DEVELOPMENTAL-CHANGES IN CHOLINEPHOSPHATE CYTIDYLYLTRANSFERASE ACTIVITY AND MICROSOMAL PHOSPHOLIPID FATTY-ACID COMPOSITION IN ALVEOLAR TYPE-II CELLS

Cholinephosphate cytidylyltransferase is the rate-limiting enzyme in t he choline pathway of phosphatidylcholine synthesis in fetal and adult lung. To examine the developmental changes in cytidylyltransferase ac tivity, subcellular fractions were prepared from freshly isolated type II cells from 19 and 21 d gestation fetal rats, newborn (0 h, 3 h, 3- 12 h, +1, +2, +7 postnatal day), and adult rats and the fractions assa yed for cytidylyltransferase activity. Cytidylyltransferase activity p er cell was low during late fetal gestation, but rose rapidly during t he first 3 h after birth, predominantly due to an increase in microsom al enzyme activity. Microsomal and cytosolic enzyme specific activity increased 6 and 3.9 fold, respectively, from birth (0 h) to +1 postnat al day. The subcellular distribution of enzyme activity changed f rom 40% microsomal in fetal type II cells to 85% microsomal at 3-12 h, and 60% in this compartment in adult type II cells. Although evidence sug gests that membrane lipid may affect enzyme activity, developmental ch anges in type II cell membrane phospholipid fatty acid composition hav e not been previously studied. Capillary gas chromatography analysis o f phospholipid fatty acids extracted from microsome fractions revealed a 3-fold increase in both total saturated and unsaturated phospholipi d fatty acids f rom day 19 to day 21 gestation. There was a further 62 % increase in total saturated fatty acids during the first postnatal d ay, concomitant with the peak in microsomal cytidylyltransferase activ ity.