Bilayer Properties of Ceramides: Role of Interfacial Hydroxylation, Acyl Chain Length, and Chain Unsaturation

Abstract

Ceramides are structurally one of the simplest classes of sphingolipids and contain a large number of heterogeneous but very closely related molecules. In mammals they are produced by six different ceramide synthases and expressed in different cellular compartments or tissue types where they are believed to execute distinct functions. Natural ceramides contain a long-chain sphingoid base to which a fatty acyl moiety is amide linked. There is a wide variation in the length of the N-linked acyl chains, with even increasing levels of divergence introduced by unsaturations, hydroxylations and branching that may occur at different positions of the N-linked chain. We have examined the effect of the structural variations on the bilayer properties of different naturally occurring ceramides in mixed bilayers together with N-palmitoyl-Dᴅ-erythro-sphingosylphosphorylcholine (PSM), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and cholesterol. We performed differential scanning calorimetry and various fluorescence based measurements to study the ceramides’ interactions with PSM and their lateral segregation into ceramide-rich domains in POPC-bilayers, as well as the effect of cholesterol on such domains. Our results indicated that the saturated and hydroxylated ceramide species form domains with higher degree of chain packing and thermal stability than unsaturated species in fluid POPC-bilayers. The major differences were detected from the presence of cis-double bonds and their specific position in the acyl chain.