Investigation of human low-density lipoprotein by H-1 nuclear magnetic resonance spectroscopy: Mobility of phosphatidylcholine and sphingomyelin headgroups characterizes the surface layer

The resolution of the trimethyl headgroup resonance of phosphatidylcholine (PC) and sphingomyelin (SM) in the intact human low-density lipoprotein (LD L) H-1 NMR spectrum at 600 MHz enabled the investigation of LDL surface str ucture and phospholipid-apoB interactions. We have previously shown that a higher proportion of PC headgroups (25-35% of total PC in LDL) compared to SM were tightly bound to apoB and therefore NMR-invisible [Murphy, H. C., e t al, (1997) Biochem. Biophys. Res. Commun. 234 (3), 733-737]. In the prese nt study, we have investigated the mobility of phospholipid (PL) headgroups , using H-1 NMR spin-spin (T-2) relaxation measurements, in LDL isolated fr om nine volunteers. We show that both PC and SM exist in two additional and distinct environments indicated by the biexponential behavior of the relax ation decays in each case. The data showed that 36% of PC headgroups had a short T-2 component, mean T-2 of 31 ms, and 64% had a longer T-2 component of 54 ms. Approximately 15% of SM headgroups had a short T-2 component (mea n T-2 Of 27 ms) and 85% had a longer T-2 component of 78 ms. Therefore the majority of SM headgroups (85%) were more mobile than PC (P < 0.001) and si nce PC headgroups in organic media were more mobile than SM, we conclude th at the characteristic high mobility of LDL SM is not an intrinsic property but arises from a high degree of order in molecular packing of the surface PL of human LDL. We suggest that because PC and SM interact differentially with cholesterol and possibly with neighboring phospholipids, this results in the formation of relatively long-lived microdomains of PL in vivo.