CTP:phosphocholine cytidylyltransferase activation by oxidized phosphatidylcholines correlates with a decrease in lipid order:: A 2H NMR analysis

The enzyme CTP:phosphocholine cytidylyltransferase (CT) binds reversibly to membranes and is active only in its membrane-bound form. Membrane lipid composition influences the equilibrium between its soluble and membrane-bound forms. Whereas the enzyme is not activated by phosphatidylcholine (PC vesicles, it is activated by PC vesicles that have been oxidized with HClO4 [Drobnies, A. E., et al. (1998) Biochim. Biophys. Acta 1393, 90-98], Here we explore the mechanism of activation of CT by a PC oxidized with lipoxidase, Multilamellar vesicles (MLVs) containing greater than or equal to 5 mol % oxidized 1-palmitoyl-2-arachidonoylPC (PAPC) progressively activated the enzyme, which was fully activated by 25 mol % oxidized PC. The effect of oxidized PAPC on lipid order was investigated by H-2 NMR, using MLVs containing PAPC perdeuterated on the palmitoyl chain. Spectral depaking generated order parameter profiles along the sn-1 chain. The average order parameter (S-CD) in the plateau region at 37 degrees C decreased from 0.18 to 0.15 with increasing percent of oxidized PAPC (0-25%), The change in S-CD was even greater near the end of the palmitoyl chain. CT activation was inversely related to lipid order. The major component of the lipoxidase-oxidized PAPC was purified and characterized by mass spectrometry and NMR. This component, 1-palmitoyl-2-(11,15-dihydroxy)eicosatrienoylPC (dihydroxyPAPC), incorporated into PAPC MLVs, also stimulated CT activity and reduced the lipid order parameter. Both effects were reversed by egg sphingomyelin. We propose that CT activation by oxidized PAPC is mediated by effects on lipid packing perturbations. This is the first study to report the effects of a purified oxidized PC on the orientational order along the acyl chain and to correlate the lipid disordering of the oxidized PC with the activation of a membrane-associated regulatory enzyme.