Lipid Compositions of Liquid-Ordered and Liquid-Disordered Phases in Ternary Membranes of Sphingomyelin, Cholesterol, and Dioleoylphosphatidylcholine Determined by 2H NMR: Stearoyl-Sphingomyelin Compared with Its Palmitoyl Counterpart.

Abstract

Sphingomyelin (SM) and cholesterol are the major lipids in the signaling platforms of cell membranes, known as lipid rafts. In particular, SM with a stearoyl chain (C18-SM) is abundant in specific tissues such as the brain, the most cholesterol-rich organ, whereas the distribution of palmitoyl (C16)-SM is ubiquitous. Here, we reveal the differences between palmitoyl- and stearoyl-SM in lipid-lipid interactions based on the tie lines obtained from the 2H solid-state NMR spectra of bilayer systems composed of SM/dioleoylphosphatidylcholine/cholesterol 33:33:33 and 40:40:20. Lipid probes carrying position-selective deuterations, 10',10'-d2-SM, 24-d1-cholesterol, and 6″,6″-d2-dioleoyl-phosphatidylcholine, were incorporated into the membranes. 2H NMR peaks from these probes in the membranes directly provide the lipid compositions of the liquid-ordered (Lo) and liquid-disordered (Ld) regions. Without using bulky fluorescent groups, these probes allow us to obtain the end points of the tie lines in a ternary phase diagram based on the lever rule. Consequently, the tie lines of the stearoyl-SM membranes were steeper than those of the palmitoyl-SM membranes, indicating that cholesterol content was higher in the Lo domains of stearoyl-SM, regardless of the total concentration of unsaturated phospholipids. When comparing the content of unsaturated lipids in the Lo domain, the stearoyl-SM membranes had a higher content than palmitoyl-SM membranes. These results revealed that stearoyl-SM is suitable for stabilizing biologically functional microdomains in cholesterol-rich organs, whereas palmitoyl-SM may be better suited for stabilizing domains in tissue membranes with normal cholesterol content. The small but significant differences in the lipid interactions between stearoyl-SM and palmitoyl-SM may be related to the spatiotemporal formation of functional domains in biological environments.