Interaction of cholesterol with sphingomyelins and acyl-chain-matched phosphatidylcholines: A comparative study of the effect of the chain length

In this study we have synthesized sphingomyelins (SM) and phosphatidylcholi nes (PC) with amide-linked or sn-2 linked acyl chains with lengths from 14 to 24 carbons. The purpose was to examine how the chain length and degree o f unsaturation affected the interaction of cholesterol with these phospholi pids in model membrane systems. Monolayers of saturated SMs and PCs with ac yl chain lengths above 14 carbons were condensed and displayed a high colla pse pressure (similar to 70 mN/m). Monolayers of N-14:0-SM and 1(16:0)-2(14 :0)-PC had a much tower collapse pressure (58-60 mN/m) and monounsaturated SMs collapsed at similar to 50 mN/m. The relative interaction of cholestero l with these phospholipids was determined at 22 degrees C by measuring the rate of cholesterol desorption from mixed monolayers (50 mot % cholesterol; 20 mN/m) to beta-cyclodextrin in the subphase (1.7 mM). The rate of choles terol desorption was lower from saturated SM monolayers than from chain-mat ched PC monolayers. In SM monolayers, the rate of cholesterol desorption wa s very slow for all N-linked chains, whereas for PC monolayers we could obs erve higher desorption rates from monolayers of longer PCs. These results s how that cholesterol interacts favorably with SMs (low rate of desorption), whereas its interaction (or miscibility) with long chain PCs is weaker. In troduction of a single cis-unsaturation in the N-linked acyl chain of SMs l ed to faster rates of cholesterol desorption as compared with saturated SMs . The exception was monolayers of N-22:1-SM and N-24:1-SM from which choles terol desorbed almost as slowly as from the corresponding saturated SM mono layers. The results of this study suggest that cholesterol is most likely c apable of interacting with all physiologically relevant (including long-cha in) SMs present in the plasma membrane of cells.