CHOLESTEROLS INTERFACIAL INTERACTIONS WITH SPHINGOMYELINS AND PHOSPHATIDYLCHOLINES - HYDROCARBON CHAIN STRUCTURE DETERMINES THE MAGNITUDE OF CONDENSATION

Cholesterol's interfacial interaction with different sphingomyelins and phosphatidylcholines has been investigated using a Langmuir film balance. The average molecular area of cholesterol/sphingomyelin (SM) or cholesterol/phosphatidylcholine (PC) mixed monolayers was determined as a function of film composition from the force-area isotherms measured at 24 degrees C. In contrast to previous results [Lund-Katz, S., Laboda, H. M., McLean, L. R., & Phillips, M. C. (1988) Biochemistry 27, 3416-3423], little difference was observed in equimolar cholesterol's ''condensing effect'' of SMs compared to PCs when their phase state was similar and when their hydrocarbon structural differences were.-minimized, For PCs, this meant that one acyl chain had to be long and capable of assuming an extended conformation and thus configurationally similar to the long-chain base of SM. This condition facilitated strong van der Waals attractive interactions with cholesterol's planar steroid ring and was satisfied when the sn-l acyl chain of PC was either myristate or palmitate. Under these conditions, the structural requirements of the sn-2 chain of PC were mitigated. For instance, at equimolar cholesterol, almost no difference was observed in the apparent molecular area condensations of 1-palmitoyl-2-oleoyl-PC and 1-palmitoyl-2-arachidonoyl-PC at surface pressures between 10 and 40 mN/m. In contrast, the apparent molecular area condensations of dioleoyl-PC and diarachidonoyl-PC were substantially reduced under identical experimental conditions. The results are discussed in terms of the relative importance of phospholipid/sphingolipid hydrocarbon and headgroup structure in determining the extent of interaction with cholesterol. Conclusions were based on investigation of egg SM, bovine brain SM, N-oleoyl-SM, dipalmitoyl-PC, dimyristoyl-PC, 1-myristoyl-2-palmitoyl-PC, 1-palmitoyl-2-oleoyl-PC, 1-palmitoyl-2-arachidonoyl-PC, dioleoyl-PC, dinervonoyl-PC, diarachidonoyl-PC, and diphytanoyl-PC as well as comparison with cholesterol's condensation of various galactosylceramide molecular species [Ali, S., Smaby, J. M., Brockman, H. L., and Brown, R. E. (1994) Biochemistry 33, 2900-2906].