Membrane Lipids and Cell Regulation 
John A. Glomset, M.D. — Investigator 
Dr. Glomset is also Professor of Medicine and Biochemistry at the University of Washington School of 
Medicine. He received his M.D. and M.D. /Ph.D. degrees from the University of Uppsala, Sweden. He then 
joined the Department of Medicine at the University of Washington. He received an honorary M.D. degree 
from the University of Oslo for his discovery of a plasma enzyme, lecithin:cholesterol acyltransferase 
(LCAT). Dr. Glomset is a member of the National Academy of Sciences. 
ONE of the major research areas in my labora- 
tory concerns the phosphorus-containing 
lipids (phospholipids) of animal cell mem- 
branes. In particular, we have been focusing at- 
tention on the biosynthesis and function of phos- 
pholipids that contain a polyunsaturated fatty 
acid, arachidonic acid. This fatty acid or its pre- 
cursor, linoleic acid, must be present in the diet 
if animals are to reproduce, develop, grow, and 
maintain their health. The basis for this require- 
ment is not fully understood, but most of the ara- 
chidonic acid that becomes available to animal 
cells is rapidly incorporated into membrane phos- 
pholipids. Furthermore, in response to various 
stimuli, a small amount of the arachidonic acid is 
released from the phospholipids and converted 
into products that can trigger many different cell 
functions. 
The mechanisms that promote the incorpora- 
tion of arachidonic acid into cell membrane 
phospholipids remain to be characterized, but it 
is clear that the metabolic pathways involved are 
not the classical ones described in most text- 
books. For example, phosphatidylinositol (PI), a 
phospholipid that plays an important role in cell 
signaling, typically contains high amounts of ara- 
chidonic acid. As much as 80 percent of the PI 
present in animal cell membranes consists of a 
molecular species that contains arachidonic acid 
and a saturated fatty acid, called stearic acid. Ex- 
periments with radioactive precursors have 
shown that cells form this exceptional species of 
PI, 5n-l-stearoyl-2-arachidonoyl (SA) PI, several 
hours after forming Pis that contain other fatty 
acids. 
To investigate the special metabolic pathway 
that forms SA PI, we recently examined the pos- 
sibility that animal cells in culture might be 
able to incorporate a radioactive precursor, sn-2- 
arachidonoyl monoacylglycerol, into this phos- 
pholipid. Our experiments showed that to be the 
case. Furthermore, follow-up incubation experi- 
ments with cell fractions identified two new en- 
zyme activities that seemed to be involved. A 
monoacylglycerol kinase activity could catalyze 
the phosphorylation of sn-2-arachidonoyl mono- 
acylglycerol to form the phospholipid sn-2- 
arachidonoyl lysophosphatidic acid, and a stear- 
oyl transacylase could convert the lysophospha- 
tidic acid into SA phosphatidic acid. In the 
presence of appropriate cofactors, other enzymes 
could convert the phosphatidic acid into the 
corresponding species of PI. Thus it appeared 
that we had discovered a new pathway of PI 
biosynthesis. 
More recent experiments have suggested that 
additional steps may contribute to the pathway. 
We have been able to solubilize the stearoyl trans- 
acylase from membranes and examine its specific- 
ity. Surprisingly, the enzyme can use SA PI as a 
stearoyl donor in the transacylase reaction. When 
it does, a major product of the reaction is sn-2- 
arachidonoyl lysophosphatidylinositol. If the 
enzyme forms this product in intact cells, the ly- 
sophosphatidylinositol might well have some spe- 
cial function or metabolic fate. To investigate this 
possibility we are currently conducting a search 
for enzymes that catalyze the conversion of lyso- 
phosphatidylinositol into PI. If we find some, we 
will characterize their activities in order to de- 
fine the pathway more completely. Then we will 
attempt to localize the various enzymes within 
cells in order to explore the pathway's intracellu- 
lar role. 
One potential role of pathways that form ara- 
chidonic acid-containing phospholipids may 
relate to the fine structure of animal cell mem- 
branes. Thus the arachidonic acid in phospho- 
lipids might conceivably affect their ability to 
pack tightly with one another in membranes. We 
began to investigate this possibility some time 
ago in parallel with our studies of phospholipid 
biosynthesis. We used a computer-based ap- 
proach to examine the effects of arachidonic acid 
on the conformation and packing properties of 
model phospholipids in simulated monolayers. 
The results of this molecular modeling approach 
have suggested that arachidonic acid-containing 
phospholipids may be able to form straight con- 
formations and pack together in regular, tight 
arrays. 
We are currently attempting to test this possi- 
bility, in collaboration with Howard Brockman of 
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