Respiratory distress syndrome in premature infants is due to inadequate secretion of which one of the following lipids?

Phosphatidylcholine or Lecithin i. This is a nitrogen-containing phospholipid. The word lecithin is derived from the Greek word, lekythos = egg yolk. It contains glycerol. ii. The alpha and beta positions are esterified with fatty acids. Usually the fatty acid attached to the beta-carbon is a PUFA molecule iii. The phosphoric acid is added to the third position, to form phosphatidic acid. The phosphate group is esterified to the quaternary nitrogen base, Choline (Fig.7.8). The molecules of lecithin exist as zwitterions (pl = 6.7). Action of Phospholipases Phospholipases are enzymes that hydrolyse phospholipids. Different phospholipases are involved in the hydrolysis of specific bonds in lecithin . Phospholipase A2 acts on an intact lecithin molecule hydrolysing the fatty acid esterified to the beta (second) carbon atom. The products are Lysolecithin and fatty acid. Lysole- cithin is a detergent and hemolytic agent. The enzyme is present in the venom of viper snakes. The hemolysis and consequent renal failure seen in viper poisoning could be thus explained. Actions of other phospholipases are shown in Figure 7.8. The products formed in each case may be summarized as follows: Phospholipase A2 Lecithin ------------- Lysolecithin + fatty acid Phospholipase A1 Lecithin ------------- Acyl glycerophosphoryl- choline + fatty acid Phospholipase C Lecithin ------------- 1,2 diacylglycerol + Phosphorylcholine Phospholipase D Lecithin ------------- Phosphatidic acid + choline Lung Surfactants Normal lung function depends on a constant supply of lung surfactants. It is produced by epithelial cells. It decreases surface tension of the aqueous layer of lung and prevents collapse of lung alveoli. Constituents of surfactants are dipalmitoyl lecithin, phosphatidylglycerol, cholesterol and surfactant proteins A, B and C. During fetal life, the lung synthesizes sphingomyelin before 28th week of gestation. But as fetus matures, more lecithin is synthesized. The lecithin-sphingomyelin (LS) ratio of amniotic fluid is an index of fetal maturity. A ratio of 2 indicates full lung maturity. Low levels of surfactant leads to respiratory distress syndrome (RDS), which is a common cause of neonatal morbidity. Respiratory Distress Syndrome (RDS) It is due to a defect in the biosynthesis of dipalmitoyl lecithin (DPL), the main pulmonary sur- factant. Premature infants have a higher incidence of RDS because the immature lungs do not synthesize enough DPL.Ref: DM Vasudevan - Textbook of Biochemistry, 8th edition, page no: 80