REGULATION OF CTP-CHOLINE-PHOSPHATE CYTIDYLYLTRANSFERASE BY POLYUNSATURATED N-3 FATTY-ACIDS

Disaturated phosphatidylcholine (DSPC) is the most distinctive surface -active lipid in pulmonary surfactant. The feeding of docosahexanoic a cid (DHA) 22:6 n-3 has recently been described to elevate the levels o f DSPC in rodent lung. The purpose of the present study was to determi ne the mechanisms by which this n-3 fatty acid might regulate CTP:chol ine-phosphate cytidylyltransferase, a key enzyme required for phosphat idylcholine (PC) synthesis. Cytidylyltransferase exists in lung cytoso l as a large lipid-associated aggregate (H form) which is active, and as an inactive, low-molecular-weight species (L form). Fatty acids in vitro stimulate and aggregate the inactive L form to the active H form . Short-term (2-h) and longterm (24-h) exposure of fetal lung explants to DHA (150 mu M) stimulated choline incorporation into PC by 54 and 64%, respectively. The fatty acid also enhanced DSPC synthesis by 88%. These changes were associated with an increase in the activity of cyt idylyltransferase by 63% after addition of DHA to the explant medium. In vitro, DHA (50 mu M) stimulated L form nearly 15-fold and appeared to be a more potent activator and aggregator of the enzyme than either linoleic 18:2 n-6 or arachidonic 20:4 n-6 acids. The effect of DHA on L-form activation was comparable, however, with other members of the n-3 family. Kinetic studies revealed that DHA increased the maximum ve locity of enzyme reaction for cytidylyltransferase, although it did no t alter the Michaelis constant of the enzyme for CTP. These observatio ns provide in vitro evidence that n-3 fatty acids may play an importan t role in the regulation of surfactant PC biosynthesis.