In Vivo Modification of Phospholipid Polar Head Groups and Alterations in Membrane Activities and Cellular Events

Abstract

Assesment of the functional role of the polar head group portion of membrane phospholipids has approached this problem by using lipid analog precursors to alter the composition of the polar head groups. We have shown that N-isopropyl-ethanolamine (IPE), a competitive inhibitor of choline, can be incorporated into 1,2-diacyl-sn-glycero-3-phosphoisopropylethanol (pohsphatidyl-IPE) of membranes from cultures L-M cells and rat liver. After IPE treatment, we observed profound inhibition of certain enzymatic activities (e.g., Δ9 acyl-desaturase and 5’-nucleotidase), but not in others (e.g., NADH-cytochrome c reductase, NADPH-cytochrome c reductase, and succinic dehydrogenas). The changes in enzyme activities do not appear to be directly related to membrane fluidity or the formation of phosphatidyl-IPE.

Experiments initiated to determine the time-dependent changes in metabolism cause by IPE in order to gather a more complete assessment of how IPE might be affecting the enzyme activities revealed that the following changes occurred in the IPE-treatead cells: (1) immediate inhibition of both the cellular uptake of [³H]choline and its incorporation into phosphatidylcholine, (2) a decrease in the incorporation of [³H]thymidine into DNA as early as 2 h after initiating the choline block, (3) inhibition of the cellular uptake of [³H]uridine and incorporation into RNA 15 to 24 h after addition of the IPE, and (4) stimulation of the cellular uptake of [³H]leucine and an inhibition of its incorporation into protein, which reached a maximum (68% of controls) 8 h after IPE treatment.

Collectively, our data indictate that the effect of the choline analog, IPE, on selected enzyme activities associated with the membranes is not cause by the presence of an unnatural polar head group in the membrane where the enzyme resides, but instead is apparently manifested via a derangement in the metabolism of phosphatidylcholine or choline, and this serves an important signal in the modulation of the expression of specific membrane-bound enzyme activities.

To whom correspondence should be addressed