Membrane properties of D-erythro-N-acyl sphingomyelins and their corresponding dihydro species

We have prepared acyl chain-defined D-erythro-sphingomyelins and D-erythro- dihydrosphingomyelins and compared their properties in monolayer and bilaye r membranes. Surface pressure/molecular area isotherms of D-erythro-N-16:0- sphingomyelin (16:0-SM) and D-erythro-N-16:0-dihydrosphingomyelin (16:0-DHS M) show very similar packing properties, except that the expanded-to-conden sed phase transition (crystallization) occurs at a lower surface pressure f or 16:0-DHSM. The measured surface potential was generally about 100 mV les s for 16:0-DHSM monolayers compared to 16:0-SM monolayers. The condensed do mains (crystals) that formed in 16:0-SM monolayers as a function of compres sion displayed star-shaped morphology when viewed under an epifluorescence microscope. 16:0-DHSM monolayers did not form similar crystals upon compres sion. I6:0-DHSM was degraded much faster by sphingomyelinase from Staphyloc occus aureus than 16:0-SM (10-fold difference in enzyme activity needed for comparable hydrolytic rate). Cholesterol desorption from 16:0-DHSM to cycl odextrin was slightly slower (similar to 20%) than the rate measured from 1 6:0-SM monolayers (at 60 mol % cholesterol). The bilayer melting temperatur e of 16:0-DHSM was 47.7 degreesC (DeltaH 8.3 kcal/mol) whereas it was 41.2 degreesC for 16:0-SM (DeltaH 8.1 kcal/mol). Cholesterol/16:0-DHSM bilayers (15 mol % sterol) had more condensed domains than comparable 16:0-SM bilaye rs, as evidenced from the quenching resistance of DPH in DHSM membranes. We conclude that cholesterol interacts more favorably with 16:0-DHSM and that the membranes are more condensed than comparable 16:0-SM-containing membra nes.