Interaction of cholesterol with various glycerophospholipids and sphingomyelin

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Abstract
The influence of cholesterol on the phase behavior of glycerophospholipids and sphingomyelins was investigated by spin-label electron spin resonance (ESR) spectroscopy. 4-(4,4-Dimethyl-3-oxy-2-tridecyl-2-oxazolidinyl)butanoic acid (5-SASL) and 1-stearoyl-2-[4-(4,4-dimethyl-3-oxy-2-tridecyl-2-oxazolidinyl)butanoyl]-sn-glycero-3-phosphocholine (5-PCSL) spin-label ESR spectra as a function of temperature were taken as empirical indicators of cholesterol-induced changes in the acyl chain motions in the fluid state. The Amax values of 5-PCSL exhibit a triphasic dependence on the concentration of cholesterol for phosphatidylcholines and bovine brain sphingomyelin. We interpret this dependence as reflecting the existence of liquid-disordered, ld, liquid-ordered, lo, and coexistence regions, ld + lo. The phase boundary between the ld and the two-phase region and the boundary between the lo and the two-phase region in the phosphatidylcholine-cholesterol systems coalesce at temperatures 25-33.degree.C above the main-chain melting transition temperature of the cholesterol-free phosphatidylcholine bilayers. In the case of bovine brain sphingomyelin, the ld-lo phase coalescence occurs about 47.degree.C above the melting temperature of the pure sphingomyelin. The selectivity of interaction of cholesterol with glycerophospholipids of varying headgroup charge was studied by comparing the cholesterol-induced changes in the Amax values of derivatives of phosphatidylcholine, phosphatidic acid, phosphatidylethanolamine, phosphatidylglycerol, and phosphatidylserine spin-labeled at the fifth position of the sn-2 chain. The Amax data were analyzed assuming (a) an equimolar stoichemistry for the phospholipid-cholesterol association and (b) that the chemical exchange between cholesterol-associated and cholesterol-free phospholipids in a bilayer is fast on the ESR time scale. The relative strength of cholesterol association with glycerophospholipids of varying chain length and chain composition and with sphingomyelins was assessed by analyzing the Amax data of 5-SASL in seven glycerophospholipid and three sphingomyelin multilayers. The results indicate that the interaction of cholesterol with sphingomyelin is the strongest. Increasing headgroup negative charge leads to a stronger association of cholesterol with phospholipids.