The enzymes of lecithin biosynthesis in human newborn lungs. 3. Phosphorylcholine glyceride transferase.

Abstract

Extract: Phosphorylcholine glyceride transferase (EC. 2.7.8.2), the enzyme responsible for the final step of lecithin biosynthesis via the cytidine diphosphorylcholine (CDP-choline) pathway, is active in human neonatal lung tissue. A crude homogenate enzyme preparation in phosphate buffer was made from lung samples obtained at autopsy. The synthesis of product was linearly dependent upon protein concentration and linear with time for 30 min. The Michaelis constant for CDP-choline was 1.4 × 10-5 m. Optimal activity was attained at pH 7.5–8.0, at 35–39°, in the presence of Mg. Whole homogenate was 20–50 times as active as supernatant. No effect of oxygen or cysteine could be demonstrated. Triton and calcium inhibited the enzyme. The difference in enzyme activity between premature infants (3.53 × 10-7 mM lecithin/mg protein) and infants of longer (>32 weeks) gestation periods (1.76 × 10-7 mM lecithin/mg protein) is statistically significant at the 95% confidence limit. Speculation: Phosphorylcholine glyceride transferase could play a significant role in the relation between lung lecithin biosynthesis and respiratory distress syndrome of the human neonate. Characterization and study of the enzyme in human lung may lead to definition of its role as the possible regulator of pulmonary surfactant biosynthesis. There may be a relation between the level of enzyme activity and gestational age.