucial. It exists in two forms and can be stimulated to be converted to the more active H-form by the presence of phosphatidylglycerol. This would give PG a specific role; it has no unique ability to affect the surface tension, but it stimulates the synthesis of DPPC.