AJP - Lung Cellular and Molecular Physiology, Vol 267, Issue 6 641-L648, Copyright © 1994 by American Physiological Society
Regulation of CTP:choline-phosphate cytidylyltransferase by polyunsaturated n-3 fatty acids
R. K. Mallampalli, R. G. Salome and A. A. Spector
Department of Internal Medicine, University of Iowa College of Medicine, Iowa City 52242.
Disaturated phosphatidylcholine (DSPC) is the most distinctive
surface-active lipid in pulmonary surfactant. The feeding of docosahexanoic
acid (DHA) 22:6 n-3 has recently been described to elevate the levels of
DSPC in rodent lung. The purpose of the present study was to determine the
mechanisms by which this n-3 fatty acid might regulate
CTP:choline-phosphate cytidylyltransferase, a key enzyme required for
phosphatidylcholine (PC) synthesis. Cytidylyltransferase exists in lung
cytosol 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 long-term (24-h) exposure of fetal lung explants to
DHA (150 microM) 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
cytidylyltransferase by 63% after addition of DHA to the explant medium. In
vitro, DHA (50 microM) 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 velocity of enzyme
reaction for cytidylyltransferase, although it did not alter the Michaelis
constant of the enzyme for CTP. These observations provide in vitro
evidence that n-3 fatty acids may play an important role in the regulation
of surfactant PC biosynthesis.
